The Relationship between Primary Gross Tumor Volume and Tumor Response of Locally Advanced Rectal Cancer: pGTV as a More Accurate Tumor Size Indicator.

Purpose: To identify the clinical predictive factors of tumor response and to evaluate the significance of primary gross tumor volume (pGTV), obtained from radiotherapy planning, in predicting tumor response. Materials and Methods: We retrospectively analyzed data of consecutive locally advanced rectal cancer (LARC) patients who were treated with neoadjuvant chemoradiotherapy (nCRT) followed by radical surgery at our institution between March 2009 and December 2017. We identify independent predictors of tumor response to nCRT by statistical analysis. Disease-free survival (DFS) starting from the time of surgery was calculated by the Kaplan-Meier method, and log-rank tests were performed to compare DFS between patients with superior and inferior tumor response. Results: Overall, 185 LARC patients received nCRT, of whom 89 (48.11%) achieved superior tumor response. Diminutive pGTV (p = 0.038) and distance from the anal verge (DAV) (p = 0.006) were independent predictive factors of superior tumor response. Meanwhile, pGTV can be regarded as an independent predictor of pathologic complete response (pCR) (p = 0.036). The log-rank test revealed that DFS was longer in the diminutive pGTV group than in the giant pGTV group (p = 0.001). Conclusions: pGTV, as a measure of tumor size, is not only an important prognostic indicator but also an independent predictive factor of tumor response, even pCR.