Rectum

GIRectumRectal cancerIMRTVMATConventional fractionationShort-courseImage fusionMRI
Overview

Pre-operative radiotherapy for rectal cancer: a long course of 45–50.4 Gy with chemotherapy, or a short course of 25 Gy in 5 fractions. The target is the tumor, the mesorectum and the nodes along the pelvic sidewall, defined on MRI, and the planning work is in keeping small bowel and bladder dose down in a pelvis that changes shape every day.

Start here: I’m planning my first GI case — a step-by-step learning roadmap.

Schedules

Long course treats the pelvis to 45 Gy in 25 fractions with concurrent capecitabine or 5-FU, and brings the tumor to about 50–50.4 Gy. The boost can be given two ways:

  • Sequential boost — a second plan of 5.4 Gy in 3 fractions to the tumor and mesorectum at that level, for 50.4 Gy in 28. This is the approach of RTOG 0822 and many US plans.
  • Simultaneous integrated boost (SIB) — one plan in 25 fractions, with the boost volume at a higher dose per fraction (one protocol uses 50 Gy in 25 to the boost and 45 Gy to the elective volume).

Short course is 25 Gy in 5 fractions, usually without concurrent chemotherapy. It is often part of total neoadjuvant therapy, with chemotherapy before or after it. The organ limits differ completely from long course, so the constraint table has a column for each.

Organ-preserving (watch-and-wait) approaches change what happens after radiotherapy more than how it is planned. Your protocol sets the schedule, the boost approach and the dose to each volume.

Simulation & Setup

The pelvis changes shape from day to day with bladder and bowel filling. Setup choices and image fusion decide how well the plan matches what is there on treatment.

Supine or prone
Prone on a belly board can move small bowel out of the pelvis; supine is common with IMRT and VMAT.

What is it?

Supine with knee and ankle supports (often in a vacuum bag), or prone on a belly board that lets the small bowel fall forward out of the pelvis.

Why does it matter?

Prone on a belly board can reduce small bowel dose, but it is less comfortable and less reproducible for some patients, and it doesn't suit everyone — for example patients with a stoma or when the inguinal nodes are treated. Supine is common with IMRT and VMAT, which can shape dose around the bowel instead.

Bladder filling
A consistently full bladder moves small bowel out of the pelvis and keeps the front of the target stable.

What is it?

A comfortably full, reproducible bladder at simulation and on every treatment day.

Why does it matter?

A full bladder pushes small bowel up and out of the pelvis. Changes in filling move the front of the mesorectal volume, which is why some protocols add an internal margin at its anterior edge.

Planning CT and markers
The contrast CT shows the vessels the nodal volumes follow, and a marker ties it to the clinical exam.

What is it?

A thin-slice CT from above the pelvis to below the perineum, with IV contrast to show the pelvic vessels and nodes, optional oral contrast for small bowel, and a radiopaque marker at the anal verge or lower edge of the tumor.

Why does it matter?

The vessels are the landmarks for the nodal volumes, and the marker ties the planning CT to what the physician found on examination and endoscopy — especially important for low tumors.

Image fusion — MRI, sometimes PET
Rectal MRI fused with the planning CT, with PET/CT in selected cases.

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 rectal cancer this is the dedicated rectal staging MRI, with PET/CT added in selected cases. Either the diagnostic MRI or a planning MRI can be used, read together with the clinical exam and endoscopy findings.

Why does it matter?

The mesorectum and perirectal nodes are best seen on MRI. The diagnostic MRI is usually taken in a different position and with different bladder and rectal filling, so a bony match may not line up the tumor — check the registration in the region of the tumor. One national guideline asks the treating oncologist to approve it.

Daily imaging
Daily cone-beam CT with a soft-tissue check, because the target can move independently of the pelvic bones.

What is it?

Cone-beam CT, daily for short course and recommended daily for long course, matched to bone and then checked against soft tissue.

Why does it matter?

The rectum and mesorectum can move independently of the pelvic bones as the bladder and rectum fill, so a good bony match doesn't guarantee the target is covered. Looking at the soft tissue is what catches it.

Target Delineation

What each volume represents. Expansions, nodal regions and dose levels come only from your protocol.

GTV — Primary tumor and involved nodes

Definition

The rectal tumor and any involved nodes seen on MRI, CT, PET or endoscopy. One template includes the whole rectal circumference at the level of the tumor.

Why it matters

The GTV sets where the boost goes and anchors the craniocaudal extent of everything else, so it depends heavily on a good MRI fusion.

Contouring resources

Organ contouring guides →

Relevant protocols

CTV — Mesorectum and nodal regions

Definition

The mesorectum, presacral space and internal iliac and obturator nodes, with external iliac or inguinal nodes added when the tumor involves nearby organs or the anal canal.

Why it matters

Rectal cancer spreads first within the mesorectum and along the internal iliac chain. The international consensus guideline and the RTOG anorectal atlas define the boundaries of each region.

Contouring resources

Organ contouring guides →

Relevant protocols

Boost volume — Long course only

Definition

The GTV with a margin, including the mesorectum and presacral space at those levels, treated to the higher dose — as a separate sequential plan or as the high-dose level of an SIB.

Why it matters

The higher dose is aimed at the gross tumor, where recurrence is most likely, without raising the dose to the whole pelvis.

Contouring resources

Organ contouring guides →

PTV — Planning target volume

Definition

The CTV and boost volume expanded for setup uncertainty — and, in some protocols, for bladder-driven movement at the front of the mesorectum.

Why it matters

The PTV often overlaps the bladder and small bowel. Deciding whether coverage or an organ limit gives way in that overlap is part of the planning discussion.

Contouring resources

Organ contouring guides →

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.

Organ45–50.4 Gy / 25–28 fxLong course25 Gy / 5 fxShort course
Small BowelUse the limits that match how you contour bowel — individual loops or the bowel cavity.
  • V35 Gy <180 cc, V40 Gy <100 cc, V45 Gy <65 cc1
  • No volume ≥50 Gy1
  • Bowel cavity V45 Gy <195 cc2
  • Loops D200 cc <20 Gy, D20 cc <25 Gy3
  • Bowel cavity D200 cc <23 Gy3
BladderIf overlap with the target makes max <50 Gy unachievable, minimize the V50 Gy.
  • V40 Gy <40%, V45 Gy <15%1
  • Max <50 Gy1
  • D45% <21 Gy3
Femoral Heads
  • V40 Gy <40%, V45 Gy <25%1
  • Max <50 Gy1
—
  • RTOG 0822 set its limits for the 45 Gy in 25 fraction IMRT phase, followed by a 5.4 Gy boost; its variation-acceptable levels are higher (e.g. small bowel V35 Gy <230 cc, bladder V45 Gy <30%).
  • Short-course limits come from single-institution protocols (bowel loops and bladder: Glasgow; bowel cavity: Leeds). The source gives no formal short-course limits for the femoral heads or genitalia.
  • Limits are for plan optimization; target coverage generally takes priority.
References (3)
  1. RTOG 0822 — RTOG 0822 protocol — A Phase II Evaluation of Preoperative Chemoradiotherapy Utilizing Intensity Modulated Radiation Therapy (IMRT) in Combination with Capecitabine and Oxaliplatin for Patients with Locally Advanced Rectal Cancer. Protocol document, RTOG 0822 Source
  2. QUANTEC summary — Use of normal tissue complication probability models in the clinic. Int J Radiat Oncol Biol Phys. 2010;76(3 Suppl):S10–S19 Source
  3. RCR rectal IMRT guidance — National rectal cancer intensity-modulated radiotherapy (IMRT) guidance. RCR Clinical Oncology publication Source

Values quoted from RTOG 0822, QUANTEC summary, RCR rectal IMRT guidance and eviQ 1863 (last reviewed 14 June 2024; review due 30 June 2026), checked by DoseLab on 2026-09-29. Verify against your protocol, institutional policy and physician directive before clinical use.

Search all GI constraints in the Constraint Finder →

Protocols & Literature

Clinical protocolSource Verified

Phase 2 RTOG trial of preoperative IMRT with capecitabine and oxaliplatin for locally advanced rectal cancer. Treatment was 45 Gy in 25 fractions with IMRT to the pelvis, then a 3D conformal boost of 5.4 Gy in 3 fractions. Section 6.5 lists the IMRT planning constraints for small bowel (absolute volumes), femoral heads and bladder, and section 6.7 the variation-acceptable levels. DoseLab read sections 6.1–6.7 of the IROC Houston copy (version date 17 April 2008, update 30 March 2010). The trial results were published by Hong et al. in 2015.

View Source Details
Professional guidelineSource Verified

UK national guidance for rectal IMRT and VMAT. It covers pre-treatment imaging (dedicated rectal MRI, with PET-CT in selected cases), fusing the diagnostic or planning MRI with the planning CT and having the oncologist approve the registration, target volume definitions and an atlas, SIB strategies, beam and arc arrangements, CBCT matching, and organ limits for long-course and short-course treatment with either bowel loops or a bowel cavity. The short-course limits come from single-institution protocols (Glasgow and Leeds). DoseLab read the January 2021 PDF.

View Source Details
Clinical resourceSource Verified
NRG Oncology Protocol Development Template for Radiation Therapy — Gastrointestinal Cancer: Lower Gastrointestinal Cancer
Yock A, Parker W · NRG Oncology Medical Physics Subcommittee / Radiation Oncology Committee · 2021

The template NRG Oncology gives investigators for the radiation therapy section of a lower GI (rectal) trial. Its example uses 45 Gy in 25 fractions with a 5.4 Gy sequential boost, or a single-phase simultaneous integrated boost, and recommends rigid registration of MRI and PET with the planning CT. Values in it are examples to be edited for each trial.

View Source Details
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.

View Source Details
Clinical resourceSource Verified

Endorsed eviQ protocol for pre-operative long-course chemoradiation of rectal adenocarcinoma. It covers simulation (supine or prone on a bellyboard, MRI and PET/CT to aid tumor delineation), IMRT/VMAT with a simultaneous integrated boost or 3D conformal treatment with a sequential boost, target volumes referenced to the international consensus guideline, and an organs-at-risk table. Its bladder and femoral head values are credited to RTOG 0822. DoseLab's rectal table reproduces selected values. Its review was due 30 June 2026, so a newer version may appear.

View Source Details
Clinical resourceSource Verified

Endorsed eviQ protocol for pre-operative short-course radiotherapy (25 Gy in 5 fractions) for rectal cancer. It notes that few published protocols give organ limits for short-course IMRT or VMAT, and takes its bowel and bladder values from the Royal College of Radiologists' national rectal IMRT guidance. It also asks for care in sparing the anal canal for tumors 6 cm or more from the anal verge. Its review was due 30 June 2026, so a newer version may appear.

View Source Details

All GI literature →