Pocket Oncology (Pocket Notebook Series), 1st Ed.

EXTERNAL BEAM RADIATION THERAPY

Sewit Teckie, Ryan M. Lanning, and Simon N. Powell

What is External Beam Radiation Therapy (EBRT)?

• EBRT = therapeutic RT delivered by an external machine to treat malignancies & some benign conditions

• EBRT is a local Rx (exceptions: TBI = total body irradiation & TSEB = total skin electron beam)

• Contrast w/brachytherapy: RT delivered by a source inside/near the tumor

How Does a Patient Proceed From Consultation to Starting EBRT?

• Consultation visit: Radiation oncologist evaluates need for RT, schedules simulation

• Simulation: Pt undergoes a planning CT scan, ± PET or MRI, in the treatment position

1. Pt immobilized using variety of devices

2. Treatment “isocenter” (point RT beam interaction) placed on the CT scan

3. Isocenter & alignment tattoos placed on the pt

4. Images transferred to a treatment-planning computer

• Set volumes: MD contours target volumes & nl structures on the planning CT scan; if simple 2D treatment, MD also places desired RT beams on the CT

• Write prescription: MD prescribes a specific dose & fractionation of RT

• Treatment planning:

1. If 2D, physics staff verifies the dose & beams, & EBRT can begin quickly

2. If 3D or more complex treatment, physics staff generates custom plan unique for each pt; can take up to 1 wk of treatment planning & QA review

• Treatment plan reviewed & approved by MD

• Pt set-up on treatment machine: A “dry-run” session to double-check all treatment & machine parameters

• EBRT begins d after set-up session

Terminology of EBRT: Delivery Types

• Conventional: 2D RT w/2 beams; used for simple treatments & lower total doses of EBRT eg, whole-brain RT, palliation for bone mets

• 3D conformal RT (3DCRT): 3D treatment plan using more than 2 beams to conformally treat a target region eg, pelvis for rectal cancer

• IMRT: Use of an inverse-planning algorithm to create a computer-generated 3D RT plan using multiple beam angles;optimizes treatment of target tissue while sparing critical nl tissues eg, head & neck cancers, prostate cancer. Requires contouring/delineation of both treatment volumes & nl tissues

• Image-guided radiation therapy (IGRT): Daily or weekly 2D or 3D imaging to ensure treatment is precisely delivered; often used along w/IMRT, stereotactic radiosurgery (SRS), & stereotactic body RT (SBRT) to allow smaller margins on treatment volumes

• 4D CT: Used to account for pt internal motion from breathing, ensuring that tumor volume is not outside of the beams during treatment eg, lung cancer, upper abdominal cancers

• Respiratory gating: Technique used to deliver RT only when a pt’s breathing falls w/in certain phases of respiratory cycle. Used for upper abdominal tumors.

• SRS/SBRT: Separate chapter

Dose & Fractionation

• Units of RT dose = Gy = 1 J/kg

• Fractions = Number of sessions over which the total RT dose is delivered

• Dose & fractionation are determined according to multiple factors: Intrinsic tumor radiosensitivity, proximity of tumor to critical nl tissues, data from randomized trials, use of sequential or concurrent chemotherapy, pt convenience

• Typically, RT is delivered in daily fractions over a number of wks

• Fractionation terminology:

• Hypofractionation = using higher RT dose per fraction, over fewer fractions eg, SBRT, radioresistant tumors such as melanoma

• Hyperfractionation = using smaller RT dose per fraction, over more fractions eg, limited-stage SCLC

Side Effects of EBRT

• During RT, pts seen in clinic at least once a wk by attending MD. S/e are actively managed during these visits

• Acute s/e (<90 d): Site-specific, listed in table below

• Fatigue & mild skin erythema are most common acute s/e

• Late s/e (>90 d): Site-specific & age-specific

• Secondary malignancies: Overall absolute risk of secondary RT-induced malignancy in adults is <1%. Risk ↓ w/↑ age

Children have ↑ risk of secondary malignancy. Latency after RT: >7 y for solid tumors, >2 y for liquid tumors (Lancet Oncol 2011;12:353–360)

Follow-up after EBRT

• First post-treatment H&P occurs 4–8 wks after EBRT, depending on treatment site & type of treatment

• Imaging is a regular part of follow-up, including CT, PET, MRI

• Laboratories as indicated (eg, TSH after H&N RT, ESR/LDH for lymphomas)

• Pt is then seen every 2–6 mos, alternating w/other treating physicians