Radiolabeled thymidine: A sensitive tracer for early tumor response and recurrence after irradiation

This study evaluated the sensitivity of a radiolabeled thymidine tracer for assessment of early tumor response and recurrence after irradiation. Metho ds: SW707 human colon carcinoma implanted into nude mice was irradiated wit h 6 or 20 Gy. Tumor volume was determined for an interval of 14 d. At 4, 8 and 24 h and at 2, 3, 7, 10 and 14 d after irradiation, [C-14]thymidine upt ake into the tumor was determined with a liquid scintillation counter and t he intratumoral distribution of [C-14]thymidine was visualized and evaluate d semiquantitatively by autoradiography using a phosphor imager. Results: I n both groups, tumor volume decreased until day 7 after irradiation; afterw ard, regrowth occurred in only the group that had received 6 Gy. A decrease in thymidine uptake was found as early as 8 h after irradiation. On day 3 after irradiation, thymidine uptake increased again in the 6-Gy group, befo re the increase in tumor volume, but remained unchanged in the 20-Gy group. Also on day 3, multiple foci of thymidine uptake suggesting proliferation preceding tumor recurrence were seen on autoradiographs from the 6-Gy group but not from the 20-Gy group. Histological findings correlated with the re sults of autoradiography Conclusion: The results show that radiolabeled thy midine is a sensitive tracer for assessment of early tumor response and rec urrence after irradiation. The rapid decrease in uptake, however, does not allow any prediction about tumor recurrence.