Brachytherapy: Applications and Technique, 1st Edition

2. Technical Physics Support for Brachytherapy

 

Robert A. Cormack

Jorgen L. Hansen

Desmond A. O'Farrel

Alexandra J. Stewart

The field of radiation therapy originated with brachytherapy using radium sources to deliver a dose. However, brachytherapy is now largely viewed as a subfield within radiation oncology with the number of brachytherapy practitioners dwarfed by the number of radiation oncologists with experience in treating with external beam. With this in mind, it is good to review some of the needs of a brachytherapy service that may be beyond those routinely considered to be part of an external beam clinic. These differences arise from the nature of the radiation sources, the invasive nature of the procedures, and the needs of treatment planning.

General Resources

Sources

Brachytherapy has come a long way since the early years of the twentieth century when radium needles were placed in proximity to accessible tumors. The advent of the nuclear age provided a selection of β and γ sources having a range of energies and half-lives. Some isotopes commonly used in radiation oncology are listed in Table 2.1. Unlike that from a linear accelerator, the radiation from these sources cannot be turned off, and shielding and distance from the source provide safety to patient and staff.

A treatment team routinely involved in planning and delivery of brachytherapy must be familiar with the nature and shielding requirements of the various sources. The team should always recall that other staffs, within and beyond the treating department, do not necessarily share that knowledge. The variety of procedures from permanent or temporary implants, inpatient or outpatient, low dose rate (LDR) to high dose rate (HDR) through inpatient temporary LDR suggest that these patients will impact a number of areas beyond the treatment department including, operating rooms, recovery rooms, patient rooms, emergency rooms, radiology scanners, pathology labs, and even the morgue. Staff in all these locations and more will undoubtedly have questions about implants and radiation. Communication with these groups is essential and may be facilitated by the institution's Radiation Safety Officer.

Table 2.1 Isotopes and their Physical Properties

Isotope

Emission Type

Half-life

Energy (keV)

198Au

γ

2.7 d

412

131Cs

γ

9.7 d

34 max

137Cs

γ

30 y

662

125I

X-ray

59 d

27–35

192Ir

γ

74 d

290–608

32P

β

14 d

1,710

103Pd

X-ray

17 d

20–23

90Sr - 90Y

β

29 y

2,280 max

LBNL Isotopes Project. http//:IE.LBL.gov/TOI.

Core Staff for Department

There are a number of roles that must be filled in a brachytherapy group. The brachytherapy practitioner will be supported by nursing, treatment-planning, radiation safety, and radiation physics staff. Depending on the types of treatment, the core group may also require a radiology technologist or a radiation therapist. Whereas a single person may fill more than one of these roles, care must be taken to ensure there exists a means for independent checks of key treatment parameters. There is a great deal of information available from each staff member's professional society. A list of such societies in the United States and other helpful organizations appears in Table 2.2.

Renewable Supplies Needed for a Brachytherapy Department

A substantial number of applicators in different sizes and configurations are required for treatment of different sites. Applicators are usually categorized as gynecologic, intraluminal, vascular, interstitial, surface, and special applicators. The number and type of applicators routinely stocked will be dictated by the clinical focus of the department.

For most gynecologic applicators, an applicator fixation system is required. This system prevents applicator motion during verification and treatment. Radiopaque dummy strands are needed for visualizing applicators or catheters on x-rays or computed tomography (CT) scans. Radiopaque markers are useful for visualizing scars and anatomic reference marks on the patient. Surface applicators can be custom made from commercial material similar to Freiburg Flab (Nucletron) or Harrison, Anderson, Mick (HAM) applicators (Mick Radio-Nuclear Instruments Inc.) or made in-house from thermoplastic material. Examples of these applicators are shown in Chapters 3, 5 and 10. Surgical and nursing supplies will be needed if invasive procedures are performed.

Table 2.2 Professional Societies and Relevant Organizations

American Association of Physicists in Medicine
American Association of Medical Dosimetrists
American Brachytherapy Society
American College of Radiology
American Society of Therapeutic Radiation Oncology
Nuclear Regulatory Committee
State Department of Public Health
U.S. Food and Drug Administration
National Committee for Radiation Protection

Treatment Planning

The rapid development in imaging techniques has aided the brachytherapist in more accurate delineation of structures of interest. Modern brachytherapy planning is moving from reference point dosimetry to clinical target volume dosimetry. Most radiation oncology departments have a CT scan simulator in the department and many are adding computed tomography-positron emission tomography (CT-PET) or magnetic resonance imaging (MRI) capabilities. A detailed understanding of the data from the studies, combined with an understanding of the anatomy seen, is crucial for the accuracy of brachytherapy. The potential for movement of applicator and organ with respiration, and between fractions, and during long LDR hospitalizations must be recognized, planned for, and accounted for. A number of image modalities offer support for the placement, delivery, or evaluation of brachytherapy.

X-ray

A pair of x-ray films can be used to localize the position of applicators, seeds, or other radiopaque landmarks within the patient. In the event that filming on a simulator within the department is not an option, jigs of known geometry and reference points can be used. Soft tissue detail is not available, nor is three-dimensional anatomic information. However, the configuration of the sources, with respect to bony anatomy, surgical clips, contrast-filled catheters and the like, can be determined (see Fig. 2.1).

While dosimetry derived from planar films may be suboptimal, x-rays or fluoroscopy has an important role in the placement of implants. During a procedure, x-rays offer a noninvasive, nondistorting means of verifying seed deployment during a permanent prostate implant, catheter placement for an endobronchial treatment, and general needle configuration for interstitial implants. X-ray imaging also offers verification of applicator placement and configuration.

Figure 2.1 Chest x-ray to plan palliative endobronchial high dose rate brachytherapy. The reference points are the anatomic landmarks. These must be correlated with the clinical findings at endoscopy to prevent a geographic miss of the tumor.

Ultrasound

Ultrasound offers a relatively low cost means of soft tissue visualization to assist in placement of an implant (see Fig. 2.2). Widely used in prostate cancer, the use of transrectal ultrasound gives the experienced user a workable image to define the target. With this modality, adjacent normal structures, the urethra, rectum, and bladder, can readily be identified. In this setting, anterior, posterior, and lateral margins are readily identifiable but the base and apex require a much more subtle clinical investigation. Some institutions use an in-house algorithm to define these anatomic landmarks. Ultrasound also has useful gynecologic applications as a portable quick way of visualizing paracervical and paravaginal anomalies during intracavitary or interstitial applications. It can also be used to ensure that the bladder is not punctured during interstitial needle placement. Intravascular ultrasound can also assist in localizing and measuring regions of interest for delivering vascular brachytherapy.

Computed Tomography

CT simulation has, in many departments, supplanted the use of orthogonal films. The accrued axial CT images and reconstructed sagittal and coronal images have made seed, strand, and applicator identification much easier. Normal tissues and target volumes can also be modeled using contouring. A limitation is the image artifact from high Z materials, such as applicators or seeds, which can saturate the image gray scale, yielding large areas with no data or erroneous data. This can be corrected with the use of increasingly available CT scan compatible applicators. CT imaging has greatly simplified the postimplant evaluation of permanent implants (see Fig. 2.3).

Figure 2.2 A moveable ultrasound unit provides useful real-time image guidance for prostate brachytherapy. Other uses of brachytherapy include imaging guidance of tandem and ovoid placement and intracavitary partial breast implant placement.

Figure 2.3 The computed tomography (CT) scanner fixed in the middle of a dedicated brachytherapy suite allows for optimal image guidance of a variety of brachytherapy procedures. It can also be used for routine treatment-planning scanning. Multiple applicators specifically adapted so as not to distort the CT scan or MRI image are available.

Magnetic Resonance Imaging

While not widely used as the modality for actual implantation, but used rather for reference, the MRI with its multitude of imaging techniques can be used to delineate differences in soft tissues not readily seen on ultrasound or CT scanning. MRI is particularly valuable for imaging of the pelvis. This may allow the brachytherapist to limit the extent of the implant. Distortions occur in MR images because of the presence of metal applicators, trochars, and needles. MRI compatible hardware is increasingly becoming commercially available. Examples of these applicators and images are found in Chapters 8 and 9.

Documentation

Radiotherapy Prescription

This is the formal written instruction to deliver the radiation dose to the clinical target area. Whereas there may be some overlap and potential redundancy in creating a separate written directive for brachytherapy, there are many advantages. Many brachytherapy cases also receive external beam radiation therapy. The written prescription is the correct document to incorporate both prescriptions and alert all parties to the potential combined effect of these modalities. It is important to state whether concurrent chemotherapy has been given so that this can be accounted for in the dose prescribed. Figure 2.4 is the standard written prescription form currently in use at the DFCI BWH Cancer Center.

 

Figure 2.4 This is a standard radiation prescription that can be used for both external beam and brachytherapy. It is important to indicate the cumulative dose from different courses of radiation and brachytherapy.

Written Directive

This is the formal written instruction from the physician to the physics staff and therapists to apply brachytherapy methodology to the implant to achieve a required dose of radiation to a specified site. The directive should have the patient's identity clearly stated. It should state the isotope, dose rate, site to be treated, type of applicator, and type of implant: Temporary or permanent. The directive should specify either the dose and prescription point, or the number of sources, source activities, and duration of the implant. The written directive currently in use at the DFCI BWH Cancer Center is shown in Figure 2.5.

 

Figure 2.5 This is a standard written directive for brachytherapy. This is distinct from the radiation therapy prescription, though the prescription includes an indication of the brachytherapy dose. The elements of the written directive assist the brachytherapy team to deliver the prescribed dose precisely. Note that there is a special section to make amendments to the directive. The directive also has space for a diagram of applicator placement, channel assignment, or other important details. The right-hand columns are for recording each high dose rate fraction and is to be signed by the authorized user. This document also details the cumulative dose from the brachytherapy.

 

Figure 2.6 The low dose rate brachytherapy implant checklist is a two-page form to document all phases of planning, source preparation, equipment availability, loading, unloading, source return, and paperwork.

 

Isotope Recording Forms

Whereas HDR systems may have intrinsic documentation of the dwell times, channels, and activity planned and delivered, for very low dose rate (VLDR) and LDR these important issues need to be documented. The record should clearly demonstrate which source went where, when, for how long and when it was withdrawn and by whom. As distinct from the prescription or directive, these generally document the detailed placement and removal of specific amounts and forms of isotope used in the delivery of brachytherapy. Figures 2.6 to 2.7 are the LDR implant checklist (two pages) and is recorded in an isotope handling form, Figure 2.8. The record of placement and removal of the radioactive implant for the patient chart.

Brachytherapy Quality Assurance

Each center should develop a quality assurance (QA) process. This should ensure that all necessary documentation is present in the patient's record. A safe methodology is to insist that no isotope may be delivered without the minimum information, such as confirmation of patient identification, informed consent, a prescription, a written directive, having been recorded as being present. Regular peer review of new cases and changes in planned therapy is standard in most departments for external beam therapy, and brachytherapy should receive the same scrutiny. Brachytherapy cases can either be integrated into this process or reviewed separately. In a center that has a large number of brachytherapy cases, regular brachytherapy-specific chart rounds may need to be conducted. Nontreating physicians should review all relevant chart materials. Representatives of each supporting specialty such as nursing, physics, planning, and administration should be involved in the review process. Evaluation of the adequacy of the implant as well as the suitability of the patient and stage of disease is necessary. An example of the DFCI BWH Cancer Center brachytherapy chart QA form is seen in Figure 2.9. The minutes of these meetings should be related to the larger QA processes of the Department of Radiation Oncology and the treating hospital or clinic.

High Dose Rate

Introduction

HDR machines use high activity sources to deliver a treatment in the course of minutes. The high activity requires that HDR machines operate only in rooms that have adequate shielding. Because treatment fractions are brief, the HDR allows some treatments to be delivered as outpatient procedures and may offer advantages to the medical management of patients. At this point in time, a number of HDR afterloaders are available (see Figs. 2.10, 2.11, 2.12 and 2.13).

Facilities and Equipment

The optimal configuration for an HDR clinic is a dedicated shielded treatment room. The configuration of the brachytherapy suite at the DFCI BWH Cancer Center is shown in Figure 2.14. However, a dedicated shielded brachytherapy suite is not possible or feasible in most clinics. Linear accelerator rooms are adequately shielded for a 10 curie (Ci) iridium 192 (192Ir) source, and can therefore be wired and configured for HDR brachytherapy. Visual and audible monitoring is required to monitor patients during treatments, and a patient entertainment system (radio, TV, CD, and/or DVD) aids in increased patient comfort during long-duration treatment procedures. Anesthesia may be required for some HDR procedures—either local or general. This must include remote monitoring if a patient is to receive HDR treatment while under general anesthesia (see Table 2.3).

The high activity of the radioactive source used for HDR brachytherapy necessitates a dedicated afterloader system. Manufacturers typically perform the periodic source replacements and machine maintenance procedures. A computerized treatment-planning system is required. This system should be able to work intimately with the afterloader, and in most configurations delivers a computer file with the treatment plan to the afterloader control system. HDR comes with proprietary software intrinsic to the system on which treatment planning must be performed. A secondary computer system is required for QA of the treatment plan.

Figure 2.7 The low dose rate brachytherapy implant checklist is a two-page form to document all phases of planning, source preparation, equipment availability, loading, unloading, source return, and paperwork.

Figure 2.8 This isotope handling form is the official medical record of the actual transfer of the specific isotope to the patient and back to the hot lab. The details of isotopes, activity, form, time of loading and (for temporary implants) unloading, personnel involved, signatures of authorized users loading and unloading are all recorded.

 

Figure 2.9 The Brachytherapy Chart Quality Assurance (QA) Form documents adherence to the QA requirements. The first five elements must be completed before a treatment can start. Note that there is space to document the physician and physicist peer review, as well as the presence of the brachytherapy completion note.

Figure 2.10 Nucletron Corporation's Microselectron afterloader with bail out container positioned in a dependent area under the indexer plate. The afterloader houses an iridium source of nominal 10 Ci activity. Shielding protects staff and the patient when the source is not extended. High dose rate units have independent built-in radiation detectors. The container has sufficient shielding to safely hold the source in the event it becomes lodged in an applicator and cannot return to the shielded area of the afterloader.

 

Figure 2.11 Varian VariSource high dose rate afterloader.

Figure 2.12 Varian GammaMed high dose rate afterloader.

An imaging system is required for treatment verification. It documents the applicator used and its correct placement. A conventional simulator is a fast and adequate method for this purpose. If contouring is desired for dose–volume histogram generation, a CT or MR scanner is required. Most clinics have film capabilities, but digital imaging is becoming more common.

Quality control and radiation safety instrumentation are required. A well counter (see Fig. 2.15) is used for verification of source activity. Room monitors (see Fig. 2.16) and survey meters (see Fig. 2.17) are used to ensure staff and patient safety when the source is assumed to be in a retracted position. Redundant radiation monitors provide parallel security against exposure. Radiation badges are required to document the safety of the staff, and real-time personnel monitors are needed during emergency procedures. Emergency equipment including long forceps and scissors are required in case a source becomes stuck in an exposed position (see Fig. 2.18.) These should be clearly visible and easily accessible, for example, on the wall above the treatment unit. It is advisable to purchase several sets of these instruments.

 

Figure 2.13 Isodose control flexitron high dose rate afterloader.

To start the HDR program, specific training of therapists, planners, and physicists is required. A specific quality management program (QMP) for this modality is needed. This must include descriptions of the equipment, treatment preparation, delivery and recovery, safety procedures, commissioning, and periodic maintenance.

Written Procedures

Written procedures document essential QA aspects of therapy. In the United States, radiation therapy departments are advised to have a written QMP referring to all procedures used in the department, and this is often a regulatory requirement. This describes a system to quality check the procedures and allows methods for improvements (see Table 2.4). The QMP must be updated for all new equipment, isotopes, and modalities. These updates must be approved by institutional and government agencies.

The afterloader QMP procedure details the frequency of checking source activity, timer accuracy and linearity, positional accuracy, applicator integrity and geometry, safety interlocks, and emergency retraction functionality. Safety interlocks may be evaluated daily while other features may only need to be evaluated on a monthly basis or after any source exchange or major maintenance on the device. There should also be a regular schedule of QA for all applicators, connectors, and other equipment used in conjunction with the device.

When a prescription has been written and a plan has been created, a quality control procedure needs to be in place to verify that the plan conforms to the directive and that the treatment output is within a predetermined tolerance. A secondary computer system with knowledge of the source type and strength is essential to double-check the integrity of the treatment plan.

Figure 2.14 Floor plan of the high dose rate procedure and treatment room at the DFCI BWH Cancer Center. Lead and high-density concrete were incorporated in the plan to provide adequate shielding so that any adjacent area is unrestricted. Efficiency can be gained by having walls in common with adjacent linear accelerator bunkers.

Table 2.3 Facilities and Equipment

Treatment room (dedicated or linear accelerator room)
High dose rate afterloader
Treatment-planning system
Secondary computer for plan verification
Conventional simulator, computed tomography scan, and/or magnetic resonance imaging scanner
Film processor or digital imaging system
Radiation safety equipment
Closed-circuit TV and audible monitoring system
Patient entertainment system
Anesthesia equipment
Emergency equipment

 

Figure 2.15 An example of a well counter and electrometer used to calibrate brachytherapy sources. A countertop shield is also seen in the picture. The shield reduces dose to the radiation safety staff while manipulating and calibrating the sources.

Figure 2.16 A wall-mounted radiation detector. It has a selectable sensitivity, visual, and audible indication of detected levels of radiation. Such a detector should be visible from the entrance point to the room. It may also be connected to a secondary indicator outside the room.

 

Figure 2.17 This typical survey meter is used to screen every patient for high dose rate (HDR) before and after the treatment. It can give a reliable quantitative reading in millirem per hour. Experience teaches that the meter takes a minute or two to settle at a monitor reading. Before a HDR treatment, the patient's level is recorded for baseline. The room is declared safe at the end of the procedure when the survey level is the same as the baseline level.

Before a patient is treated, the parameters of the anticipated treatment must be verified. The patient ID, assumed source activity, prescription dose, applicator type and size, and planned treatment time need to be carefully scrutinized. In addition to secondary calculations, it is advisable to have a means for a “sanity check” of the plan before the treatment is delivered. This does not imply an independent calculation (which should have already been carried out), but rather implies an evaluation that the pending treatment is consistent with the goal of the directive based on previous experience. With this in mind, it is advisable to accumulate summary statistics or characterize dwell times for each type of applicator.

Figure 2.18 Long handled instruments, a pair of scissors, and two forceps, for manual loading of low dose rate isotopes and as part of the emergency kit for high dose rate. All manipulation of sources should be done quickly, in an organized manner, and as far away from the hands and body as feasible. These techniques reduce the dose to hands.

Table 2.4 Quality Assurance Documentation

Quality management plan
Acceptance QA procedure
Yearly QA procedure
Monthly QA procedure
Daily QA procedure
Treatment plan QA
Pretreatment QA
Emergency procedure
Chart QA

QA, quality assurance.

Before the first treatment fraction has been delivered, certain information needs to be present in the treatment chart. Written informed consent of the patient, a signed prescription, a signed written directive, a pathology report, and patient identification information are essential. During treatment and after the final fraction has been delivered, the treatment chart needs to be checked for completeness of treatment documentation and appropriate signatures. Uniform documentation for written directives, prescriptions, and patient's informed consent can also minimize errors and miscommunications. Figure 2.19 is the HDR Treatment Worksheet for the DFCI BWH Cancer Center.

The documentation of staff training needs to be maintained. Therapists need to be evaluated periodically on their knowledge of treatment and emergency procedures. Physics and medical staff need to be trained regularly in the specifics of the various treatment modalities as well as the emergency procedures. Most importantly, all members of the HDR team must be prepared for the scenario of a source stuck in the out position. This procedure needs to be rehearsed quarterly for the actions needed to rapidly isolate the source and applicator so as to render the patient and staff safe. The bailout container (as shown in Fig. 2.10) must be in a predefined position in front of the HDR afterloader. Long handled instruments need to be accessible and visible in the clinic at all times for this purpose.

End of High Dose Rate Treatment

HDR afterloaders generally have built-in radiation monitors. Rooms for HDR should have additional independent monitors with visual and audible alarms. Every time the treatment room is used after the isotope is exposed, the first team member entering the room should survey the room for radioactivity and verify the return of the source to its storage location. An integral part of this requirement is to make sure that an initial survey of the room and the patient is performed. A patient may have had a recent isotope study and have a residuum of activity still measurable. If this is not detected before treatment, it can cause much confusion and a false emergency.

The patient does not need any radiation protection advice on discharge. The patient should be given specific postprocedure nursing instructions as required.

Unforeseen Circumstances

A vitally important part of the program is to develop emergency procedures. These will differ with dose rate and modality. However, common elements are worthy of mention.


For HDR, the chance of an incident where the isotope is jammed out of the afterloader is small. If it were to happen, there would be potentially fatal implications for the patient and risk of dangerous dose to staff. The elements of the emergency “source out” protocol must be reviewed by all staff involved in HDR delivery at least yearly. Such procedures will be specific to the site and applicators used. As a brachytherapy practice grows, the emergency procedures should be reexamined each time a new treatment site is considered to determine if additional equipment should be added to an emergency response kit. It is advisable to 
practice the steps for a fast applicator removal with only the equipment in the emergency kit on a regular basis.

Figure 2.19 DFCI BWH Cancer Center High Dose Rate (HDR) Treatment Worksheet.

The event that an HDR source will be stuck out is unlikely. If it were to happen, rapid and organized reaction will minimize risk to patient and staff alike. Clinical teams must take preparedness seriously. This preparedness must be a condition of credentialing to practice HDR brachytherapy. The DFCI BWH Cancer Center practice is to have the senior physicist schedule regular in-service training sessions. These are supplemented by surprise challenges to the team with feedback given to review all responses. Yearly recertification of readiness is integral to the maintenance of HDR certification. The manufacturer's instructions for remote retraction of the source must be followed. If these fail, the designated individual (physicist/physician) must enter the room and a rapid manual retraction is attempted. If this fails, isolation of the applicator with stuck source must be made. Careful attention to positioning of the emergency bail out receptacle is a necessary part of every single HDR fraction (Fig. 2.10). The receptacle must be large enough to receive the applicator being used, must have an easily closable lid, and must be made of suitable material to properly shield the staff from the high activity iridium source. Personal instantaneous dosimeters must be available and ready at all times to provide timely information about potential increased exposure in this situation to the responders.

Low Dose Rate

Introduction

Although the use of HDR (>1,200 cGy per hour) is increasing, LDR (40 to 200 cGy per hour) and VLDR (<40 cGy per hour) remain the trusted tools of modern brachytherapy. In particular, there has been a significant upsurge in VLDR brachytherapy for early stage prostate cancer over the past 15 years. Gynecologic LDR applications for certain disease types and stages remain common. There are many examples of other sites where brachytherapy's signature falloff can be exploited for which HDR is not yet accessible or proven. All of the original brachytherapy research was conducted with LDR radiation.

Facilities and Equipment

LDR brachytherapy may be delivered with sources from an institution's permanent inventory of long-lived isotopes, or from sources with shorter half-lives that are ordered for specific procedures. A secure location must be used to receive, store, calibrate, and prepare the sources. In most centers, a secured room is designated as the “hot lab.”

Source Management

Equipment

The hot lab should be equipped with the essentials for safe handling of radioactive sources. Source manipulation should be carried out with long handled devices, and there should be a sufficient variety of such devices appropriate to the types of sources used. Specifically, long handled forceps and scissors, as shown in Figure 2.18, increase the distance between the sources and the operator's hands. Lead shields should be present at any location where sources will be exposed on a regular basis. The minimum equipment includes lead shields at the workbench, preferably with a leaded-glass viewing panel built in as shown in Figure 2.20. For any facility with a permanent inventory of sources, there should be a shielded safe for organized storage (Fig. 2.20). Portable shielded containers should also be available for transportation within the institution (see Fig. 2.21). Geiger counters (see Fig. 2.22) and survey meters (Fig. 2.17) are necessary and serve complementary roles. Geiger meters are highly effective in locating the occasional dropped seed and ensuring no sources have inadvertently been left out. Survey meters provide a quantitative measure of radiation levels. Access to the room should be restricted and automated logging of access to the room is an important part of security for the sources.

 

Figure 2.20 A source safe for a permanent inventory of 137Cs sources. The safe includes shielding and organized storage of sources. A shield is immediately in front of the door to shield the radiation safety staff accessing the sources. Leaded glass eases visualization of the sources while providing some shielding. Long instruments are mandatory in this setting also.

Calibration

Any source ordered and delivered to a radiation therapy department must be first checked for tampering and the structural integrity of the transportation device. The calibration declared on the bill of lading must be verified by the radiation safety team. Lower energy isotopes, such as iodine 125 (125I) or palladium (103Pd), may be batch-assayed using a well chamber and an electrometer. If the sources are batch-assayed, a subsample should be assayed individually. Ten percent is widely used for the size of the subsample.

Figure 2.21 A transport container for moving 192Ir sources within the institution. This provides shielding to move the sources from hot lab to the patient room.

 

Figure 2.22 This typical Geiger counter is crucial to the safe practice of brachytherapy. Normally it is used for low dose rate and very low dose rate implants where there is some chance that a single seed or strand may fall to the floor. In the operating room environment, it also plays an important role in retrieving sources that may have been taken up in the surgical suction system. The needle reading levels fluctuate widely, and are not routinely used for quantitative measurements. There is an audible option, but this usually is left off, as it can unduly upset bystanders.

Transportation

A brachytherapy service requires transport devices suitable for the energy and type of emission being transported. For 192Ir and 137Cs, a large metal safe or pig (see Fig. 2.23) is used. This can be mounted on a sturdy trolley because the lead casing makes it rather heavy. Isotopes such as 125I with relatively low energy are transported in stainless steel rings in the case of strands or in small shielded vials similar in size to an aspirin bottle in the case of loose seeds. A visual survey of the cart as it leaves the hot lab indicates whether it is secured and all appropriate lids are closed. It is wise if possible to reduce anxiety to the general public by using service elevators and corridors en route to the patient's room.

Following completion of the treatment plan, the sources need to be prepared. In the case of strands, the sources that were not custom ordered may need to be trimmed to the desired length. The sources must then be labeled to designate their name, activity, and where they are to be introduced in the delivery system.

Planning

The physics of brachytherapy is dominated by the inverse square falloff of dose over distance from the source, or D  1/r2. This formula, the energy, the nature of the emission, and the half-life of the selected radioisotope (Table 2.1) provide the brachytherapist with the permutations of energy and dose rate needed for treatment planning. Before the advent of modern computer-aided calculations, treatment planning was supported by empirical tabulations, which are still widely available for consultation. These paradigms outlined simple rules for dose calculations. The practitioner would make a best estimate of the area or volume to be treated and the required dosage and would then read or interpolate the data in the tables to arrive at a solution. In practice, reproducing the exact parameters was rarely easy. The development of image-supporting treatment-planning systems has allowed the physician to preplan optimal treatment plans, view plans interactively at the time of implanting, and postoperatively reconstruct and assess the actual delivered dosimetry.

Of course, the most important component of any brachytherapy treatment plan is good geometrical placement of the sources. Even a skilled practitioner may be stymied by the challenging tissue interfaces encountered in the interstitial setting, for example, pubic arch interference in transperineal prostate brachytherapy. The use of needle-guiding templates (see Chapter 9), beveled steerable needles, and interactive real-time imaging help immensely with the geometry problem.

Figure 2.23 The cesium transporter is mounted on a three wheel trolley for transport from the hot lab to the patient room, where it remains for the duration of the implant.

There are many treatment-planning systems available. Most offer a brachytherapy treatment-planning module piggybacked on to an (more widely used) external beam package and activated by supplemental license. See Table 2.5 for a checklist of desirable features on a brachytherapy treatment system.

Planning can be performed at three different time points in the treatment course depending on the disease site and/or physician preference. The differences between these planning techniques can be illustrated using VLDR prostate brachytherapy because this brachytherapy modality can use any of these planning techniques.

1. Preplanning. A preplanning process is widely used in trans rectal ultra sound–guided permanent prostate implants. Images are accrued using a transrectal ultrasound probe at a pretreatment visit. The gland is imaged using a referenced setup that mimics the implanting geometry, notably using a reference grid and incorporating a similar ultrasound probe. Preplanning allows the preparation of a treatment plan in a more sedate setting, and facilitates preloading of the needles in the hot lab or ordering of preloaded needles from an external vendor. This may decrease the patient time under anesthesia, physician time, and radiation exposure within the operating room. Changes in geometry and tumor configuration may still occur and can be adjusted for during the procedure. To accommodate these changes spare preloaded needles, typically with two to four seeds, can be made available for selection.

Table 2.5 Treatment-Planning System Requirements

Transparent and independently verifiable output data
Ability to import or construct common commercial seed models
Ability to incorporate standard dose modeling guidelines, for example, TG43
Ability to import/export image (computed tomography scan, magnetic resonance-positron emission tomography), structure, and dose data sets
Ability to coregister image sets
Robust contouring and reconstruction tools
Seed finding and catheter identification software

2. Interactive or real-time planning. The patient is immobilized and anesthetized using a local or general anesthetic. Images are generally acquired using a transrectal ultrasound probe (the use of interactive planning with CT scan or MRI imaging is described in Chapter 8) and a treatment plan generated. The needles are loaded in the operating room and the patient is treated. This approach has the benefit that the patient is exactly in the same position, therefore minimizing the setup error and eliminating the time taken to reproduce the patient's original position. There is also minimal seed wastage as needles are made only when required. More sophisticated planning systems allow an iterative dosimetry approach with alteration of the plan according to actual needle placement rather than intended needle placement.

3. Postplanning. The patient will often have orthogonal views taken in the operating room or immediately after operation to ensure that there have been no major problems with seed positioning. However, the patient will undergo a more formal dosimetric evaluation with CT scan and/or MRI simulation any time from 1 day to 6 weeks postprocedure. Where anomalies occur, the physician can add further seeds if required.

Postplanning is the basic approach used in afterloading techniques. This allows the physician to insert an applicator with a known geometry, for example, a tandem and ovoids or a template-assisted interstitial implant. If the geometry is fixed to known standards, the experienced practitioner can, to a very high degree, “envision” the resultant dose distribution. LDR isotopes of a known strength can then be implanted before the planning process on the basis of experience. The planner can then digitize planar films or import image data sets and reconstruct the geometries and display actual doses. If discrepancies are found, the physician can eliminate or add isotope or adjust the duration or activity of the implant until the required distribution is realized.

Insertion

Upon arrival in the patient's room, usually a shielded private room, the patient must be positively identified and the implant visually inspected for integrity and position. Before insertion, the patient and room should be surveyed to ensure that the room is clear of radiation at time zero. Again, background radiation must be accounted for in cases when the patient may have an isotope load on board. Figure 2.24 shows a typical setup for a cart to carry cesium and iridium from the hot lab to the patient's room.

With regard to the safety of radiation, poorly choreographed insertion of LDR implants results in unnecessary exposure to personnel. Insertion is best achieved with the physician well positioned with respect to the patient and the physicist handling the sources. It helps greatly if these two participants clearly arrange the order of insertion, such as whether they prefer to lead with the left hand or the right hand. Vocal cues like “ready to load catheter number 14” with the reply “number 14 ready and out” helps both to keep track of the progress of the implant and confirms adherence to the loading pattern. This also eliminates the situation when the source has been pulled from the lead pig but the loader is not yet ready to receive it, therefore unnecessarily exposing all parties to extra dose. When the order and positioning of sources is predetermined, mobile lead shields can be optimally positioned and the speed of the implanting is optimized.

When dealing with two isotopes with different characteristics, for example, tandem and interstitial gynecologic applicators where 137Cs slugs are to be introduced to the tandem and 192Ir strands into the interstitial needles. In this scenario, the tandem should be loaded last to minimize the dose from the higher-energy isotope to the physician.

 

Figure 2.24 This typical cart setup is prepared with Geiger counter, marking pen, long instruments, radiation warning signs, and information for posting on the door.

Monitoring Shielding and Safety

When the implant is in place within the patient, survey meter measurements should be taken at 1 m from the patient, behind the shields and at the outside surface of the door. The radiation safety staff should record these measurements, and these should be posted in the room to alert hospital staff to the presence of radioactivity, with the trefoil international sign used for radiation exposure (see Fig. 2.25).

Figure 2.25 The international warning sign for radioactivity, the trefoil, must be displayed prominently, along with important radiation safety information and contact numbers, on the room where an implant is occurring and in any location where isotope is stored.

 

In the LDR setting, the duration of the implant is typically 3 to 5 days, and therefore the caregivers and family members need to be protected from undue radiation exposure. When temporary removal in not an option, this can be achieved by surrounding the patient's bed with mobile lead shield and limiting the time that staff or family members can spend with the patient each day. The nursing staff will appreciate it if shielding is provided on the side where the intravenous pump is mounted. This may need to be adjusted at frequent intervals to provide pain medication and hydration to the patient. The patient should be monitored by visual inspection to ensure that the implant has not moved by inadvertent patient or staff interference.

If an unanticipated call is made to the brachytherapy team, time may be of the essence. It is important to leave enough equipment at the bedside so that the isotope may be removed as soon as possible. This list should include a survey meter with enough battery charge to last the anticipated length of the implantation, a set of long instruments, including a pair of scissors, forceps, wire cutters, a shielded receptacle for isotope storage, and a small cart to hold these instruments.

Before LDR brachytherapy is implemented, it is crucial to orient all nonradiation oncology staff to the process. The usual method for this is through in-service sessions with small groups, where a slide presentation is made, discussion is initiated, and questions are answered.

When patients are admitted to the hospital for brachytherapy, they can be admitted to the service of the radiation oncologist or to the service of the disease site being treated, for example, gynecologic oncology. In either case, the radiation oncologist and the physics staff have significant responsibilities in addition to the regular medical welfare of the patient. It is rare to have an in-house radiation staff on call team. It is vital to have a radiation oncology contact available 24 hours a day to give advice and to respond rapidly should an emergency arise. Appropriate contact information, should be posted clearly in the medical record and on the door of the isolation room. For implants that span many days when different members of the team may be on call for different days, these numbers need to be updated and prominently recorded.

Written Procedures

Paperwork specific to LDR and VLDR include an isotope recording form. A list of contact numbers for the radiation oncology team should be filed in the patient's chart for dealing with any emergency. There should be clear documentation posted on to the door highlighting the presence of radioactive materials, the international radiation hazard sign, the duration of the implant and the emergency contact numbers of the appropriate staff. The addition of a check sheet is a worthwhile idea, particularly where there is a high rotation of staff to the brachytherapy service, where the institute has a teaching mission, or where such implants are infrequent.

Removal and Discharge

Once again, preplanned choreography for removal is optimal. The most hazardous material should be removed and secured first. When all the sources are safely in the pig, the cart should be moved out of the room. The physicist should then return to the room and survey the patient, the bedding, and the surroundings to ensure exposure has returned to background levels.

At this point, a clear audible declaration “the patient and room are clear of radiation” is warranted. The nursing staff may now enter the room to assist with the removal of the delivery apparatus. The cautionary paperwork can be removed and the bed shields removed to a storage location. The isotopes are then returned to the hot lab for safekeeping. Short half-life isotopes are returned to a central well in the middle of the delivery pig and at a suitable date returned to the manufacturer for reprocessing.

Patients need to be informed about permanent radioactive implantation. Every clinic needs a system to educate about the isotope used and the possibility of low level radiation emanating from them for some period of time. The patient needs to know if there is any risk to family and other contacts. Clear guidance as to the risks to adults, children, and pregnant contacts is important. Methods of minimizing dose to others need to be given. A useful adjunct to the guidance is that the patient be discharged with an information sheet. This should contain the date and type of isotope implanted and any specific instructions based on these parameters. The DFCI BWH Cancer Center form is seen in Figure 2.26.

Figure 2.26 This patient information form from the DFCI BWH Cancer Center is part of the postimplant education for patients and families.



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