Anal Cancer Target Delineation and Setup

In anal cancer, target volume delineation and field setup are driven by three decisions that affect the entire plan: where the lesion sits in relation to the dentate line and anal verge, how much pelvic and inguinal nodal drainage must be electively covered, and how PET/CT is reconciled with physical examination and anatomic imaging. The chapter stays focused on exactly those practical points and follows them through boosts, margins, and final plan review.

For the broader framework, see our complete clinical guide to target volume delineation and field setup. If you want to compare how the logic changes in another gastrointestinal primary, our gastric cancer target delineation article is a useful contrast.

Anatomy and patterns of spread

The practical starting point is anatomy. The anal canal is about 4 cm long and extends from the anorectal ring proximally to the anal verge distally.

The anal verge is the junction between the nonkeratinized squamous epithelium of the distal anal canal and the keratinized hair-bearing perianal skin. The dentate line marks the embryologic transition from proximal endoderm to distal ectoderm, and that transition matters because histology and lymphatic drainage change with it. Squamous cancers arising proximal to the anal verge are managed as anal canal cancers; tumors arising distal to the verge are managed as perianal skin cancers.

Lymphatic drainage of the anal canal

The chapter starts with lymphatic drainage because elective coverage follows anatomy. Distal lesions, tumors near the dentate line, and proximal canal disease do not drain the same way.

Primary tumor location	Draining lymphatics
Distal anal canal, perianal skin, and anal verge	Superficial inguinal Femoral External iliac
Anal canal just proximal to the dentate line	Internal pudendal Hypogastric Obturator Inferior and middle hemorrhoidal
Proximal anal canal and distal rectum	Perirectal Superior hemorrhoidal

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.1)

This drainage map is what keeps elective coverage honest. Distal lesions push the inguinal chain to the front of the plan, while more proximal disease shifts the emphasis toward perirectal, superior hemorrhoidal, and pelvic nodal basins.

Diagnostic workup relevant for target volume delineation

For anal cancer, the physical examination is part of planning, not just staging. The chapter expects detailed documentation of tumor size, position relative to the anal verge, sphincter function, adjacent-organ invasion on pelvic examination, and the status of the inguinal nodes.

When an inguinal node looks suspicious but is only borderline in size, the chapter recommends biopsy confirmation. The reason is practical and strong: nearly 50% of suspicious nodes can still represent reactive hyperplasia. In day-to-day work, that is exactly the kind of situation where tissue confirmation can prevent unnecessary escalation of GTV-N and CTV-HR.

PET/CT is recommended both for staging and for treatment planning because it helps define the extent of gross disease. The chapter is careful, though, not to let PET overrule everything else. Low uptake should not supersede the physical examination or abnormalities seen on CT or MRI. That hierarchy is sensible. PET is valuable, but it cannot erase convincing anatomic disease.

Simulation and daily localization

Reproducibility matters as much as contour quality. The preferred setup is supine, with the arms on the chest, using a body mold.

Prone positioning on a belly board can displace bowel anteriorly, but the chapter points out two drawbacks: it is less reproducible and it complicates bolus placement. A radiopaque marker should be placed at the anal verge. CT simulation should use intravenous contrast and slice thickness of 3 mm or less so the pelvic vessels and gross tumor volume are easier to define. PET/CT fusion should be performed when available, and MRI may also help.

Bladder filling strategy should be chosen deliberately. A full bladder may keep bowel out of the pelvis, whereas an empty bladder may be easier to reproduce. For image guidance, the recommendation is daily orthogonal kilovoltage imaging and weekly cone-beam CT to verify soft-tissue alignment. If bladder or rectal filling varies substantially, more frequent cone-beam imaging is justified.

Target volume delineation and treatment planning

The short answer is that modern anal planning revolves around IMRT, but only if contouring is disciplined. Conventional 3D conformal treatment was historically difficult because the plan had to cover both the pelvis and the inguinal nodes, and the photon-electron thunderbird technique used to be the classic solution.

RTOG 0529 established that IMRT was feasible in a multi-institution setting and showed lower grade 2 or higher hematologic toxicity plus lower grade 3 or higher gastrointestinal and dermatologic toxicity than the historical RTOG 9811 controls. The catch is important: noncompliant target delineation is associated with a higher risk of recurrence. In practice, that makes IMRT the better option only when the team is willing to contour carefully and audit the plan.

Suggested target volumes for gross and microscopic disease

This is the working table of the chapter. It separates gross disease, high-risk and low-risk clinical target volumes, and the final planning expansion.

Target volume	Definition and description
Gross tumor volumes (GTV-P, GTV-N)	GTV-P: all gross primary disease on examination and imaging. GTV-N: all nodes ≥1.5 cm, PET-positive, and/or biopsy-proven. In the absence of biopsy, nodes that remain in doubt should be included as GTV-N. Nodes ≤3 cm may be separated as GTV-Na, and nodes >3 cm as GTV-Nb.
Clinical target volumes for gross disease (CTV-P, CTV-N)	CTV-P: GTV-P plus a 1.5-2.5 cm expansion, excluding uninvolved bone, muscle, or air. CTV-N: GTV-N plus a 1.0-1.5 cm expansion, again excluding uninvolved bone, muscle, or air.
High-risk clinical target volume (CTV-HR)	Should cover CTV-P, CTV-N, the entire mesorectum, perirectal nodes, and bilateral internal iliac nodes inferior to the inferior border of the sacroiliac joint. If inguinal or external iliac nodes are involved, those regions move into CTV-HR; upper internal iliac nodes are also included when involved. To cover internal iliac nodes, draw a 0.7 cm margin around the internal iliac vessels, excluding muscle and bone. To cover external iliac nodes in inguinal- or external-iliac-positive disease, add 1 cm anterolaterally around the vessels and include adjacent small nodes. To cover inguinal nodes, contour the entire inguinal compartment, including small vessels and adjacent nodes bounded by muscle and bone. Draw a 1.8 cm-wide volume between the external and internal iliac vessels to cover the obturator nodes. Add 1-1.5 cm anteriorly into the bladder to account for bladder and rectal filling variation.
Low-risk clinical target volume (CTV-LR)	Should cover uninvolved internal iliac nodes superior to the inferior border of the sacroiliac joint, as well as uninvolved external iliac and inguinal nodes. Keep the 0.7 cm margin around the internal iliac vessels. Add 1 cm anterolaterally around the external iliac vessels and include adjacent small nodes. Contour the full inguinal compartment, including small vessels and adjacent nodes bounded by muscle and bone.
Planning target volumes (PTV)	Each CTV should be expanded by 0.5-1 cm, depending on setup confidence, imaging frequency, and the use of IGRT.

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.2)

Figure 19.3 shows how that framework plays out in a clinical T2N0 case treated with definitive chemoradiotherapy. The patient was simulated supine with PET/CT using 2.5 mm slices. PTV-LR and PTV-HR received 40 Gy at 1.6 Gy per fraction and 45 Gy at 1.8 Gy per fraction in 25 fractions, followed by a sequential boost of PTV-P to 50.4 Gy at 1.8 Gy per fraction for 28 total fractions.

Elective nodal regions in the RTOG anorectal contouring atlas

The RTOG atlas turns pelvic anatomy into reproducible contouring regions. For day-to-day planning, that structure is often what keeps a complex anal plan organized.

Volume	Key highlights
CTV-A (perirectal, presacral, internal iliac)	Lower pelvis: the inferior border should be 2 cm below gross disease and include the entire mesorectum; the volume does not need to extend more than a few millimeters beyond the levator muscles unless there is extension into the ischiorectal fossa. Mid pelvis: includes rectum, mesorectum, internal iliac nodes, and a 1 cm margin into the bladder for daily bladder variation; posterolaterally it reaches the pelvic sidewall muscles or bone; at minimum the posterior portion of the internal obturator vessels should be included; use a 7-8 mm soft-tissue margin around the internal iliac vessels while trimming uninvolved muscle and bone. Upper pelvis: the superior extent is the bifurcation of the common iliac vessels into the external and internal iliacs, approximately at the sacral promontory; keep a 7-8 mm soft-tissue margin and at least 1 cm anteriorly, especially if vessels or small nodes are visible.
CTV-B (external iliac region)	The border with the inguinal region is somewhat arbitrary. The consensus places it at the inferior extent of the internal obturator vessels, with the superior pubic rami as the bony landmark. A 7-8 mm soft-tissue margin around the iliac vessels is recommended, with at least 1 cm anteriorly, while trimming uninvolved muscle and bone.
CTV-C (inguinal region)	The inferior extent should be 2 cm below the saphenous-femoral junction. The border between CTV-B and CTV-C is approximately the upper border of the superior pubic rami. The entire inguinal compartment should be contoured, including small vessels and lymph nodes, while excluding uninvolved muscle and bone.

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.3)

The chapter also references the Australasian GI Trials Group atlas, which expands the elective framework to seven regions: mesorectum, presacral space, internal iliac nodes, ischiorectal fossa, obturator nodes, external iliac nodes, and inguinal nodes. That broader map is especially useful when nodal involvement or local extension makes the plan less routine.

AGITG atlas: seven elective regions cited in the chapter

The chapter also points to the Australasian GI Trials Group atlas. The extracted prompt degrades the original table formatting, but the chapter still makes the principal anatomic landmarks explicit enough to guide coverage.

Region	Main landmarks emphasized in the chapter
Mesorectum	Rectosigmoid junction cranially; anorectal junction caudally; anterior relation to bladder and seminal vesicles in men, and bladder, uterus, vagina, and cervix in women.
Presacral space	Sacral promontory at L5/S1 cranially; inferior border of the coccyx caudally; sacroiliac joints laterally.
Internal iliac nodes	Common iliac bifurcation near L5/S1 as the cranial reference; contouring around the internal iliac vessels; extra anterior reach when visible vessels or small nodes are present.
Ischiorectal fossa	Apex formed by levator ani, gluteus maximus, and obturator internus; anal verge as the caudal reference; 1-2 cm anterior extension when needed to cover the sphincteric region.
Obturator nodes	Relationship to the obturator canal and obturator internus; the chapter also reinforces that a 1.8 cm volume between the internal and external iliac vessels helps cover this chain.
External iliac nodes	Common iliac bifurcation cranially; the region between the roof of the acetabulum and the superior pubic rami caudally; additional anterolateral margin around the vessels.
Inguinal nodes	Complete inguinal compartment with small vessels and adjacent lymph nodes; the text also recommends a 2 cm radial margin around the femoral vessels, a 1 cm margin around the saphenous-femoral junction, a 3 cm medial and lateral margin along the lower inguinal ligament, and the anal margin as the caudal border.

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.4)

One of the more useful warnings in the chapter is that major anal contouring guidelines do not agree completely on inguinal borders. It cites data showing that 10-29% of involved inguinal nodes may lie outside recommended borders. To better cover that chain, the text recommends a 2 cm radial margin around the femoral vessels, a 1 cm radial margin around the saphenous-femoral junction, and a 3 cm medial and lateral margin along the lower inguinal ligament, with the anal margin used as the caudal border.

Figure 19.4 shows why that matters. In a clinical T3N1a patient with bilateral inguinal nodal involvement, the bilateral inguinal and external iliac nodes were included in CTV-HR because the inguinal disease was bilateral. PTV-LR and PTV-HR again received 40 Gy and 45 Gy in 25 fractions, and PTV-P plus PTV-N then received a sequential boost to 54 Gy at 1.8 Gy per fraction in 30 total fractions.

Suggested dose and fractionation schemes

The exact prescription depends on technique, but the chapter anchors current practice in the RTOG 9811 and RTOG 0529/Transaustralian designs. That comparison is useful when deciding between sequential boosts and integrated dose painting.

Target volume	RTOG 9811 [3]	RTOG 0529 [2] / Transaustralian [5]
PTV-P	T1N0: 45-50.4 Gy at 1.8 Gy/fraction T2N0: 50.4 Gy at 1.8 Gy/fraction N+ or T3-T4: 54-59.4 Gy at 1.8 Gy/fraction	T1N0: not included on RTOG 0529 T2N0: 50.4 Gy at 1.8 Gy/fraction N+ or T3-T4: 54 Gy at 1.8 Gy/fraction
PTV-N	54-59.4 Gy at 1.8 Gy/fraction	50.4 Gy at 1.68 Gy/fraction if node is ≤3 cm 54 Gy at 1.8 Gy/fraction if node is >3 cm
PTV-HR	45 Gy at 1.8 Gy/fraction	T2N0: 42 Gy at 1.5 Gy/fraction N+ or T3-T4: 45 Gy at 1.5 Gy/fraction
PTV-LR	30.6-36 Gy at 1.8 Gy/fraction Alternatively, 40 Gy at 1.6 Gy/fraction SIB may be used	A low-risk PTV was not used on RTOG 0529

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.5)

The postoperative example is worth keeping in mind because it shows that not every anal-adjacent case follows the classic definitive pattern. This patient had a pathologic T1, 1.0 cm, clinical N0M0 squamous carcinoma of the perianal skin resected with a close 0.1 cm margin. The perianal surgical bed was treated as CTV-P with a 1.5-2 cm margin, and the at-risk inguinal nodes were treated as CTV-HR because of concern for microscopic residual disease and nodal spread. PTV-HR and PTV-P received 45 Gy in 25 fractions, followed by a sequential PTV-HR boost to 55.8 Gy in 31 total fractions.

Plan assessment

A plan is not ready just because the targets are drawn. The chapter sets a clear benchmark: ideally, at least 95% of each PTV should receive 100% of the prescription dose, and the maximum dose within the PTV should not exceed 10%.

When a sequential boost is used for gross disease, every individual plan should be reviewed before the summed plan. That is the only reliable way to catch hot spots or undercoverage in a specific PTV before they get hidden inside the composite dose display.

Organ-at-risk dose constraints

Final plan review is not just about target coverage. The chapter uses QUANTEC and RTOG 0529 as direct references for bowel, bladder, femoral heads, iliac crest, and external genitalia.

Organ at risk	Constraints
Small bowel	QUANTEC V15Gy < 120 cc (individual loops) V45Gy < 195 cc (entire potential space within the peritoneal cavity) RTOG 0529 V30Gy < 200 cc V35Gy < 150 cc V45Gy < 20 cc Dmax < 50 Gy
Large bowel	RTOG 0529 V30Gy < 200 cc V35Gy < 150 cc V45Gy < 20 cc
Bladder	QUANTEC Dmax < 65 Gy V65Gy < 50% RTOG 0529 V35Gy < 50% V40Gy < 35% V50Gy < 5%
Femoral heads	RTOG 0529 V30Gy < 50% V40Gy < 35% V44Gy < 5%
Iliac crest	RTOG 0529 V30Gy < 50% V40Gy < 35% V50Gy < 5%
External genitalia	RTOG 0529 V20Gy < 50% V30Gy < 35% V40Gy < 5%

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.6)

The chapter also flags pelvic bone marrow as an emerging organ at risk in patients receiving concurrent chemoradiotherapy. At present, the pelvic bones are used as a surrogate, divided into three subsites: lumbosacral spine, ilium, and low pelvis. Suggested constraints are mean dose <28 Gy, V10 <90%, and V20 <75%, but the authors explicitly say these limits have not been prospectively validated and should not supersede other planning goals. The lumbosacral spine may be the most active marrow subsite, so limiting dose there may be enough to reduce hematologic toxicity.

For the full series context, go back to the complete clinical guide to target volume delineation and field setup. If you want another gastrointestinal comparison, the gastric cancer target delineation article is a good next read.