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Radiolabeled Thymidine: A Sensitive Tracer for Early Tumor Response and Recurrence After Irradiation

Department of Nuclear Medicine, Technische Universität München, München, Germany

Correspondence: For correspondence or reprints contact: Reingard Senekowitsch-Schmidtke, MD, PhD, Nuklearmedizinische Klinik und Poliklinik, der Technischen Universität München, Ismaninger Str. 22, 81675 München, Germany.

ABSTRACT

This study evaluated the sensitivity of a radiolabeled thymidine tracer for assessment of early tumor response and recurrence after irradiation. Methods: SW707 human colon carcinoma implanted into nude mice was irradiated with 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, [¹⁴C]thymidine uptake into the tumor was determined with a liquid scintillation counter and the intratumoral distribution of [¹⁴C]thymidine was visualized and evaluated semiquantitatively by autoradiography using a phosphorimager. Results: In both groups, tumor volume decreased until day 7 after irradiation; afterward, 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 increasead gain in the 6-Gygroup, before 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 results of autoradiography. Conclusion: The results show that radiolabeled thymidine is a sensitive tracer for assessment of early tumor response and recurrence after irradiation. The rapid decrease in uptake, however, does not allow any prediction about tumor recurrence.

Key Words: radiolabeled thymidine • tracer for tumor response • tracer for tumor recurrence • cell proliferation