Anal Canal

GIAnal canalAnal cancerIMRTVMATSIBDose paintingImage fusionMRIPET
Overview

Definitive chemoradiation for anal cancer with dose-painted IMRT: several dose levels in one plan, from the primary tumor to the pelvic and inguinal nodes. The planning work is in covering a wide, concave target while sparing the bowel, bladder, bone marrow, genitalia and the skin of the groin and perineum.

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Schedules

Anal cancer is treated with definitive chemoradiation — usually 5-FU or capecitabine with mitomycin — and surgery is kept for disease that persists or recurs. Treatment is almost always dose-painted IMRT or VMAT: one plan with several dose levels delivered at once, following RTOG 0529.

The dose depends on stage. In the NRG example protocol, the primary receives 50.4 Gy in 28 fractions for T2N0 disease and 54 Gy in 30 fractions for larger or node-positive disease; involved nodes receive 50.4 or 54 Gy depending on size; and the elective nodes receive 42 Gy in 28 or 45 Gy in 30. Other protocols use similar levels, sometimes with a higher dose to bulky disease.

Treatment breaks lower the chance of cure, so protocols ask to keep the overall treatment time short — one limits it to 42 days. That makes skin care and a plan the patient can tolerate part of the planning goal.

Simulation & Setup

The target runs from the pelvic nodes down to the anal verge and out to the groins, so setup has to serve both the perineum and the inguinal nodes.

Position
How the patient lies affects the groin skin folds, inguinal node coverage and how reproducible the setup is.

What is it?

Usually supine — often in a "frog-leg" position with the knees apart — with a custom immobilization device for the legs. Prone is also used.

Why does it matter?

Frog-leg reduces the skin folds at the top of the inner thigh and groin, which lowers the risk of moist desquamation there. Supine suits inguinal nodal treatment and is generally more reproducible. For male patients, placing the genitalia the same way every day matters; for female patients, a vaginal dilator can help spare the vaginal wall.

Markers and bolus
A marker ties the planning CT to the clinical exam, and bolus keeps disease at the skin from being under-dosed.

What is it?

A radiopaque marker at the anal verge or the lower edge of the tumor, and bolus considered over the perianal or inguinal regions when disease reaches the skin.

Why does it matter?

The marker links the planning CT to what the physician felt on examination. Without bolus, a superficial tumor near the skin would be under-dosed in the build-up region.

Planning CT
The contrast planning CT is where the nodal regions are drawn, so the vessels need to be visible and the bladder and rectum reproducible.

What is it?

A thin-slice CT in the treatment position (or a PET/CT simulation), with IV contrast to show the nodes and vessels and a comfortable, reproducible bladder.

Why does it matter?

The nodal regions are drawn around the vessels. One template suggests repeating the scan if the rectum is very distended, because it would shift the target and nearby organs.

Image fusion — MRI and PET/CT
Pelvic MRI and FDG-PET/CT fused with the planning CT.

What is it?

Image fusion aligns an outside scan with the planning CT so the target can be drawn using information the CT doesn't show. For anal cancer the scans are pelvic MRI and FDG-PET/CT, registered rigidly to the planning CT. The simulation itself can also be a PET/CT in the treatment position.

Why does it matter?

The fused scans help define the primary tumor and the involved nodes, and in a dose-painted plan the involved nodes get their own dose level, so finding every one matters. Leg, bladder and rectal position usually differ from the diagnostic scans, so check the registration at the primary and at each node rather than relying on bone.

Daily imaging
Daily cone-beam CT tracks changes in bladder and rectal filling and in tumor size across the course.

What is it?

Cone-beam CT, daily where possible, matched to bone with the soft tissue reviewed.

Why does it matter?

Daily CBCT shows changes in bladder and rectal filling, and swelling or shrinkage of the tumor, over the six weeks of treatment.

Target Delineation

What each volume represents. Dose levels, expansions and nodal regions come only from your protocol.

GTV — Primary and each involved node

Definition

The primary tumor as found on examination and imaging, and each involved node as a separate GTV.

Why it matters

Dose painting gives the primary and each node their own dose level, often based on stage or node size, so every node needs its own contour.

Contouring resources

Organ contouring guides →

Relevant protocols

CTV — primary — Anal canal and sphincters

Definition

The primary GTV with a margin, including the whole anal canal from the anorectal junction to the anal verge and the internal and external sphincters.

Why it matters

The tumor spreads along the canal and into the sphincters, so the whole canal is treated rather than just the visible tumor.

Contouring resources

Organ contouring guides →

Relevant protocols

Elective nodal CTV — Pelvic and inguinal nodes

Definition

The mesorectum, presacral space, internal and external iliac (including obturator) and inguinal nodes, at a lower dose level.

Why it matters

Anal cancer drains both to the pelvic nodes and to the groins, which is why anal fields are wider than rectal fields and reach the inguinal regions.

Contouring resources

Organ contouring guides →

Relevant protocols

PTVs — One per dose level

Definition

Each CTV expanded for setup uncertainty, usually trimmed a few millimeters inside the skin — except in the perineum, where the tumor itself may reach the skin.

Why it matters

Trimming stops the optimizer pushing dose into the skin to cover a PTV that extends into air, which would make skin reactions worse.

Contouring resources

Organ contouring guides →

Dose Painting in Practice

A dose-painted plan has several PTVs, each with its own prescription, delivered together. A few things make these plans different from a single-dose plan:

  • Naming carries the dose. Protocols name structures with their dose level — for example PTVp_5400, PTVn_5040, PTVn_4500 — so the plan, the constraints and any review all refer to the same volume.
  • Overlaps are handled by priority: where a lower-dose PTV overlaps a higher-dose one, the higher dose wins, and the lower-dose volume is evaluated outside it.
  • Dose gradients between levels need room. Where two dose levels meet, or where a node sits next to the bowel, the plan needs space to step down, and coverage of the lower level just outside a higher one is easy to lose.
  • Priorities decide trade-offs. One template lists the PTVs first, then the bowel space, bone marrow, femoral heads, genitalia and bladder — so a bowel goal outranks a bladder goal when both can't be met.

Organs at Risk & Dose Constraints

Click an organ for its anatomy, why it matters and contouring resources. Superscripts link to the study behind each value.

Organ50.4–54 Gy / 28–30 fxDose-painted IMRT
Small BowelAn NRG template applies the same values to the whole bowel space.
  • Loops V30 Gy <200 cc, V35 Gy <150 cc, V45 Gy <20 cc1
  • Max <50 Gy1
Large Bowel / Sigmoid
  • Loops V30 Gy <200 cc, V35 Gy <150 cc, V45 Gy <20 cc1
Bladder
  • V35 Gy <50%, V40 Gy <35%, V50 Gy <5%1,2
Femoral Heads
  • V30 Gy <50%, V40 Gy <35%, V44 Gy <5%1,2
Pelvic Bone MarrowApplied to the iliac crests in one protocol and to all pelvic bones in another.
  • V30 Gy <50%, V40 Gy <35%, V50 Gy <5%1,2
External Genitalia & Perineum
  • V20 Gy <50%, V30 Gy <35%, V40 Gy <5%1,2
  • These are optimization goals; target coverage generally takes priority, and the bladder, genitalia and bowel goals are often hard to meet.
  • The same bladder, femoral head, bone marrow and genitalia values appear as example criteria in the NRG anal cancer template, which bases its examples on RTOG 0529 and the EA2182 trial.
References (2)
  1. RTOG 0529 — RTOG 0529: a phase 2 evaluation of dose-painted intensity modulated radiation therapy in combination with 5-fluorouracil and mitomycin-C for the reduction of acute morbidity in carcinoma of the anal canal. Int J Radiat Oncol Biol Phys. 2013;86(1):27–33 Source
  2. NRG anal RT template — NRG Oncology Protocol Development Template for Radiation Therapy — Gastrointestinal Cancer: Anal Cancer. NRG Oncology protocol template Source

Values quoted from eviQ 1860 (last reviewed 14 June 2024; review due 30 June 2027), checked by DoseLab on 2026-09-29. Verify against your protocol, institutional policy and physician directive before clinical use.

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Protocols & Literature

Landmark trialUnder Review

Phase 2 RTOG trial of dose-painted IMRT with 5-FU and mitomycin-C for anal cancer. It did not meet its primary endpoint, a reduction in grade 2+ acute gastrointestinal and genitourinary toxicity. Its normal-tissue dose goals are quoted by later protocols; DoseLab could not open a readable copy of the protocol document, so values credited to it are shown as the quoting protocol gives them. Citation checked against Europe PMC.

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Clinical resourceSource Verified
NRG Oncology Protocol Development Template for Radiation Therapy — Gastrointestinal Cancer: Anal Cancer
Yock A, Parker W, et al. · NRG Oncology Medical Physics Subcommittee / Radiation Oncology Committee · 2025

The template NRG Oncology gives investigators for writing the radiation therapy section of an anal cancer trial. Its example text and values are based on RTOG 0529 and the ECOG-ACRIN DECREASE trial (EA2182), and are meant to be edited for each protocol. It covers immobilization (a supine "frog-leg" position is recommended), CT or PET/CT simulation, rigid registration of MRI and PET to the planning CT, standard structure names, dose compliance tables and planning priorities.

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AAPM reportUnder Review

AAPM Task Group 132 report on image registration and fusion in radiotherapy. It reviews rigid and deformable registration, points out that every registration carries an uncertainty that should be understood and communicated, and gives recommendations for commissioning and quality assurance of registration software and for checking individual registrations. Citation checked against Europe PMC.

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Clinical resourceSource Verified
Anal definitive EBRT chemoradiation (eviQ ID 1860)
eviQ, Cancer Institute NSW · 2024

Endorsed eviQ protocol for definitive chemoradiation of anal cancer with IMRT and a simultaneous integrated boost: 50.4 Gy in 28 fractions for non-bulky disease or 54 Gy in 30 fractions for bulky primary or nodal disease, with elective nodes at a lower dose level. Target volumes follow the AGITG contouring atlas. Its organs-at-risk table states that it is based on RTOG 0529 and QUANTEC. It stresses keeping overall treatment time short and avoiding treatment breaks.

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