PURPOSE: We studied the prognostic value of thymidylate synthase (TS) expression in primary colorectal cancer (CRC) and the role of TS expression as a predictor of chemotherapeutic benefit in patients treated with adjuvant chemotherapy.

PATIENTS AND METHODS: TS expression was immunohistochemically assessed on tumor sections from 862 patients with CRC Dukes’ stages B and C enrolled onto randomized trials evaluating fluorouracil (5-FU)-based adjuvant chemotherapy.

RESULTS: TS expression was an independent prognostic factor for disease-free (P = .05) and overall survival (P = .05). In the subgroup treated with surgery alone, TS was an independent prognostic factor for disease-free (P < .001) and overall survival (P = .001), whereas this was not the case in the subgroup of adjuvantly treated patients. Patients whose tumors expressed high TS levels had a tendency to improved outcome after adjuvant therapy (not significant). The group whose tumors expressed the highest TS grade, grade 3 (34% of the patients), had a significantly longer disease-free survival if they were treated with adjuvant therapy compared with surgery alone (multivariate analyses, P = .02), whereas patients whose tumors expressed low TS levels (28% of the patients) had an impaired outcome after adjuvant therapy (multivariate analyses: disease-free survival, P = .01; overall survival, P = .01).

CONCLUSION: TS expression predicts for survival independent of Dukes’ stage in patients with CRC treated with surgery alone. The study indicates that patients with high TS levels may benefit from adjuvant 5-FU–based chemotherapy. However, patients with low TS levels seem to have a worse outcome when treated with adjuvant chemotherapy.