Assessment of tumor viability by ²⁰¹Tl SPECT in VX-2 tumor-bearing rabbits after irradiation: Comparison with x-ray CT and pathohistologic findings

In order to evaluate ²⁰¹Tl SPECT for the assessment of tumor viability after radiation therapy. Twenty rabbits were implanted with VX-2 tumor in the left femoral muscles and were classified into four groups as follows: normal control group (N), 20 Gy irradiated response group (A), 20 Gy irradiated non-response group (B), and 40 Gy irradiated group (C). Eight days after implantation, ²⁰¹Tl SPECT and CT studies were performed to assess the changes in ²⁰¹Tl uptake and tumor volume at 0, 1, 2, 3, and 4 weeks after single irradiation. Histologic specimens were also taken. In group N tumors continued to increase in volume, but ²⁰¹Tl uptake showed a steady decrease after 2 weeks. In group A tumors continued to increase in volume, but ²⁰¹Tl uptake showed a transient decrease at 2 weeks, then a steady increase. In group B tumors showed a transient increase in volume at 2 weeks, then a continuous decrease, but ²⁰¹Tl uptake showed a continuous decrease throughout the study. In group C there was a continuous decrease both in volume and ²⁰¹Tl uptake with the later more prominent. ²⁰¹Tl uptake was decreased earlier than tumor volume both in group B and C, at one week that was significant (p < 0.01). ²⁰¹Tl uptake by tumor reflects the burden of viable tumor on histology; therefore ²⁰¹Tl SPECT is of great use in assessing the therapeutic effects on tumors.