(S013) Dosimetric Analysis of Treating Malignant Pleural Mesothelioma Using Volumetric Modulated ARC Therapy

Abstract

In the treatment of malignant pleural mesothelioma (MPM), there is evidence to support the use of pleurectomy/decortication (P/D) as the primary treatment. This presents a unique challenge to the radiation oncologist to treat the pleura while limiting the amount of radiation given to the underlying lung. Treatment with Intensity-Modulated Pleural Radiation Therapy (IMPRINT) has been shown to be efficacious and safe (Rimner et al. JCO 2016). In this study we examined our institution’s experience in treating MPM with this technique. Forty patients who received P/D, then were treated with Volumetric Modulated Arc Therapy (VMAT) from 2010-2016. Dosimetric parameters from treatment plans were correlated to the incidence of radiation pneumonitis (RP). Statistical analysis was performed using MATLAB V8.1. Significant correlation was seen between the incidence of ≥ grade 3 RP and the absolute volume of the ipsilateral lung spared of 20 Gy (p = 0.013) and 30 Gy (p = 0.041). There was also a significant correlation between the percentage of ipsilateral lung spared of 20 Gy (p = 0.006) and 30 Gy (p = 0.011). The absolute volume and percentage volume of the ipsilateral lung to be spared from ≥ 20 Gy or 30 Gy to reduce the incidence of ≥ grade 3 RP was at least 55 cc or 200 cc and 11% or 29% respectively (p<0.05). Heart dose did not correlate with the incidence of ≥ grade 3 or RP. This is the largest cohort reported of patients treated with P/D and IMPRINT using VMAT. Our findings show, in contrast to other reports, that a significant factor in reducing the incidence of radiation pneumonitis is the reduction of dose to the underlying lung, with approximately 200 cc of unaffected lung needing to receive less than 30 Gy.