Thallium-201 SPECT in the evaluation of early effects on brain tumors treated with stereotactic irradiation.

Purpose: To determine whether thallium-201 SPECT can predict response to stereotactic irradiation (STI) earlier than magnetic resonance imaging (MRI), the change in tumor size measured by MRI was compared with the change in tumor activity measured by T1-201 SPECT before and after STI. Materials and Methods: Twenty-one tumors in 16 patients with intracranial tumors were treated by STI. T1-201 SPECT was performed within 1 week before the beginning of STI and within 1 week after the end of STI in all patients. All patients underwent MRI within 1 week before the beginning of STI, and 14 patients (19 tumors) underwent MRI within 1 week after the end of STI. Follow-up MRI was performed 1 to 2 months after the end of STI in 14 patients (16 tumors). The activity of T1-201 in the tumor divided by that of the uninvolved symmetric area was defined as the Tl-index. The change in tumor size immediately and I to 2 months after STI was compared with the change in T1-index immediately after STI. Results: No significant relationship between the ratio of tumor size immediately after STI and the ratio of T1-index immediately after STI was found. A significant correlation (r = 0.69, P <0.05) between the ratio of tumor size 1 to 2 months after STI and the ratio of Tl-index immediately after STI was found. Conclusions: This study suggests that T1-201 SPECT immediately after STI can predict treatment response I to 2 months after STI, and that T1-201 KPECT can he an early indicator of treatment response.