Institutional variation in the thoroughness of pathologic assessment and pathologic complete response rates for locally advanced rectal cancers treated with neoadjuvant chemoradiation.

696Background: A pathologic complete response (pCR) after neoadjuvant therapy and surgical excision is associated with a better prognosis and guides the management of patients with locally advanced rectal cancer. It is not known whether the thoroughness of pathologic assessment correlates with the finding of pCR. Methods: We introduce a surrogate measure for the thoroughness of pathologic assessment by taking the ratio of maximum residual tumor size and the number of cassettes prepared from the tumor: the Tumor Size to Cassette Ratio (TSCR). We retrospectively reviewed pathology reports from 259 patients with Stage II/III rectal cancer enrolled in a multicenter prospective clinical trial to determine whether TSCR is associated with pCR. Results: Of 247 included patients, 71 (29%) had a pCR. The pCR rate ranged from 0-45% and TSCR ranged from 0.0004 to 1.67 across the twelve trial sites. TSCR was significantly associated with pCR on univariable analysis. On multivariable analysis, TSCR remained significant...