68Ga-DOTATATE PET vs. MRI-Based Treatment Planning for Post-Operative and Intact Meningioma: A Dosimetric Analysis

Abstract

<h3>Purpose/Objective(s)</h3> The current standard for radiation treatment planning involves MRI-based image guidance to define intact disease and residual tumor. Because nearly all meningiomas express somatostatin receptor 1/2 (SSTR1/SSTR2), SSTR ligands such as ⁶⁸Ga-DOTATATE are being explored for meningioma radiotherapy treatment planning. Incorporation of ⁶⁸Ga-DOTATATE PET can assist with radiation target volume delineation and reduce the dose to organs-at-risk (OARs). We hypothesize that ⁶⁸Ga-DOTATATE PET-based treatment plans will reduce dose to OARs when compared to MRI-based plans. <h3>Materials/Methods</h3> For each patient, 7 blinded physician contours were retrospectively performed on MRI and PET. A representative MRI and PET physician contour was chosen. All treatment plans were rendered on computed tomography (CT) planning datasets, using VMAT and 6MV photon energy. Each arrangement consisted of 3 independent partial arcs. Two arcs were positioned on the coplanar axis with alternated collimator settings of 0 and 90 degrees to promote MLC sparing with a third unique vertex arc chosen independently to spare normal brain tissue and associated organs at risk (OAR). For MRI structures, a 1 cm expansion from the gross tumor volume (GTV) was used to create a clinical treatment volume (CTV) 60. A 2 cm expansion was used to create a CTV 54. For PET structures, a 1 cm expansion from the GTV was used to create a CTV 60. All CTVs were cropped from outside the brain and a 3 mm isotropic expansion was used to create planning treatment volumes (PTVs). Each plan was prescribed to 95% coverage of prescription dose to the high risk PTV, and dosimetric maximum points to 0.03cc (Dmax) were evaluated for brainstem, optic chiasm, and both optic nerves. Plans were optimized such that Dmax to brainstem remained limited to 60 Gy or less and Dmax to optic structures was limited to 54 Gy or less. <h3>Results</h3> 18 meningioma patients were included (9 post-operative, 9 intact). The mean PTV volume on MRI was 258 ccs (306 ccs for post-operative, 210 ccs for intact), and the mean PTV volume on PET was 60 ccs (97 ccs for post-operative, 91 ccs for intact). The mean radiation dose was reduced to critical structures (Table 1). The mean brain dose was reduced by an average of 42% in PET-based plans. <h3>Conclusion</h3> ⁶⁸Ga-DOTATATE PET has potential to reduce treatment volume and dose to OARs for patients diagnosed with a meningioma. Our study shows that incorporation of ⁶⁸Ga-DOTATATE PET is especially useful in both the post-operative and intact setting and may reduce treatment toxicity. PET guided radiation treatments are worthy of future prospective investigation.