Author: PathologyMCQ Editorial Team
Category: Grossing – Gastrointestinal Pathology
Last Updated: October 2026
Read Time: 12 minutes
Difficulty: Moderate (MD/DNB, FRCPath Part 2, NEET-SS)
At a glance
- Why it matters: correct orientation and peritoneal relations decide whether a tumour is pT4a or a positive circumferential margin.
- Operations: low anterior resection (LAR) spares the sphincter; abdominoperineal resection (APR) removes rectum, anus and sphincters with a permanent colostomy.
- Grossing essentials: grade the mesorectum fresh, ink the non-peritonealised surface, slice transversely and measure the tumour-to-CRM distance.
- In this post: your 7 specimen diagrams, a step-by-step grossing protocol, mesorectal grading, a 4-tab virtual slide viewer and 10 MCQs including 4 slide-based questions.
Gross anatomy of the rectum
The rectum runs about 12–15 cm from the rectosigmoid junction to the anorectal junction at the levator ani. It is wrapped posteriorly and laterally by the mesorectum, the fatty envelope carrying the superior rectal vessels and the first-station lymph nodes, bounded by the mesorectal fascia. Clinically the rectum is divided into upper (10–15 cm), middle (5–10 cm) and lower (0–5 cm) thirds by distance from the anal verge on MRI or endoscopy; the peritoneal relations below follow the same thirds.
Peritoneal relations and why they matter
| Segment | Anterior | Lateral | Posterior |
|---|---|---|---|
| Upper third | Peritoneum | Peritoneum | Mesorectum |
| Middle third | Peritoneum | Mesorectum | Mesorectum |
| Lower third | Mesorectum (extraperitoneal) | Mesorectum | Mesorectum |
APR vs LAR
| Feature | Low anterior resection (LAR) | Abdominoperineal resection (APR) |
|---|---|---|
| Goal | Sphincter-sparing total mesorectal excision (TME) | Removes rectum, anal canal and sphincters |
| Reconstruction | Colorectal or coloanal anastomosis, often with a temporary stoma | Permanent end colostomy |
| Indication | Mid and upper rectal tumours where a clear distal margin (usually 1 cm or more) can be achieved above the sphincters | Very low tumours invading the sphincters or levators, or when continence cannot be preserved |
| Specimen | Rectum with mesorectum and a distal doughnut | Rectum, mesorectum, anal canal, sphincters and perianal skin; extralevator APR (ELAPE) also includes the levators |
| Key grossing point | Mesorectal grade and distal margin | Mesorectal grade, plus the plane at the sphincter–levator level (coning) |
Orienting APR and LAR specimens
- Anterior view: the peritoneal reflection sits low on the front, forming a peritonealised “V” or triangle; look for appendices epiploicae and taeniae on the sigmoid end.
- Posterior view: the mesorectum is bulkiest posteriorly and has no peritoneal covering; it tapers distally.
- Lateral view: the peritoneum runs higher on the sides than in front, so the reflection slopes from lateral (higher) to anterior (lower).
- APR extras: anal canal, perianal skin and sphincters at the distal end; check for a waist above the sphincters.
Step-by-step grossing protocol
- Fresh and unopened: photograph anterior and posterior surfaces and grade the mesorectum (and the APR plane).
- Measure: total length, mesorectal dimensions, and the distance of the tumour from the distal margin and the dentate line.
- Ink: paint the non-peritonealised surface (the CRM); use a second colour for the peritonealised surface if desired.
- Open: open anteriorly above and below the tumour, leaving the tumour segment unopened so the CRM is not distorted.
- Fix: pin out loosely and fix in formalin for at least 48 hours (packing the lumen with formalin-soaked gauze helps).
- Slice: cut serial transverse slices 3–5 mm thick through the tumour and mesorectum; photograph them and record the closest tumour-to-CRM distance.
- Sample: the deepest invasion, the closest CRM, any peritoneal involvement, EMVI, the margins and all lymph nodes (aim for 12 or more). After neoadjuvant therapy with no visible tumour, submit the whole tumour bed.
Mesorectal and APR plane grading
| Mesorectal grade (Quirke) | Appearance | Meaning |
|---|---|---|
| Mesorectal plane (good) | Intact, bulky, smooth fascia; defects no deeper than 5 mm; no coning | Complete TME, lowest local recurrence |
| Intramesorectal plane (moderate) | Moderate bulk, irregular surface, defects not reaching muscularis propria | Nearly complete |
| Muscularis propria plane (poor) | Little bulk, defects down to the muscularis propria | Incomplete TME, high risk of CRM involvement |
For APR specimens, grade the sphincter and levator area separately: a levator plane gives a cylindrical specimen, a sphincteric plane shows a waist or coning at the levators, and an intrasphincteric or submucosal plane has perforations or missing muscle. Coning is where low tumours most often involve the CRM.
See the histology behind the gross: the virtual slide viewer below has rectal adenocarcinoma, high-grade dysplasia, a rectal NET and solitary rectal ulcer syndrome.
Virtual slides · real cases
Rectal lesions: carcinoma, dysplasia, NET and a mimic
Find the muscularis mucosae first: invasion through it is what separates carcinoma from dysplasia.
What to look for
Drag to pan · pinch or double-tap to zoom · use the + / − buttons
Reporting checklist
| Item | What to record |
|---|---|
| Specimen | LAR, APR or ELAPE; length; tumour relation to the peritoneal reflection (above, at, below) |
| Plane of excision | Mesorectal grade; APR sphincter–levator plane |
| Tumour | Size, type, grade, depth (pT), tumour deposits, EMVI, perineural and lymphatic invasion, budding |
| Margins | Closest CRM distance in mm (positive if 1 mm or less), distal and proximal margins, doughnuts |
| Nodes | Number examined (12 or more) and number involved |
| After neoadjuvant therapy | Tumour regression grade; ypTNM |
| Ancillary | Mismatch repair (MMR) immunohistochemistry or MSI status |
Featured course · Ultimate FRCPath Part 2 Study Program
Grossing is a whole FRCPath Part 2 station. Be ready for it.
The FRCPath Part 2 program trains you on the specimens examiners set, including rectal resections, with orientation, block selection and reporting walked through by practising pathologists.
- Macro and grossing stations with model answers
- Short surgicals, long cases and virtual slides
- Exam-day reporting technique
Test yourself: 10 MCQs
4 slide-based questions (adenocarcinoma, dysplasia, NET, solitary rectal ulcer) plus 6 on peritoneal relations, CRM, mesorectal grading and APR planes. Tap an answer for the explanation.
More practice: the GI pathology MCQ bank →
Exam pearls and key takeaway
- Peritoneum: upper third anterior and lateral; middle third anterior only; lower third none.
- Above the reflection: tumour on the peritoneal surface is pT4a. Below it: tumour 1 mm or less from the inked surface is CRM positive.
- LAR spares the sphincter; APR removes it with a permanent colostomy; ELAPE adds the levators to avoid coning.
- Grade the mesorectum fresh: mesorectal, intramesorectal or muscularis propria plane.
- Nodes: aim for 12 or more; report EMVI and tumour deposits.
- Key takeaway: orient, grade, ink, slice transversely and measure the CRM; these few minutes decide the patient’s local recurrence risk.
Go further
Master grossing and GI pathology
Courses, printed notes and daily practice by practising pathologists.
The Pathology MCQ app
A slide and an MCQ every day
Daily MCQ with streaks, pinch-zoom virtual slides with annotations, and your courses in one place.
Related reads: GI pathology MCQs →
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