Purpose: To evaluate the feasibility of [(11)C]-methionine positron emission tomography (MET PET) in radiotherapy (RT) treatment planning and long-term follow-up in patients with low-grade glioma.
Patients: Thirteen patients with low-grade astrocytoma and 1 with anaplastic astrocytoma underwent sequential MET PET and magnetic resonance imaging (MRI) before and 3, 6, 12, and 21-39 months after RT, respectively. Ten patients were studied after initial debulking surgery or biopsy and 4 in the recurrence phase.
Methods: A total of 58 PET scans were performed. After transmission scanning, a median dose of 425 MBq of MET was injected intravenously and emission data was acquired 20 min after injection for 20 min. The uptake of MET in tumor area was measured as standardized uptake value (SUV) and tumor-to-contralateral brain SUV ratios were generated to assess irradiation effects on tumor metabolism. Functional imaging with PET was compared with concurrent MRI in designing the RT planning volumes and in assessment of response to RT during a median follow-up time of 33 months.
Results: In 12 patients (86%), tumor area was clearly discernible in the baseline PET study. In the remaining 2 patients with a suspected residual tumor in MRI, PET showed only a diffuse uptake of MET interpreted as negative in the original tumor area. In the dose planning of RT, MET PET was helpful in outlining the gross tumor volume in 3 of 11 cases (27%), whereas PET findings either coincided with MRI (46%) or were less distinctive (27%) in other cases. In quantitative evaluation, patients with a low tumor SUV initially had significantly better prognosis than those with a high SUV. Tumor-to-contralateral brain uptake ratios of MET discriminated well patients remaining clinically stable from those who have since relapsed or died of disease.
Conclusion: Quantitative MET PET has prognostic value at the time of initial treatment planning of low-grade glioma. Some patients may benefit of RT volume definition with MET PET, which seems to disclose residual tumor better than MRI in selected cases. Stable or decreasing uptake of MET in tumor area after RT during follow-up seems to be a favorable sign.