Methyl-11C-L-methionine positron emission tomography for radiotherapy planning for recurrent malignant glioma

Abstract

Abstract Objective To investigate differences in uptake regions between methyl-11C-L-methionine positron emission tomography (11C-MET PET) and gadolinium (Gd)-enhanced MRI, and to examine the effects on dose distribution of changing the threshold value for definition of tumor boundaries.Methods Twenty consecutive patients with grade 3 or 4 glioma who had recurrence after postoperative RT between April 2016 and October 2017 were examined in the study. This study is simulation study assuming all patients received RT. Clinical target volume (CTV) was contoured using the Gd-enhanced region (CTV-Gd), T2-high region (CTV-T2), tumor/normal tissue (T/N) ratios of 11C-MET PET of 1.3 and 2.0 (CTV (T/N 1.3), CTV (T/N 2.0)), and the PET-edge method (CTV(P-E)) for stereotactic radiotherapy planning for recurrent malignant glioma after radiotherapy. Differences among CTVs were evaluated and the dose distribution for the normal brain was simulated for each CTV.Results The Jaccard index (JI) for concordance of CTV (Gd) with CTVs using 11C-MET PET was highest for CTV (T/N 2.0), with a value of 0.7. In a comparison of pixel values of MRI and PET, the correlation coefficient for cases with higher JI was significantly greater than that for lower JI cases (0.37 vs. 0.20, P = 0.007). D50% and D33% of the brain in radiotherapy planning using each CTV differed significantly (P = 0.03, P = 0.02) and those using CTV (T/N 1.3) were higher than with use of CTV (Gd). V90% and V95% for each CTV differed in a simulation study for actual treatment using CTV (Gd) (P = 1.0×10− 7, P = 3.0×10− 9) and those using CTV (T/N 1.3) and CTV (P-E) were lower than with CTV (Gd). Comparing with CTV (T2), the JI showed no significant difference with any CTV using PET. D50% and D33% of the brain were significantly higher than any CTV using PET.Conclusions The region of 11C-MET accumulation is not necessarily similar to the Gd-enhanced or T2 high region. A change of the tumor boundary using 11C-MET PET can cause significant changes in doses to the brain and the CTV.