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.