{"id":14088,"date":"2026-03-12T00:06:10","date_gmt":"2026-03-12T03:06:10","guid":{"rendered":"https:\/\/rtmedical.com.br\/tmp-en-1773284768466\/"},"modified":"2026-04-04T18:07:21","modified_gmt":"2026-04-04T21:07:21","slug":"anal-cancer-delineation-fields","status":"publish","type":"post","link":"https:\/\/rtmedical.com.br\/en\/anal-cancer-delineation-fields\/","title":{"rendered":"Anal Cancer: Target Delineation and Field Setup"},"content":{"rendered":"

Target volume delineation in anal cancer: what drives the plan<\/h2>\n

Target volume delineation in anal cancer<\/strong> hinges on a thorough understanding of anal canal anatomy and its lymphatic drainage patterns. The anal canal measures roughly 4 cm in length, extending from the anorectal ring (the palpable border of the anal sphincter and puborectalis muscle) proximally to the anal verge distally. Squamous cell carcinomas arising proximal to the anal verge are managed as anal canal cancers, while those distal to the anal verge are managed as perianal skin cancers \u2014 a distinction that fundamentally changes the field design.<\/p>\n

\"GTV-P
GTV-P delineation using co-registered PET\/CT. Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n

The dentate line (or pectinate line) marks the transition from the columnar epithelium of the proximal anal canal to the squamous epithelium distally. This embryologic junction \u2014 where endoderm meets ectoderm \u2014 is critical because it defines entirely different lymphatic drainage pathways. Proximal tumors drain preferentially to perirectal, superior hemorrhoidal, and internal iliac nodes. Distal tumors, on the other hand, drain to superficial inguinal, femoral, and external iliac nodes.<\/p>\n

For a comprehensive overview of all anatomic sites and planning strategies, see our complete guide on target volume delineation and field setup<\/a>.<\/p>\n

Lymphatic Drainage: The Foundation of Nodal Planning<\/h2>\n

The lymphatic drainage pattern of the anal canal varies directly with the location of the primary tumor, and understanding this map is essential for deciding which elective nodal regions to cover.<\/p>\n\n\n\n\n\n\n\n
Primary Tumor Location<\/th>\nDraining Lymphatics<\/th>\n<\/tr>\n<\/thead>\n
Distal anal canal, perianal skin, and anal verge<\/td>\nSuperficial inguinal, femoral, external iliac<\/td>\n<\/tr>\n
Anal canal just proximal to the dentate line<\/td>\nInternal pudendal, hypogastric, obturator, inferior and middle hemorrhoidal<\/td>\n<\/tr>\n
Proximal anal canal and distal rectum<\/td>\nPerirectal, superior hemorrhoidal<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.1)<\/em><\/p>\n

In practice, most anal canal tumors require coverage of both pelvic and inguinal chains. Whether the external iliac and inguinal nodes are classified as high risk or low risk depends on confirmed nodal involvement by biopsy or PET\/CT.<\/p>\n

Diagnostic Workup Relevant for Delineation<\/h2>\n

Physical examination remains irreplaceable in anal cancer staging. The evaluation should include: tumor size and location relative to the anal verge, sphincter function, invasion of adjacent structures via pelvic examination, and inguinal lymph node assessment.<\/p>\n

Inguinal nodes that are suspicious but borderline in size deserve biopsy confirmation. Nearly 50% of clinically suspicious nodes turn out to be reactive hyperplasia \u2014 a finding that directly shifts the CTV from high risk to low risk classification.<\/p>\n

\"Dose
Dose distribution: thunderbird technique (photon\/electron). Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n

PET\/CT<\/strong> is recommended for both staging and treatment planning to help delineate the extent of gross disease. However, areas of low uptake on PET should not supersede physical examination findings or abnormalities seen on CT or MRI. Multi-modality image integration remains the safest approach.<\/p>\n

Simulation and Daily Localization<\/h2>\n

The patient should be simulated supine with arms on chest in a body mold. Prone positioning with a belly board can displace bowel anteriorly, but it is less reproducible and complicates bolus placement. A radiopaque marker should be placed at the anal verge.<\/p>\n

CT simulation with intravenous contrast and slice thickness of 3 mm or less is required to delineate the pelvic blood vessels and gross tumor volume. When available, PET\/CT fusion or MRI can significantly assist in target volume definition.<\/p>\n

\"IMRT
IMRT plan dose distribution. Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n

Bladder filling strategy matters: a full bladder may displace bowel loops out of the pelvis, while an empty bladder may be more reproducible. Daily image guidance with orthogonal kV imaging and weekly cone-beam CT scans is recommended. If there is significant variation in bladder or rectal filling, CBCT frequency should increase.<\/p>\n

Target Volumes and Planning: From GTV to PTV<\/h2>\n

Conventional 3D conformal radiotherapy for anal canal cancers was historically complex due to the need to irradiate the pelvis and inguinal nodes simultaneously. The “thunderbird” technique was the classic approach. The RTOG 0529 trial established the feasibility of IMRT in a multi-institution setting, demonstrating lower rates of grade 2 or higher hematologic toxicity and lower rates of grade 3 or higher gastrointestinal or dermatologic toxicity compared to historical controls from RTOG 9811 using 3D conformal radiotherapy.<\/p>\n

A key point: target delineation that is non-compliant with consensus guidelines is associated with an increased risk of disease recurrence. Contouring accuracy matters as much as the delivery technique itself.<\/p>\n\n\n\n\n\n\n\n\n\n
Target Volume<\/th>\nDefinition and Description<\/th>\n<\/tr>\n<\/thead>\n
GTV-P, GTV-N<\/strong><\/td>\nGTV-P: all gross disease on physical examination and imaging. GTV-N: nodes \u22651.5 cm, PET-positive, and\/or biopsy-proven. Distinguish GTV-Na (\u22643 cm) from GTV-Nb (>3 cm)<\/td>\n<\/tr>\n
CTV-P, CTV-N<\/strong><\/td>\nCTV-P: GTV-P + 1.5\u20132.5 cm margin excluding uninvolved bone, muscle, or air. CTV-N: GTV-N + 1.0\u20131.5 cm excluding uninvolved bone, muscle, or air<\/td>\n<\/tr>\n
CTV-HR (high risk)<\/strong><\/td>\nCovers CTV-P, CTV-N, 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, include these regions. 0.7 cm margin around internal iliac vessels. 1.8 cm strip between external and internal iliac vessels for obturator nodes. 1\u20131.5 cm anteriorly into bladder<\/td>\n<\/tr>\n
CTV-LR (low risk)<\/strong><\/td>\nCovers uninvolved internal iliac nodes superior to the sacroiliac joint inferior border, plus uninvolved external iliac and inguinal nodes. Margins: 0.7 cm around internal iliac vessels; 1 cm anterolateral on external iliacs; entire inguinal compartment contoured<\/td>\n<\/tr>\n
PTV<\/strong><\/td>\n0.5\u20131 cm expansion from each CTV, depending on setup accuracy, imaging frequency, and IGRT use<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.2)<\/em><\/p>\n

Contouring Atlas: Elective Nodal Regions<\/h2>\n

The RTOG anorectal contouring atlas describes three CTV regions that should be included for all patients with anal canal cancer. The Australasian GI Trials Group Atlas details seven elective regions. There is disagreement among the RTOG, AGITG, and BNG guidelines regarding inguinal lymph node contouring \u2014 recent data show that 10\u201329% of involved inguinal nodes fall outside the recommended nodal borders.<\/p>\n

\"Clinical
T2N0 case: CTV-LR (cyan), CTV-HR (orange), CTV-P (green), GTV-P (red). Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n\n\n\n\n\n\n\n
CTV Region (RTOG)<\/th>\nDescription<\/th>\n<\/tr>\n<\/thead>\n
CTV-A<\/strong> (perirectal, presacral, internal iliac)<\/td>\nLower pelvis: inferior border 2 cm below gross disease, including entire mesorectum. Mid pelvis: rectum, mesorectum, internal iliac nodes, 1 cm margin into bladder. Upper pelvis: to the bifurcation of common iliac vessels (~sacral promontory). 7\u20138 mm margin around internal iliac vessels<\/td>\n<\/tr>\n
CTV-B<\/strong> (external iliac)<\/td>\nBorder between inguinal and external iliac at the inferior extent of the internal obturator vessels (upper edge of superior pubic rami). 7\u20138 mm margin, at least 1 cm anteriorly<\/td>\n<\/tr>\n
CTV-C<\/strong> (inguinal)<\/td>\nInferior extent 2 cm below the saphenous\/femoral junction. Entire inguinal compartment contoured including small vessels and lymph nodes<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.3 \u2014 RTOG Anorectal Contouring Atlas)<\/em><\/p>\n

To adequately cover the inguinal chain, the recommended approach includes: a 2 cm radial margin around the femoral vessels, 1 cm radial margin around the saphenous\/femoral junction, and 3 cm medial\/lateral margin along the lower inguinal ligament. The caudal border of the inguinal CTV should be at the level of the anal margin.<\/p>\n

Dose and Fractionation: Validated Schemes<\/h2>\n

There are multiple dose prescription techniques for anal cancer. Current recommendations derive from the treatment plans used in RTOG 9811 and RTOG 0529\/Transaustralian protocols.<\/p>\n

\"Clinical
T3N1a case with bilateral inguinal involvement. Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n\n\n\n\n\n\n\n\n
Target Volume<\/th>\nRTOG 9811<\/th>\nRTOG 0529 \/ Transaustralian<\/th>\n<\/tr>\n<\/thead>\n
PTV-P<\/strong><\/td>\nT1N0: 45\u201350.4 Gy at 1.8 Gy\/fraction
T2N0: 50.4 Gy at 1.8 Gy\/fraction
N+ or T3-T4: 54\u201359.4 Gy at 1.8 Gy\/fraction<\/td>\n		T2N0: 50.4 Gy at 1.8 Gy\/fraction
N+ or T3-T4: 54 Gy at 1.8 Gy\/fraction<\/td>\n<\/tr>\n
PTV-N<\/strong><\/td>\n54\u201359.4 Gy at 1.8 Gy\/fraction<\/td>\n50.4 Gy at 1.68 Gy\/fraction if node \u22643 cm
54 Gy at 1.8 Gy\/fraction if node >3 cm<\/td>\n<\/tr>\n
PTV-HR<\/strong><\/td>\n45 Gy at 1.8 Gy\/fraction<\/td>\nT2N0: 42 Gy at 1.5 Gy\/fraction
N+ or T3-T4: 45 Gy at 1.5 Gy\/fraction<\/td>\n<\/tr>\n
PTV-LR<\/strong><\/td>\n30.6\u201336 Gy at 1.8 Gy\/fraction (or 40 Gy at 1.6 Gy\/fraction with SIB)<\/td>\nLow-risk PTV not used on RTOG 0529<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.5)<\/em><\/p>\n

Clinical Examples: How Plans Come Together<\/h2>\n

The T2N0 case demonstrates definitive chemoradiotherapy with IMRT: PTV-LR and PTV-HR treated simultaneously to 40 Gy (1.6 Gy\/fraction) and 45 Gy (1.8 Gy\/fraction) in 25 fractions, respectively. Then PTV-P received a sequential boost to 50.4 Gy (1.8 Gy\/fraction) in 28 total fractions.<\/p>\n

\"Magnified
Lower pelvis detail of T2N0 case. Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n

The T3N1a case with bilateral inguinal involvement had PTV-LR and PTV-HR treated simultaneously to 40 Gy and 45 Gy in 25 fractions. The bilateral inguinal and external iliac nodes were included in CTV-HR due to bilateral inguinal involvement. PTV-P and PTV-N then received a sequential boost to 54 Gy in 30 total fractions.<\/p>\n

\"Postoperative
Postoperative T1 perianal case: CTV-P (green) and CTV-HR inguinal (orange). Source: Target Volume Delineation and Field Setup, 2nd Edition<\/figcaption><\/figure>\n

The third case illustrates a pathologic T1 (1.0 cm) N0M0 squamous cell carcinoma of the perianal skin resected with a close 0.1 cm margin. Adjuvant treatment covered the postoperative bed and inguinal lymph nodes with IMRT: PTV-HR and PTV-P to 45 Gy in 25 fractions, followed by a sequential boost of PTV-HR to 55.8 Gy in 31 total fractions. Even small tumors with close margins demand inguinal coverage.<\/p>\n

The planning approach for rectal cancer<\/a> shares some anatomic principles \u2014 particularly regarding mesorectal and internal iliac vessel coverage \u2014 but anal cancer specifically demands attention to the inguinal chain.<\/p>\n

Plan Assessment and Dose Constraints<\/h2>\n

Ideally, at least 95% of each PTV should receive 100% of the prescription dose. The maximum dose within the PTV should not exceed 110%. When using sequential boosts, each individual plan should be evaluated separately before the plan sum, checking for hot spots or under-coverage.<\/p>\n\n\n\n\n\n\n\n\n\n\n
Organ at Risk<\/th>\nConstraints<\/th>\n<\/tr>\n<\/thead>\n
Small bowel<\/strong><\/td>\nQUANTEC: V15Gy < 120 cc (individual loops); V45Gy < 195 cc (peritoneal cavity)
RTOG 0529: V30Gy < 200 cc; V35Gy < 150 cc; V45Gy < 20 cc; Dmax < 50 Gy<\/td>\n<\/tr>\n
Large bowel<\/strong><\/td>\nRTOG 0529: V30Gy < 200 cc; V35Gy < 150 cc; V45Gy < 20 cc<\/td>\n<\/tr>\n
Bladder<\/strong><\/td>\nQUANTEC: Dmax < 65 Gy; V65Gy < 50%
RTOG 0529: V35Gy < 50%; V40Gy < 35%; V50Gy < 5%<\/td>\n<\/tr>\n
Femoral heads<\/strong><\/td>\nRTOG 0529: V30Gy < 50%; V40Gy < 35%; V44Gy < 5%<\/td>\n<\/tr>\n
Iliac crest<\/strong><\/td>\nRTOG 0529: V30Gy < 50%; V40Gy < 35%; V50Gy < 5%<\/td>\n<\/tr>\n
External genitalia<\/strong><\/td>\nRTOG 0529: V20Gy < 50%; V30Gy < 35%; V40Gy < 5%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 19.6 \u2014 QUANTEC and RTOG 0529)<\/em><\/p>\n

Pelvic Bone Marrow: An Emerging Organ at Risk<\/h2>\n

Pelvic bone marrow is emerging as an important organ at risk for minimizing acute hematologic toxicity in patients receiving concurrent chemoradiotherapy. The pelvic bones serve as a surrogate for the pelvic bone marrow. The structure consists of three sub-sites: lumbosacral spine, ilium, and low pelvis.<\/p>\n

Suggested constraints include mean dose < 28 Gy, V10 < 90%, and V20 < 75%. However, these constraints have not been validated prospectively and should not supersede other planning objectives. The lumbosacral spine may be the most active sub-site of the pelvic bone marrow, and limiting dose to this region may be sufficient to reduce hematologic toxicity.<\/p>\n

The approach of minimizing pelvic bone marrow dose also applies to other sites treated with pelvic IMRT, such as vulvar cancer<\/a> and definitive gynecologic malignancies<\/a>.<\/p>\n

Final Considerations for Clinical Practice<\/h2>\n

Radiation planning for anal cancer requires careful integration of clinical information, multi-modality imaging, and consensus guidelines. The choice between different fractionation schemes \u2014 RTOG 9811 versus RTOG 0529 \u2014 should consider staging, nodal involvement, and institutional capacity for IMRT with robust IGRT. Compliance with consensus contouring atlases is not optional: adherence to guidelines correlates directly with regional control.<\/p>\n

For a comprehensive overview of delineation strategies across all anatomic sites, see our complete guide on target volume delineation and field setup<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Anal cancer target delineation: anatomy, lymphatic drainage, CTV volumes, RTOG dose schemes, and OAR constraints with IMRT.<\/p>\n","protected":false},"author":1,"featured_media":16628,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"ngg_post_thumbnail":0,"fifu_image_url":"","fifu_image_alt":"","footnotes":""},"categories":[265,269,99],"tags":[],"class_list":{"0":"post-14088","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-delineamento-volumes","8":"category-delineamento-pelve-ginecologia","9":"category-radiotherapy"},"aioseo_notices":[],"rt_seo":{"title":"Anal Cancer RT: Target Volume Delineation","description":"Target volume delineation for anal cancer radiotherapy. CTV for primary tumor, inguinal and pelvic nodal coverage, and IMRT planning.","canonical":"","og_image":"","robots":"default","schema_type":"MedicalWebPage","include_in_llms":false,"llms_label":"","llms_summary":"","faq_items":[{"q":"How is the CTV defined for anal canal cancer?","a":"The CTV includes the primary tumor with margin, mesorectum, and at-risk nodal levels including presacral, internal iliac, obturator, and inguinal nodes. The extent of inguinal coverage depends on T-stage and tumor location relative to the dentate line."},{"q":"Why is IMRT preferred for anal cancer treatment?","a":"IMRT significantly reduces acute dermatologic and hematologic toxicity compared to 3D-CRT by improving dose conformality around the complex pelvic and inguinal target geometry. RTOG 0529 demonstrated reduced grade 3+ toxicity with IMRT while maintaining efficacy."},{"q":"What dose levels are used in anal cancer RT?","a":"A simultaneous integrated boost approach is standard: 50.4-54 Gy to gross disease, 45-50.4 Gy to involved nodal regions, and 36-42 Gy to elective nodal volumes. Higher doses up to 58-59 Gy may be used for bulky T3-T4 tumors or unresectable nodes."}],"video":[],"gtin":"","mpn":"","brand":"","aggregate_rating":[]},"_links":{"self":[{"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/posts\/14088\/"}],"collection":[{"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/posts\/"}],"about":[{"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/types\/post\/"}],"author":[{"embeddable":true,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/users\/1\/"}],"replies":[{"embeddable":true,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/comments\/?post=14088"}],"version-history":[{"count":2,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/posts\/14088\/revisions\/"}],"predecessor-version":[{"id":16659,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/posts\/14088\/revisions\/16659\/"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/media\/16628\/"}],"wp:attachment":[{"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/media\/?parent=14088"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/categories\/?post=14088"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rtmedical.com.br\/en\/wp-json\/wp\/v2\/tags\/?post=14088"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}