Purpose: We assessed the prognostic importance of the level of thymidylate synthase (TS) expression in patients with primary rectal cancer and whether, for Dukes' B and C cancer patients, the benefit of chemotherapy was associated with TS expression. Patients and Methods: The level of TS expression in the primary rectal cancers of 294 of 801 patients enrolled on protocol R-01 of the National Surgical Adjuvant Breast and Bowel Project (NSABP) was immuno histochemically assessed with the monoclonal antibody TS 106. Results: Forty-nine percent of patients whose tumors had low TS levels (n = 91) were disease free at 5 years compared with 27% of patients with high levels of TS (n = 203; P <.0.1). Moreover, 60% of patients with low TS levels were alive after 5 years compared with 40% of patients with high TS levels (P <.01). The level of TS protein wets significantly associated with Dukes' stage (P <.01); patients with a more advanced Dukes' stage had a significantly higher level of TS. The level of TS expression remained prognostic for both disease-free survival (P <.01) and survival (P <.05) independent of Dukes' stage and other pathologic characteristics evaluated. Thirty-eight percent and 54% of patients with high TS levels (n = 71) were disease free and alive, respectively, after 5 years when created with chemotherapy, compared with 17% and 31%, respectively, of similar patients when treated with surgery alone (n = 64) (P < .01). No difference was noted in disease-free survival (P =.46) or survival (P =.43) in patients with low TS levels. Conclusion: The expression of TS is an important independent prognosticator of disease-free survival and survival in patients with rectal cancer. Adjuvant fluorouracil (5-FU)-based chemotherapy demonstrated significa nt improvement in disease-free and overall survival for patients with high TS levels. Prospective studies measuring TS levels will be needed to understand further the role of TS as a prognosticator of survival and chemotherapeutic benefit. (C) 1994 by American Society of Clinical Oncology.
