Pathological examination of the resection specimen is of the utmost importance in determining prognosis and the need for adjuvant therapy. In this section, the important histopathological features, the need for proforma reporting and the role of pathologists in the multidisciplinary team are discussed.

There is good evidence that staging identifies those patients who might benefit from adjuvant chemotherapy, and circumferential resection margin (CRM) reporting helps to select patients with rectal cancer who might benefit from postoperative radiotherapy (see section 9). For this reason, the pathologist is a key member of the local core multidisciplinary colorectal cancer team.

8.1 Important pathological parameters in colorectal cancer

Resection specimens for colorectal cancer need to be carefully prepared and dissected to obtain accurate assessment of the important prognostic parameters. Cohort studies have shown that tumour stage (Dukes’ or tumour, node, metastasis (TNM) system - see Annex 1) is an important prognostic parameter. The presence of clear vascular invasion outwith the bowel wall (assessed on routine haematoxylin and eosin stained preparations) is a further adverse parameter, correlating particularly with the development of hepatic metastases.^{154, 155} Ulceration of the peritoneum, defined by the presence of tumour cells directly on the surface, is another important microscopic indicator of a poor outcome.^{154, 156} Rectal cancer presents problems of its own in that much of the rectum lies embedded in the soft tissues of the pelvis. It is now clearly recognised that local recurrence of rectal cancer can be accurately predicted by pathological assessment of circumferential margin involvement in these tumours.¹⁵⁶ Evidence level 2++,2+

Pathological reporting of colorectal cancer resection specimens should include information on: tumour differentiation staging (Dukes’ and TNM systems) margins (peritoneal and CRM) extramural vascular invasion.

Specimens should ideally be received fresh in the laboratory and opened from either end up to, but not through, the tumour. The peritoneal and/or circumferential margins are marked with ink and the bowel pinned and fixed for at least 48 hours. The region of the tumour is examined by slicing in serial thin (5 mm) transverse sections to allow for optimum assessment of depth of invasion and margins. As a routine four blocks of tumour are taken for microscopy. The fat is carefully dissected to retrieve all lymph nodes.156

For rectal cancers comments should be made on the quality of the surgical mesorectal dissection and on whether or not the anterior quadrant is involved for tumours lying below the peritoneal reflection.

8.2 Reporting in colorectal cancer

Recent studies have shown that template proformas significantly increase the rate of inclusion of data items in reports of colorectal cancer resection specimens.^{157, 158} Evidence level 1+,2+

All reporting of colorectal cancer specimens should be done according to or supplemented by the Royal College of Pathologists’ minimum data set.