Pathological assessment of rectal carcinoma after preoperative therapy.

Increasingly neoadjuvant therapy is being used to improve outcomes in patients with rectal carcinoma in which the circumferential resection margins are considered to be at risk for involvement if primary surgery were to be undertaken. Assessment of the response to this approach relies on radiological examination, particularly magnetic resonance imaging (MRI) studies. Following definitive surgery, careful histological examination allows full assessment of the tumour response to these preoperative approaches. Histological examination requires careful fixation, examination of the entire area occupied by the tumour prior to down-staging and careful lymph node harvesting. Adequate fixation helps in these endeavours and the lymph node harvest appears to be unaffected by neoadjuvant chemo- or radiotherapy. Correlation between preoperative assessment of response by MRI and the subsequent histological assessment is close, but the presence of isolated residual neoplastic glands in a post-treatment fibrotic stroma is impossible to detect prior to resection. The clinical significance of these microscopic foci remains uncertain, particularly in view of the prolonged tumour doubling time associated with colorectal adenocarcinoma. The preoperative discussion with the patient requires a synthesis of their own scan results and the experience of detailed clinico-pathological studies. While MRI frequently predicts the presence or absence of residual tumour the possibility of under-staging remains and this is of crucial importance if a 'watch and wait' policy is to be adopted following apparent complete clinical and radiological remission. The significance of potential residual microscopic disease in patients with apparent radiological complete remission needs further investigation but may need to be interpreted in individual patients in the context of overall life-expectancy.