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To further define the indications for postoperative pelvic irradiation and chemotherapy, an analysis of the influence of extent of tumor invasion into perirectal fat, lymphatic or venous vessel invasion, and tumor grade on the clinical course of patients with Stage T2NO rectal cancer undergoing surgery was undertaken.From 1968 to 1985, 117 patients with Stage T3NO rectal cancer underwent resection with curative intent. No patient received neoadjuvant or adjuvant irradiation or chemotherapy. Surgical specimens were assessed for maximum depth of tumor invasion into perirectal fat, lymphatic or venous involvement, and tumor grade. After surgery the clinical course of these patients was assessed for local control, distant metastases, and survival rate.For 25 patients with tumors exhibiting favorable histologic features (well-differentiated or moderately well-differentiated carcinomas invading less than 2 mm into perirectal fat, without lymphatic or venous vessel involvement), the ten-year actuarial rates of local control and recurrence-free survival were 95 and 87 percent, respectively. In contrast, the ten-year actuarial rates of local control and recurrence-free survival were inferior (71 and 55 percent, respectively) for 88 patients with tumors exhibiting moderate to deep perirectal fat invasion, vessel involvement, or poor differentiation.In the design of future trials of rectal cancer, selection of patients with rectal cancer for postoperative adjuvant therapy should be based not only on stage, but also on depth of invasion into the perirectal fat, vessel involvement, tumor grade, and integrity of the radial resection margin. For subsets of patients with Stage T3NO rectal cancer, there may be little benefit to adjuvant therapy after surgery.