Radiotherapy

Radiotherapy techniques for anal cancer are similar but vary depending on the extent of disease.

  • The primary tumour should receive 50.4-54 Gy (depending on protocol) in 28-30 fractions
  • Involved nodes should receive 45 Gy
  • Uninvolved nodal regions should receive 30.6-36 Gy

This can be accomplished by:

  • Conventional field based planning
  • Volume based planning

Conventional Fields

There are several techniques; the main complicating factor is getting dose into the inguinal regions without treating additional tissue. I prefer a mono-isocentric approach using photon fields to boost the inguinal regions.

Phase 1 (36 Gy)

Opposed anterior and posterior fields are used; the anterior fields are extended laterally to encompass the inguinal nodal region.

  • Superior border: 2 cm above the inferior border of the sacro-iliac joints
  • Inferior border: 3 cm below the anal margin or most inferior extent of disease
  • Lateral border:
    • Posterior: 1 cm lateral to the pelvic brim
    • Anterior: Lateral to the femoral heads

Additional inguinal boost fields are applied to match the discrepancy between the anterior and posterior beams on the anterior surface; these are typically 6 MV photon fields. Their divergence matches the divergence of the posterior beam, preventing hot spots from forming. Enough dose is put through these boost beams to bring the inguinal region up to the desired dose level (36 Gy). These boost fields extend superiorly to the anterior superior iliac spine to cover all inguinal lymph nodes.

Phase 2 (45 Gy)

Fields are shrunk to cover a 3 cm expansion from the GTV. Any involved nodal groups also need to be included in this phase. Depending on the sites of these nodes, the APPA arrangement (for inguinal nodes) or a 3- or 4- field arrangement is necessary.

Phase 3 (50.4-54 Gy)

For T2 N0 disease, the GTV is boosted a further 5.4 Gy using the same 3 cm expansion for Phase 2. For T3 or N+ disease, the GTV is boosted to 54 Gy. Further nodal irradiation is not done.

Volumes

IMRT or 3D conformal techniques have the potential advantage to reduce normal tissue toxicity by using multiple beams to target the at risk areas. There is increased potential for geographic miss so it is essential to follow voluming guidelines. In general, the volumes are:

  • GTVp
    • 1 cm expansion to CTV54 (dose to 50.4-54 Gy)
  • GTVn
    • 1 cm expansion to CTV45 (dose to 45 Gy)
  • CTV36, including nodal groups below
    • Inguinal
    • External iliac
    • Internal iliac
    • Perirectal

A 3D conformal or IMRT technique is then used to treat the volumes to the desired dose.

Example Cases

T2 N0 SCC of distal anal canal

Presimulation

  • Informed consent
  • Medical oncology referral

Simulation

  • Empty rectum, full bladder
  • Simulate prone on belly board
  • Bolus around natal cleft/anal region
  • CT from xiphisternum to midshaft femur

Planning

  • Contour GTV
  • 2 phase technique
    • Phase 1: APPA with inguinal boost fields using 6 MV photons; superior 2 cm above inferior extent of SI joint, posterior 3 cm below anal verge, posterior lateral margin 1 cm lateral to pelvic brim, anterior lateral margin 1 cm lateral to femoral heads.
    • Inguinal boost fields aligned with divergency of posterior beam to increase dose to inguinal region
    • 36 Gy in 20 fractions
    • Phase 2: 3- or 4-fields with 3 cm expansion from GTV to field edge; 14.4 Gy in 8 fractions
  • Organs at risk: Small bowel (No more than 195 cc receiving > 45 Gy), femoral heads < 45 Gy

Outcomes

Assess disease at 6 weeks post treatment to ensure complete resolution.
5 year survival 80%; local control 75%; sphincter intact survival 70%

T3 N2 SCC of Mid-Anal Canal extending into perianal skin

Treat with curative intent using chemoradiotherapy

Pre-Simulation

  • Informed Consent
  • Medical Oncology Referral
  • Surgical referral for ileostomy/colostomy if patient very symptomatic

Simulation

  • Empty bowel, full bladder
  • Simulate prone with belly board

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