In rectal cancer, target volume delineation and field setup begin before the first isodose line is drawn. The chapter makes that clear from the opening pages: exam findings, endoscopy, MRI, and PET/CT all influence what belongs in the gross tumor volume, how far the high-risk volume needs to travel, and when external iliac or inguinal coverage stops being optional. For a broader site-by-site overview, see the Target Volume Delineation and Field Setup – Complete Clinical Guide.
The practical value of this chapter is that it never separates contouring from the clinical situation. A low-lying tumor is not just a location problem. It changes the relevance of the dentate line, raises attention to sphincter function, and can pull the inguinal chains into the conversation. A T4 lesion with anterior organ invasion is not just a stage label either. It changes which pelvic nodal basins deserve elective coverage and how far the field borders must move.
Diagnostic Workup Relevant for Target Volume Delineation
Physical examination remains part of the planning dataset. If the tumor is palpable, the distance to the anal verge should be recorded, and sphincter function should be documented at the same visit. That sounds basic, but the chapter is right to place it first. Those two details frame how you interpret everything else that follows on imaging.
Low tumors demand direct visualisation because the dentate line is not palpable. Endorectal ultrasound can help with depth of invasion and nearby nodes, but the text is appropriately cautious: it may under-stage or over-stage roughly 20% of patients. I would treat EUS as supportive information, not as the last word, especially when the remainder of the staging package points elsewhere.
MRI is now treated as the standard preoperative staging modality. The reasons are practical: it shows invasion into mesorectal fat, helps separate T3 from T4 disease, evaluates nodal status, confirms distance from the anal verge, and helps judge whether negative margins look achievable. The examples in Fig. 18.1 underscore a point every planner recognises immediately: once the tumor approaches the mesorectal fascia, margin awareness becomes part of contouring, not just surgery.
PET/CT also has a defined role. It can help delineate gross disease, particularly when co-registered to the planning CT. The caution in the text is equally important: areas of low PET uptake should never overrule the physical exam, endoscopy, or CT and MRI findings. In practice, that means PET can sharpen a contour, but it should not erase anatomy.
Figure 18.2 illustrates that point well. The case shows a clinical T4N0 rectal adenocarcinoma with invasion into the cervix, and the co-registered CT and PET views make the GTV easier to appreciate in axial, sagittal, and coronal planes. When the image fusion is clean, that kind of dataset improves confidence. When the biology and anatomy disagree, the chapter clearly favours the anatomical truth.
Simulation and Daily Localization
Simulation is driven by technique. Most patients treated with 3D conformal radiotherapy can be simulated prone on a belly board to displace bowel. For IMRT, the recommendation shifts to supine positioning in a body mold because reproducibility matters more than the bowel-displacement benefit you can gain from the prone setup. That is a sensible preference. If the setup is not consistent, the elegance of the plan is irrelevant.
The technical recommendations are straightforward: CT simulation with intravenous contrast, slice thickness of 3 mm or less, and optional oral contrast if small-bowel delineation is difficult. A radio-opaque marker at the anal verge is useful, and surgical scars should be wired. PET/CT simulation, or fusion with diagnostic PET/CT or MRI, can refine target definition when the anatomy or the disease distribution is not obvious enough on the planning CT alone.
Bladder management is presented as a choice rather than a rigid doctrine. A full bladder may keep more bowel out of the pelvis; an empty bladder may be more reproducible. That trade-off is real. The text does not pretend one answer fits every patient, and that is the right stance. What matters is choosing the approach that your team can reproduce every day.
For daily localisation, the recommendation is image guidance with daily orthogonal kilovoltage imaging and weekly cone-beam CT to assess soft tissue, with the exact frequency influenced by setup reproducibility. The chapter is practical here as well: soft-tissue verification matters because the target is defined by pelvic organs that do move, even when the bony setup looks stable.
Figure 18.5 shows the planning logic in a preoperative clinical T3N1b case simulated prone with PET/CT and 2.5 mm slices. The small bowel has fallen anteriorly, away from the target, and the relationship among CTV-SR, CTV-HR, GTV-N, and GTV-P becomes easy to read. The image is a useful reminder that simulation choices and contour quality are tightly linked.
Target Volume Delineation and Treatment Planning in Rectal Cancer
Before any cone-down, conventional 3D conformal treatment uses a posterior-anterior field plus opposed laterals. The standard PA field extends superiorly to the L5/S1 interspace, inferiorly to the lower edge of the obturator foramen or 3 cm below the GTV, whichever lies farther caudally, and laterally to 1.5-2 cm beyond the pelvic brim. The lateral fields share the superior and inferior borders, place the anterior edge at the posterior margin of the pubic symphysis for T1-T3 disease or at least 1 cm anterior to the anterior edge of the pubic symphysis for T4 disease, and place the posterior edge 1-1.5 cm behind the posterior border of the bony sacrum.
CT-based planning allows those historical borders to be modified once the actual target volumes are visible. The chapter still grounds the discussion in named structures: GTV-P is all gross disease seen on examination, endoscopy, and imaging, while GTV-N includes visible perirectal, mesorectal, and involved iliac nodes. The advice to include any doubtful node as GTV when biopsy is absent is deliberately conservative. In rectal planning, undercalling nodal disease is usually the more expensive mistake.
Two clinical patterns immediately change the pelvic map. Low-lying tumors force a deliberate look at the inguinal nodes. Tumors with anterior invasion into bladder, cervix, or prostate force a deliberate look at the external iliac chains. Those are not academic exceptions. They are exactly the scenarios in which elective coverage can fail if the contouring logic stays too generic.
Preoperative target volumes
The preoperative table remains the core reference for defining GTV, the high-risk microscopic envelope, the standard-risk pelvic elective volume, and the final PTV margin. It also makes clear that coverage of the obturator, external iliac, and inguinal regions depends on the disease pattern rather than a one-size-fits-all whole-pelvis template.
Table 18.1: Preoperative target volumes
The chapter separates gross disease, high-risk microscopic spread, elective pelvic coverage, and setup margin in a way that is easy to translate into a planning workflow.
| Target volume | Definition and description |
|---|---|
| Gross tumor volume (GTV) | Primary (GTV-P): all gross disease on physical examination, endoscopy, and imaging. Regional lymph nodes (GTV-N): all visible perirectal, presacral, and involved iliac nodes. Include any lymph node in doubt as GTV in the absence of a biopsy. For low-lying rectal tumors, attention should also be paid to the inguinal nodes. |
| High-risk clinical target volume (CTV-HR) | CTV-HR should cover the GTV-P and GTV-N with 1.5-2 cm margin expansion superiorly and inferiorly, excluding uninvolved bone, muscle, and air. For grossly involved external iliac or inguinal nodes, a minimum 10-15 mm GTV to CTV margin should be included. Include the entire rectum, mesorectum, and presacral space in the transverse plane at these levels. A 1-2 cm margin into adjacent organs, such as bladder, prostate, or cervix, should be added for T4 tumors. Visible mesorectal nodes on CT, MRI, and PET/CT should be included. |
| Standard-risk clinical target volume (CTV-SR) | Include the CTV-HR, entire mesorectum, and bilateral internal iliac lymph nodes. Include the external iliac and obturator nodes for T4 tumors with anterior organ involvement. Include the external iliac and inguinal lymph nodes in cases with anal canal involvement. Superior: rectum and mesorectum up to the L5/S1 interspace or 2 cm superior to gross disease, whichever is most cephalad. Inferior: pelvic floor or at least 2 cm inferior to gross disease, whichever is most caudad. Lymph nodes: draw a 0.7 cm margin around the internal iliac vessels, excluding muscle and bone. To cover the external iliac nodes for T4 lesions, draw an additional 1 cm margin anterolaterally around the vessels and include adjacent small nodes. For tumors that extend into the anal canal, cover the bilateral inguinal nodes. Draw a 1.8 cm wide volume between the external and internal iliac vessels to cover the obturator nodes. Anterior: add a 1-1.5 cm margin into bladder to account for changes in bladder and rectal filling. |
| Planning target volume (PTV) | Expand each CTV by 0.5-1 cm, depending on the physician’s comfort level with setup accuracy, frequency of imaging, and the use of IGRT. |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 18.1)
One of the strongest practical messages in Table 18.1 is the way the chapter layers the target. CTV-HR stays close to gross disease and the mesorectal compartment, then CTV-SR broadens into the internal iliac basin and, when indicated, into obturator, external iliac, or inguinal nodal territory. That tiered logic is more defensible than drawing a single broad pelvic volume and hoping it covers every scenario equally well.
Figure 18.6 demonstrates why T4 disease deserves a different nodal discussion. In the illustrated case, gross invasion into the cervix leads the CTV-SR to include the external iliac nodal region. The image helps because it shows the expansion is not arbitrary. It follows the pattern of spread implied by anterior organ involvement.
Figure 18.10 makes the low-lying scenario equally concrete. The tumor sits 2 cm above the anal verge, a left inguinal node is grossly involved on PET/CT, and the patient is treated with IMRT to cover bilateral external iliac and inguinal nodes. The chapter even shows the nodal CTV as a 10 mm expansion from the nodal GTV. This is a good example of when pelvic contouring needs to widen without becoming vague.
Postoperative planning
The postoperative framework is similar, but the anatomy you are covering is different. When an abdominoperineal resection has been performed, the entire surgical bed, including the perineal scar, belongs in the target. That single sentence from the chapter is easy to overlook, but it changes the inferior extent of the field in a major way.
Table 18.2: Postoperative target volumes
After surgery, the logic stays recognisable, but the chapter adds postoperative bed anatomy, anastomotic level, and the perineal scar when an abdominoperineal resection has been done.
| Target volume | Definition and description |
|---|---|
| Clinical target volume for gross disease or positive margin (CTV-P) | Areas of known microscopically involved margin or macroscopic residual disease plus a 1-2 cm margin, excluding uninvolved bone, muscle, or air. |
| High-risk clinical target volume (CTV-HR) | Remaining rectum, if applicable, mesorectal bed, and presacral space axially at these levels, excluding uninvolved bone, muscle, or air. For undissected grossly involved external iliac or inguinal nodes, a minimum 10-15 mm GTV to CTV margin should be included. |
| Standard-risk clinical target volume (CTV-SR) | Include the CTV-HR, entire mesorectum, and bilateral internal iliac lymph nodes. Include the external iliac and obturator nodes for T4 tumors with anterior organ involvement. Include the external iliac and inguinal lymph nodes in cases with anal canal involvement. Superior: remaining rectum and mesorectum, usually up to L5/S1, with at least 1 cm margin superior to the anastomosis, whichever is most cephalad. Inferior: pelvic floor or at least 1 cm below the anastomosis or rectal stump, whichever is most caudad. After abdominoperineal resection, include the surgical bed down to the wired perineal scar. Lateral: 0.7 cm margin around the internal iliac vessels, excluding muscle and bone. To cover the external iliac nodes for T4 lesions, draw an additional 1 cm margin anterolaterally around the vessels and include adjacent small nodes. For tumors involving the anal canal, cover the bilateral inguinal nodes. Draw a 1.8 cm wide volume between the external and internal iliac vessels to cover the obturator nodes. Anterior: add a 1-1.5 cm margin into bladder to account for changes in bladder and rectal filling. |
| Planning target volume (PTV) | Expand each CTV by 0.5-1 cm, depending on the physician’s comfort level with setup accuracy, frequency of imaging, and the use of IGRT. |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 18.2)
The rest of the postoperative logic stays consistent with the preoperative approach. The residual gross disease or positive-margin region is treated as its own target, the remaining rectum and mesorectal bed define the high-risk postoperative volume, and the internal iliac basin remains standard coverage. External iliac, obturator, and inguinal inclusion still depends on T4 anterior extension or anal canal involvement.
Figure 18.7 is a useful postoperative example because it shows a pathologic T3N2a case after abdominoperineal resection without preoperative chemoradiotherapy. The primary extended from 2-5 cm from the anal verge. In that case, the absence of small bowel near the postoperative bed allowed the high-risk target to be boosted to 55.8 Gy. The chapter immediately adds the real-world caveat: if bowel were close to the boost, the dose could be reduced. That is planning judgment in one sentence.
Elective pelvic atlases and nodal logic
The RTOG anorectal contouring atlas breaks elective coverage into three CTVs. CTV-A includes the perirectal, presacral, and internal iliac regions and should be covered in all rectal cancer cases. CTV-B adds the external iliac nodes for primary rectal tumors that invade adjacent organs or extend into the anal canal. CTV-C adds the inguinal region for tumors that extend into the anal canal. The chapter provides the detailed CTV-A borders in Table 18.3.
Table 18.3: CTV-A borders in the RTOG anorectal atlas
The chapter only details CTV-A in table form, but that detail is useful because it anchors the elective pelvis around reproducible landmarks instead of loose intuition.
| Clinical target volume | Key highlights |
|---|---|
| CTV-A: lower pelvis | Inferior: 2 cm below gross disease, including the entire mesorectum down to the pelvic floor. Lateral: does not need to extend more than a few millimeters beyond the levator muscles unless there is tumor extension into the ischiorectal fossa. For T4 tumors, include a 1-2 cm margin around identified areas of invasion. |
| CTV-A: mid-pelvis | Includes the rectum, mesorectum, internal iliac region, and 1 cm margin into the bladder for daily variation in bladder filling. Posterolateral: extends to the pelvic sidewall muscles or bone when muscles are absent. Anterior: at least 1 cm into the posterior bladder. It should also include at least the posterior portion of the internal obturator vessels. Recommend 7-8 mm margin in soft tissue around the internal iliac vessels. Trim the CTV off uninvolved muscle and bone. |
| CTV-A: upper pelvis | Superior perirectal component: at the rectosigmoid junction or at least 2 cm cephalad to macroscopic disease in the rectum or perirectal nodes, whichever is most cephalad. The entire length of the rectum should be included. Superior nodal coverage: at the bifurcation of the common iliac vessels into the external and internal iliacs, approximately at the sacral promontory. Recommend 7-8 mm margin in soft tissue around the internal iliac vessels, but at least 1 cm anteriorly, especially if vessels or small nodes are seen in this area. Trim the CTV off uninvolved muscle and bone. |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 18.3)
The chapter also notes that more recent international consensus guidelines use a common set of pelvic subsites and make several distinctions that matter in practice: abdominal cranial presacral space, ischiorectal fossa, anterior versus posterior lateral lymph nodes, and the cranial border of the lateral nodes. Based on those guidelines, it may be reasonable in select T3N0 tumors without mesorectal fascia invasion to omit lateral lymph nodes above the cranial border of the mesorectum, and in select T3N0-1 tumors to omit the anterior lateral nodes. The Australasian GI Trials Group atlas pushes the conversation further by defining seven elective regions: mesorectum, presacral space, internal iliac nodes, ischiorectal fossa, obturator nodes, external iliac nodes, and inguinal nodes.
Figure 18.9 is a good reminder that nodal anatomy does not always read like a textbook diagram. This patient had a 2 cm non-regional right common iliac lymph node confirmed by PET/CT. The image reinforces the value of integrating functional imaging when the disease extends beyond the most routine pelvic pattern.
Dose and fractionation
The chapter accepts more than one dose pathway. For preoperative T3 or node-positive disease, the common approach is 45 Gy in 1.8 Gy fractions to PTV-SR followed by a sequential 5.4 Gy cone-down boost to 50.4 Gy, or an SIB approach that reaches 50 Gy in 2 Gy fractions. For T4 disease, the high-risk volume may instead be boosted to 54-55.8 Gy in 30-31 fractions. Grossly involved lymph nodes that will not be resected, such as inguinal nodes, should be boosted to about 60 Gy in 30 fractions, while nodes destined for resection can stop at 50.4 Gy.
Table 18.5: Suggested dose and fractionation schemes
The dose table is practical rather than theoretical. It tells you what the chapter considers acceptable in the common preoperative and postoperative scenarios.
| Setting | PTV-SR | PTV-HR |
|---|---|---|
| Preoperative T3 or N+ | 45 Gy at 1.8 Gy/fx, or 45 Gy at 1.8 Gy/fx with SIB | 50.4 Gy at 1.8 Gy/fx with sequential cone-down, or 50 Gy at 2 Gy/fx with SIB |
| Preoperative T4 N0-2b | 45 Gy at 1.8 Gy/fx, or 45.9 Gy at 1.7 Gy/fx with SIB | 54-55.8 Gy at 1.8 Gy/fx with cone-down, or 54 Gy at 2 Gy/fx with SIB |
| Preoperative short course T3-4 or N+ | 25 Gy at 5 Gy/fx | Not applicable |
| Postoperative, negative margins | 45 Gy at 1.8 Gy/fx, or 45.9 Gy at 1.7 Gy/fx with SIB | 54-55.8 Gy at 1.8 Gy/fx with cone-down, or 54 Gy at 2 Gy/fx with SIB |
| Postoperative, gross disease or positive margin | 45 Gy at 1.8 Gy/fx, or 45.9 Gy at 1.7 Gy/fx with SIB | 54-59.4 Gy at 1.8 Gy/fx with cone-down, or 54-60 Gy at 2 Gy/fx with SIB and/or cone-down |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 18.5)
Short-course treatment is also represented directly in the chapter: 25 Gy in 5 Gy fractions for preoperative T3-4 or node-positive disease. Figure 18.8 shows how that looks in a clinical T3N0 case, again in the prone position with anterior bowel displacement on PET/CT simulation. The planning technique may differ, but the need for anatomically disciplined contouring does not.
When IMRT is used, simultaneous integrated boosts are an accepted option. That flexibility is useful, but the chapter adds one more planning rule that matters in the era of total neoadjuvant therapy: until stronger outcome data are available, the pre-chemotherapy primary and nodal volumes should define the target volumes. Initially suspicious nodes should stay in the boost volume, and a threatened radial margin before chemotherapy should remain inside the high-dose volume even after a major response. That is a conservative rule, and I think it is the right one.
Plan Assessment
Plan assessment in this chapter is blunt and appropriately simple. At least 95% of each PTV should receive 100% of the prescription dose, and the maximum dose inside the PTV should remain below 110%. If a sequential boost is used, each individual plan deserves review before looking at the summed dose. That is the safest way to spot hot spots or undercoverage that can disappear inside a reassuring plan sum.
The organs at risk called out explicitly are small bowel, large bowel, bladder, femoral heads, iliac crest, and external genitalia. Uniform contouring guidance from an RTOG consensus panel is available for small bowel, large bowel, bladder, and femoral heads, and the chapter lists QUANTEC and RTOG 0822 constraints for the most relevant structures.
Table 18.6: Organ-at-risk dose constraints
Plan evaluation is not complete until the bowel, bladder, and femoral heads are checked against the chapter’s QUANTEC and RTOG 0822 benchmarks.
| Organ-at-risk | Constraints |
|---|---|
| Small bowel | QUANTEC V15Gy < 120 cc for individual loops V45Gy < 195 cc for the entire potential space within the peritoneal cavity RTOG 0822 V35Gy < 180 cc V40Gy < 100 cc V45Gy < 65 cc Dmax < 50 Gy |
| Bladder | QUANTEC Dmax < 65 Gy V65Gy < 50% RTOG 0822 V40Gy < 40% V45Gy < 15% Dmax < 50 Gy |
| Femoral heads | RTOG 0822 V40Gy < 40% V45Gy < 15% Dmax < 50 Gy |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 18.6)
The constraints are not there to decorate the end of the plan. They should shape the trade-offs earlier, especially in postoperative cases and in boost scenarios where bowel may sit close to the target. That is also why the chapter’s dose-escalation examples are paired with image-based context. A number on its own never tells you whether the anatomy can tolerate it.
If you want the full multi-site framework behind this chapter, return to the Target Volume Delineation and Field Setup – Complete Clinical Guide. For a comparison of how contouring logic changes in a very different disease site, see our detailed article on nasopharyngeal carcinoma target delineation.
