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Tumour regression in the randomized Stockholm III Trial of radiotherapy regimens for rectal cancer. BACKGROUND: The Stockholm III Trial randomized patients with primary operable rectal cancers to either short-course radiotherapy (RT) with immediate surgery (SRT), short-course RT with surgery delayed 4-8 weeks (SRT-delay) or long-course RT with surgery delayed 4-8 weeks. This preplanned interim analysis examined the pathological outcome of delaying surgery. METHODS: Patients randomized to the SRT and SRT-delay arms in the Stockholm III Trial between October 1998 and November 2010 were included, and data were collected in a prospective register. Additional data regarding tumour regression grade, according to Dworak, and circumferential margin were obtained by reassessment of histopathological slides. RESULTS: A total of 462 of 545 randomized patients had specimens available for reassessment. Patients randomized to SRT-delay had earlier ypT categories, and a higher rate of pathological complete responses (11·8 versus 1·7 per cent; P = 0·001) and Dworak grade 4 tumour regression (10·1 versus 1·7 per cent; P < 0·001) than patients randomized to SRT without delay. Positive circumferential resection margins were uncommon (6·3 per cent) and rates did not differ between the two treatment arms. CONCLUSION: Short-course RT induces tumour downstaging if surgery is performed after an interval of 4-8 weeks.