Implications of IMT-SPECT for postoperative radiotherapy planning in patients with gliomas

Using MRI, residual tumor cannot be differentiated from nonspecific postoperative changes in patients with brain gliomas after surgical resection. The goal of this study was to analyze the value of 123I-alpha-methyl-tyrosine-single photon emission CT (IMT-SPECT) in radiotherapy planning of patients with brain gliomas after surgical resection.

In 66 patients with surgically resected brain gliomas (33 glioblastomas, 20 anaplastic astrocytomas, 7 anaplastic oligodendrogliomas, and 6 low-grade astrocytomas), IMT-SPECT and MRI were performed for radiotherapy planning. On the MRI/IMT-SPECT fusion images, the volume with IMT uptake was compared with the volume of the hyperintensity areas of T(2)-weighted MRI and with the volume of contrast enhancement on T(1)-weighted MRI. The regions with IMT uptake and/or MRI changes (composite Vol-MRI/IMT), regions with overlay of IMT uptake and MRI changes (common Vol-MRI/IMT), area with IMT uptake without MRI changes (increase Vol-MRI/IMT), and area with only MRI changes (Vol-MRI minus IMT) were analyzed separately. The planning target volume and boost volume defined using MRI information alone was compared with the planning target volume and boost volume defined by also using the SPECT information.

Focally increased IMT uptake was observed in 25 (38%) of 66 patients, contrast enhancement on MRI was outlined in 59 (89%) of 66 patients, and hyperintensity areas on T(2)-weighted MRI were found in all 66 investigated patients. The mean composite Vol-T(2)/IMT was 73 cm(3). The relative increase Vol-T(2)/IMT, mean relative common Vol-T(2)/IMT, and mean relative Vol-T(2) minus IMT was 4%, 6%, and 90% of the composite Vol-T(2)/IMT, respectively. The mean composite Vol-T(1)/IMT was 14 cm(3) and the mean relative increase Vol-T(1)/IMT, mean relative common Vol-T(1)/IMT, and mean relative Vol-T(1) minus IMT was 21%, 4%, and 64% of the mean composite Vol-T(1)/IMT, respectively. In 19 (29%) of 66 patients, the focal IMT uptake was located outside the MRI changes. In this subgroup, the mean residual volume defined by focal IMT uptake in MRI/IMT-SPECT images, mean Vol-T(1), and mean Vol-T(2) was 19 cm(3), 10 cm(3), and 70 cm(3), respectively. The mean relative increase T(2)/IMT was 14% and T(1)/IMT was 61%. In this subgroup, the additional information of SPECT led to an increase in boost volume (mean relative increase BV-IMT) by 20%.

In patients with surgically resected brain gliomas, the size and location of residual IMT uptake differs considerably from the abnormalities found on postoperative MRI. Because of the known high specificity of IMT uptake for tumor tissue, the findings on IMT-SPECT may significantly modify the target volumes for radiotherapy planning. This will help to focus the high irradiation dose on the tumor area and to spare normal brain tissue.