The percentage of mesorectal infiltration as a prognostic factor after curative surgery for pT3 rectal cancer

The aim of this study is to evaluate the prognostic value of a novel variable - the percentage of mesorectal infiltration (PMI) - in pT3 rectal cancer.A cohort of 241 patients with pT3 rectal adenocarcinoma, operated on between February 2002 and May 2019, was selected for the analysis. Data concerning patient, treatment and tumour characteristics were collected. The depth of mesorectal infiltration (DMI) and the distance between the deepest invasion and the circumferential resection margin (CRM) were measured. The PMI was calculated using a formula combining these parameters.Neoadjuvant therapy was administered in 33.2% of cases. A complete mesorectal excision was achieved in 74% of patients. The CRM was affected in 24 patients (9.9%). The 5-year actuarial local recurrence (LR), overall recurrence (OR) and overall survival (OS) rates were 7.5%, 22.9% and 72.4%, respectively. The PMI was significantly associated with worse oncological outcomes regarding LR (p = 0.009), OR (p = 0.001) and OS (p = 0.016) rates. A cut-off value of PMI >60% had the highest specificity (80%) for LR (p = 0.026), OR (p = 0.04) and OS (p = 0.07).The PMI has an adverse prognostic impact on the oncological results following surgery for pT3 rectal cancer. It allows prediction of the risk of both LR and distant recurrence with higher accuracy than the DMI or the distance to the CRM. A PMI >60% may be used as a cut off value while subclassifying pT3 rectal tumours. It may influence decision-making while establishing adjuvant treatment and the follow-up schedule.