Proton therapy is an advanced modality of external radiation therapy that allows to keep low the radiation burden on the healthy tissues both in front and beyond the target. The aim of this study was to assess, for primary brain tumors, the potential role of PET/CT to better define proton treatment planning and the impact of PET/CT fused to morphological MRI in dose escalation guided for tumor control probability.
According to a joint decision with proton radiation oncologist, two kinds of patients were enrolled for treatment: 1. affected by primary GBM after surgery; 2. affected by low grade gliomas after partial resection for mass reduction. All patients were submitted to morphological MRI with contrast enhancement and to two PET/CT studies (DOPA and FDG). We decided to employ both DOPA and FDG to identify biological heterogeneity inside the tumor to potentially deliver modulated and inhomogeneous radiation fields, following the concept of dose painting. Moreover, to better define both the volume and the molecular biologic information we applied fusion imaging and this complete data were provided to radiation oncologist that compared them to the standard ones, as established just by morphological MRI. Therefore, two dose paintings by contours (DPBC), for dose escalation, were assessed.
FDG and DOPA PET/CT studies provided different and complementary data about the behavior of primary brain tumors, respect to anatomical imaging. According to previous published experiences on DOPA and FDG, these tracers represent, for brain tumors, respectively, cellular density and higher-grade disease. Moreover, FDG is able to give further information about the function of normal brain tissue surrounding the lesion. Fusion imaging (MRI and PET/CT with both tracers) was the best modality, respect to contouring obtained just by MRI, in the target delineation for proton therapy and changed the standard contouring determining boost sub volumes in the overall planned area of treatment.
To our knowledge, this is the first experience of fusion imaging with three modalities (MRI and PET/CT both with DOPA and FDG) for radiation treatment planning in proton therapy. Up to now we do not have enough data about the impact of this technique as an improvement in radiation outcome, but we can argue that a personal dose painting both for high and low grade tumors may be more efficacy for survival, respect to standard planning, based only to anatomical imaging. We may expect that the overall information (anatomy, cellular density, grading and normal brain activity) provided by the introduction of multimodal imaging in planning assessment will be an useful approach not only for a better result in cancer control, but also for a better quality of life of patients, respect to previous modalities of radiotreatment planning.

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