This is the most relevant published phase III study, to my knowledge. Prompt surgery after CRT works the best.
Effect of Interval (7 or 11 weeks) Between Neoadjuvant Radiochemotherapy and Surgery on Complete Pathologic Response in Rectal Cancer: A Multicenter, Randomized, Controlled Trial (GRECCAR-6) - PubMed
Purpose A pathologic complete response (pCR; ypT0N0) of a rectal tumor after neoadjuvant radiochemotherapy (RCT) is associated with an excellent prognosis. Several retrospective studies have investigated the effect of increasing the delay after RCT. The aim of this study was to evaluate the...
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