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

To investigate differences in uptake regions between methyl-¹¹C-L-methionine positron emission tomography (¹¹C-MET PET) and gadolinium (Gd)-enhanced magnetic resonance imaging (MRI), and their impact on dose distribution, including changing of the threshold for tumor boundaries.

Twenty consecutive patients with grade 3 or 4 glioma who had recurrence after postoperative radiotherapy (RT) between April 2016 and October 2017 were examined. The study was performed using simulation with the assumption that all patients received RT. The clinical target volume (CTV) was contoured using the Gd-enhanced region (CTV(Gd)), the 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 RT planning. Differences among CTVs were evaluated. The brain dose at each CTV and the dose at each CTV defined by ¹¹C-MET PET using MRI as the reference were evaluated.

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% of the brain in RT planning using each CTV differed significantly (P = 0.03) and that 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 and 3.0 × 10^-9, respectively) and those using CTV (T/N 1.3) and CTV (P-E) were lower than with CTV (Gd).

The region of ¹¹C-MET accumulation is not necessarily consistent with and larger than the Gd-enhanced region. A change of the tumor boundary using ¹¹C-MET PET can cause significant changes in doses to the brain and the CTV.