Tumor proliferation in rectal cancer following preoperative irradiation.

This study examines the effect of preoperative irradiation on tumor proliferation in rectal cancer. One hundred twenty-two patients with locally advanced rectal cancer received 45 to 50 Gy of preoperative irradiation followed by surgery. Pretreatment tumor biopsies and postirradiation surgical specimens were scored for proliferative activity by assaying the extent of Ki-67 and proliferating-cell nuclear antigen (PCNA) immunostaining and the number of mitoses per 10 high-power fields (hpf). Preirradiation and postirradiation proliferative activity was determined and correlated to clinical outcome. There was an overall reduction in the tumor proliferative activity of rectal cancer after irradiation compared with its preirradiation state. Decreases in the activity of all three markers of tumor proliferation (Ki-67 and PCNA immunostaining, and mitotic counts) were observed in irradiated tumors compared with pretreatment biopsies. Postirradiation tumor proliferative activity was associated with pathologic tumor stage. A high level of proliferative activity was observed in tumors downstaged to the rectal wall (T1-2) compared with tumors that retained transmural penetration (T3-4). Multivariate analysis indicated that postirradiation proliferative activity and stage were independently associated with survival following surgery. Patients with tumors that exhibited elevated proliferative activity postirradiation had improved survival compared with patients with tumors that showed less proliferative activity. Moderate- to high-dose preoperative irradiation decreases both the tumor size and proliferative activity of rectal cancers. Elevated postirradiation tumor proliferative activity correlates strongly with improved survival. This may aid in identifying high-risk patients following preoperative irradiation and surgery.