Pathways of spread in rectal cancer: a reappraisal of the true routes to distant metastatic disease

Rectal cancer can spread in a number of ways which have been previously recognised and validated as prognostic markers. These routes of spread are not adequately recognised in the stage grouping of the tumour-node-metastasis system, which focuses predominantly on the depth of invasion and nodal status, thus limiting its prognostic accuracy. Tumour spread involving veins occurs in 40% of patients. Venous channels have greater direct access to distant sites by means of a vascular ‘anatomical highway’. This rapid spread of tumour cells to distant metastatic sites by veins cannot occur by means of lymph node pathways. Thus, lymph nodes have been overestimated in their importance. Distinction between local tumour spread (lymph node metastases, perineural and lymphatic invasion) and tumour spread mediated by a direct vascular pathway to distant dissemination (extramural venous invasion and tumour deposits) must be made as the implications for prognosis and choice of treatment are not likely to be equal. Improved precision of radiological and pathological assessment is needed to scrutinise and carefully document each route of tumour spread. Only with this accurate information will it be possible to correctly weight each feature and develop a more prognostically accurate staging method that would allow separation of true high- and low-risk groups and subsequent improvements in patient care.