Abstract

A histological diagnosis of pT4 stage is used as the basis for offering adjuvant therapy to patients with colorectal cancer. Histological features diagnostic of pT4 stage in colorectal cancer include: (1) presence of tumour perforation, (2) invasion of an adjacent organ, (3) direct or discontinuous tumour extending to the non-peritonealised resection margin, and (4) tumour breaching the visceral peritoneum. Histological parameters to identify these features are not always entirely straightforward, making recognition of pT4 stage difficult at times. The presence of a cellular mesothelial reaction and/or inflammation extending to the serosal surface causes considerable confusion and may result in upstaging pT3 tumours. Conversely, involvement of the non-peritonealised margin in colonic tumours other than rectal cancers is at a risk of being understaged. Audits have shown that significant differences remain in the frequency with which serosal involvement is found between individual pathologists and multidisciplinary teams. The recognition of the criteria that define pT4a and pT4b subsets in pT4 stage to highlight different therapeutic connotations is not universally accepted, and this is likely to affect future reporting of the subsets. The review article highlights the contentious issues in histopathological recognition of pT4 tumour stage based on microscopic parameters in colorectal cancer.