Radiation Oncology: A Question-Based Review

Squamous Cell and Basal Cell Carcinomas

Boris Hristov and Roland Engel

image Background

Name the most common pathologic subtypes of squamous cell carcinoma (SCC).

Bowen Dz (SCC in situ), keratoacanthoma, −adenosquamous, verrucous carcinoma (low grade in anogenital, oral, or plantar foot), spindle cell carcinoma

Name the most common pathologic subtypes of basal cell carcinoma (BCC).

Noduloulcerative, superficial, sclerosing morpheaform, infiltrative, pigmented, fibroepithelial tumor of Pinkus, and basosquamous

What area of the body is at highest risk for SCC and BCC?

The H&N region is at greatest risk for SCC and BCC.

What is the most common genetic mutation in both SCC and BCC?

In SCC and BCC, p53 mutations are the most −common.

What features of SCC confer a high risk of mets?

Large, deep lesions with PNI and those that appear on dorsum of hands, lips, ears, penis, or sites of chronic infection, ulceration, or RT have a high risk of mets.

What type of UV rays are most responsible for causing skin cancer?

UVB (medium wave) rays are most responsible for skin cancer.

What is more common: basal or squamous cell skin cancer?

Basal cell (80%) is more common than squamous cell (20%).

What is the sex predilection for skin cancers?

Males are more commonly affected than females (4:1).

What are the major risk factors for skin cancers?

Major risk factors include sun exposure, chronic irritation, genetic disorders, and immunosuppression.

What genetic/inherited disorders are associated with skin cancer?

Phenylketonuria, Gorlin syndrome (PTCH), −xeroderma pigmentosa, and albinism have a genetic/inherited association with skin cancer.

What is the incidence of PNI and mets with BCC?

1.     PNI: 1%

2.     Mets: <0.1% (nodes > distant sites)

What is the incidence of PNI and mets with SCC?

1.     PNI: 2%–15%

2.     Mets: nodes: 1%–30% (1% grade 1, 10% grade 3, 30% from burns); distant: 2% (lung > liver > bones)

What are the major determinants of LN spread for SCC?

Poor differentiation, size/depth (>3 cm/>4 mm), PNI/LVI, location (lips, scars/burns, ear), and recurrent lesions

What LN regions are most commonly involved in SCC?

The upper cervical and deep parotid regions (with the H&N as the most frequent site) are most commonly involved in SCC.

Sun exposure at what stage of life correlates with BCC vs. SCC?

1.     BCC: early in life/childhood

2.     SCC: decade preceding Dx

What is Bowen Dz?

Bowen Dz is SCC in situ.

What is erythroplasia de Querat?

Erythroplasia de Querat is Bowen Dz of the penis.

What is a Marjolin ulcer?

Marjolin ulcer is SCC arising in a burn scar.

Which is more common when the ear is the primary site: BCC or SCC?

1.     External ear: BCC more common

2.     Internal/canal: SCC more common

image Workup/Staging

On what is the latest AJCC T staging based for SCC/BCC?

1.     T1: ≤2 cm (<2 high-risk features)

2.     T2: >2 cm (≥2 high-risk features)

3.     T3: invasion of maxilla, orbit, or temporal bone

4.     T4: skeletal invasion, PNI of skull base

Per the latest AJCC classification, to what other site is N staging for skin cancer similar?

Skin cancer N staging is similar to that of the H&N:

1.     N1: single, ipsi ≤3 cm

2.     N2a: single, ipsi 3–6 cm

3.     N2b: multiple, ipsi ≤ 6 cm

4.     N2c: bilat or contralat LNs ≤6 cm

5.     N3: LNs >6 cm

What defines stage groupings I, II, III, and IV?

1.     Stage I: T1N0

2.     Stage II: T2N0

3.     Stage III: T3N0 or T1-3N1

4.     Stage IV: N2-3, or T4, or M1

What defines a stage II(I) pt?

Stage II(I) represents a pt with stage II Dz that is also immunosuppressed (I). This is not part of formal staging but is allowed for studies/tumor registries.

What are considered high-risk features per the latest AJCC staging guidelines?

High-risk features include >2-mm DOI/thickness, Clark level ≥IV, + PNI, poor differentiation, an ear or hair-bearing lip site.

Where is the “H” zone anatomically, and why is it important?

The H zone is located in the mid face where the embryologic fusion lines lie (high risk for deep i−nvasion and high risk for LR).

For what is a “pearly papule” lesion pathognomonic?

BCC lesions often appear as pearly papules.

How does SCC appear on the skin?

SCC is flesh toned and variably keratotic.

On H&P, what aspects should be the focus of the exam?

Palpate extent of tumor, CN exam for H&N lesions, regional LN evaluation, audiometry/otoscopy for −cancers of the ear, and CT/MRI to verify extent

image Treatment/Prognosis

Name 5 Tx options for SCC and BCC of the skin.

SCC/BCC of the skin Tx options:

1.     Mohs surgery

2.     Electrosurgery

3.     Cryotherapy

4.     Topical chemo

5.     RT

Describe Mohs surgery.

In Mohs surgery, a superficial slice of skin is taken and then sectioned into quadrants. Additional layers are taken in the quadrants that show persistent Dz.

When are cryotherapy and curettage appropriate for skin cancer?

Cryotherapy and curettage are suitable for small superficial BCC or superficial well-differentiated SCC.

When is topical 5-FU used for SCC?

Topical 5-FU is used when the lesion is confined to the epidermis.

How can Bowen Dz be treated?

Bowen Dz Tx: surgery, cryotherapy, topical 5-FU, or RT (400 cGy × 10 [40 Gy])

What is the most commonly employed surgical technique for skin cancer?

Mohs micrographic surgery is the most common surgical technique for skin cancer.

When is Mohs surgery contraindicated for skin cancer?

Mohs surgery is not used for central face lesions >5 mm or ear/scalp lesions >2 cm.

What % of BCC recurs if margin+ at the time of resection?

BCC recurs in 30% for a + lat margin and >50% for a + deep margin.

What % of SCC recurs if margin+ at the time of resection?

Nearly 100% of SCCs recur if margin+.

When is RT preferred as the primary Tx modality for skin cancer?

RT is preferred for lesions of the central face that are >5 mm at the nasal ala, lip, or eyelid and lesions >2 cm on the ears, forehead, or scalp. RT is also preferred for pts >55 yo.

What is the best predictor of LC after definitive RT?

T stage is the best predictor for LC (T1, 95%; T3, 50%).

What is the LC after definitive RT for BCC vs. SCC?

LC is similar for BCC and SCC with T1 lesions (95%) and lower for SCC than BCC if T2 (75%–85%) or T3 (50%).

What should be done if a BCC or SCC is resected to +margins?

Re-excise or use adj RT if margins are positive.

What are 4 indications for adj RT to the primary site with skin cancer?

Indications for adj RT to the primary in skin cancer:

1.     +Margin

2.     PNI of named nerve

3.     >3-cm lesion and/or T4

4.     Parotid SCC

What are relative contraindications to RT in the Tx of skin cancers?

Relative contraindications to RT for skin cancer include areas prone to trauma (hand dorsum or belt line) or with poor blood supply (below knees/elbows), age <50 yrs, post-RT recurrence, Gorlin syndrome, CD4 count <200, high occupational sun exposure, and exposed area of bone/cartilage.

When should adj nodal RT be considered after surgical resection for skin cancers?

Consider adj nodal RT after surgical resection for multiple (>3) + LNs, large (>3 cm) LNs, ECE, a preauricular site (high rates of parotid LN involvement), and PNI. (Garcia-Serra A et al., Head Neck 2003)

What fields and dose schemes are typically employed with photons/orthovoltage in the Tx of skin cancers?

Treat the tumor/tumor bed + 1–2 cm to 60–66 Gy (2 Gy/fx), especially for involvement of cartilage, or to 50–55 Gy (2.5–3 Gy/fx). For the elderly/poor performance status, 3.5–4 Gy × 10 or 10 Gy × 2 can be used.

What is the margin/dose modification if electrons are used? Why?

If electrons are used, add an additional 0.5-cm margin on the skin surface and use 10%–15% higher daily/total dose b/c of bowing out of isodose curves and a lower RBE (0.85–0.9) of electrons.

What is the Rx point if orthovoltage (100–200 kV) RT is employed?

Dmax (90% of the IDL has to encompass the tumor). Do not use this if the lesion is >1 cm deep.

When treating with electrons, how deep should the 90% IDL extend in relation to the lesion?

The IDL should extend at least 5–10 mm deeper than the deepest aspect of the lesion.

When treating skin lesions with electrons, what rule is typically employed to choose the correct beam energy?

Electron energy (in MeV) should be >3 times the lesion depth (i.e., a 9-MeV beam is needed for a 2-cm lesion depth)

What is the RT volume if a named nerve is involved by SCC?

The RT volume should include nerve retrograde to the skull base. Consider IMRT/elective nodal RT.

Where do basosquamous skin cancers occur? What is the Tx paradigm?

Basosquamous skin cancers occur on the face. These are treated like SCC, as they have similar rates of nodal mets.

What kind of shields are used, and where should they be placed? Why?

Wax-covered lead shields are used b/c of backscattered electrons with low E beams. They are typically placed behind/downstream of tumor, as hotspots occur upstream of the shield.

How is SCC of the pinna approached?

For SCC of the pinna, use RT with 2.5–4 Gy/fx to 45–65 Gy in 2–3 fx/wk to the lesion + a 1–2-cm margin (using bolus and shielding as necessary).

When should surgery be recommended for pinna lesions?

Recommend surgery when cartilage is involved or with tumor extension to the canal.

When is adj RT generally recommended for ear lesions?

1.     Pinna: if +margin, ≥T3

2.     Middle ear/mastoid: if ≥T2

Can definitive RT be done for canal/middle ear lesions?

Yes. Good outcomes have been reported with RT alone for T1 lesions. (Ogawa K et al., IJROBP 2007)

What RT fields/margins are used for middle ear/canal lesions?

For middle ear/canal lesions, include the entire canal/temporal bone + a 2–3-cm margin and ipsi regional nodes (preauricular, postauricular, level II).

How should fx size be tailored for skin cancer depending on the RT field size used?

The larger the field size, the smaller the fx size should be.

If simple excision is performed for BCC, what are the min margins required?

1.     <1-cm tumor: 4 mm

2.     >1-cm tumor: 5–10 mm

What is the 1st step to take when the pt has a +margin after excision?

Send the pt back to the surgeon to be evaluated for −re-excision if the margin is positive.

What is the preferred RT modality for bone-invasive skin cancer? For cartilage invasion?

Megavoltage photons are the preferred Tx modality for bone invasion b/c of a more homogenous −distribution compared to orthovoltage due to the f-factor; however, this is not so with cartilage invasion, as orthovoltage beams have little difference in distribution in cartilage regardless of energy.

If treating recurrent BCC or morpheaform BCC, what type of margins should be used?

Since these tumors infiltrate more extensively, an extra 0.5–1-cm margin should be added on the surface.

How long should be the wait for a skin graft to heal before starting RT?

6–8 wks of healing time is required after skin grafting before RT can be initiated.

How should an SCC of the mastoid be treated?

Treat SCC of the mastoid with mastoidectomy or temporal bone resection → PORT.

How should the RT doses be modified based on tumor size/extent for ear primaries?

Conventional fx of 1.8–2 Gy:

1.     Small thin lesions < 1.5 cm: 50 Gy

2.     Larger tumors: 55 Gy

3.     Min cartilage/bone involvement: 60 Gy

4.     Cartilage/bone involvement: 65 Gy

image Toxicity

What are some toxicities expected after RT for skin cancer?

Telangiectasia, skin atrophy, hyperpigmentation, skin necrosis, fibrosis, osteonecrosis, chrondritis; xerostomia/hearing loss for the ear

If cartilage is in the RT field, what should the dose/fx be kept below?

The dose should be kept at <3 Gy/fx to reduce chondritis.

What is the incidence of skin necrosis after RT?

Skin necrosis occurs in 3% of pts (in 13% if fx size is >4–6 Gy).

To what dose should middle ear/canal lesions be limited? Why?

Limit middle ear/canal lesions to 65–70 Gy b/c of higher rates (>10%) of osteoradionecrosis with doses >70 Gy.

What must be done to reduce the toxicities to normal tissues from skin irradiation in the H&N region?

To reduce toxicities to normal tissues, use lead shielding to block the lens, cornea, nasal septum, teeth, and gums. Use dental wax on the side from which the beam enters to absorb backscatter.

Per the latest NCCN guidelines, what should be the follow-up intervals for pts with nonmelanoma skin cancers?

NCCN nonmelanoma skin cancer follow-up intervals:

1.     Complete skin exam for life at least once/yr

2.     For local Dz: H&P q3–6mos for yrs 1–2, q6–12 mos for yrs 3–5, then annually

3.     For regional Dz: H&P q1–3mos for yr 1, q2–4mos for yr 2, q4–6mos for yrs 3–5, then q6–12mos for life