Predictive factors of tumor shrinkage and histological regression in patients who received preoperative radiotherapy for rectal cancer

In order to determine the reliable predictors of response to radiotherapy for rectal cancer, we assessed apoptosis, p53 and p21 in biopsy specimens collected before treatment, and investigated the relationships to the histological effect of irradiation and the degree of tumor shrinkage. Ninety-three patients with advanced rectal adenocarcinoma were treated with preoperative irradiation of 20 Gy in 10 fractions in combination with intraoperative electron beam irradiation of 15 Gy. We evaluated tumor grade, pathological tumor regression, biological markers associated with apoptosis and proliferation [apoptotic index (AI), p53, p21 and Ki67], and the degree of tumor shrinkage. Considering positivity of p21 and of apoptosis and negativity of p53 as favorable factors in tumor shrinkage, we compared the degree of shrinkage among the patients using the number of favorable factors as the score. The degree of shrinkage was 41.5±8.5% in patients with three favorable factors, 31.4±9.7% in patients with two favorable factors and 26.5±11.2% in patients with one favorable factor. However, there was no significant difference in the proportion of marked regression according to the number of favorable factors. Histological examination of apoptosis, p21 and p53 in biopsy specimens and scoring were considered to be useful predictive methods for assessing the efficacy of radiotherapy for rectal cancer. (authors)