Compensator‐based intensity‐modulated radiation therapy for malignant pleural mesothelioma post extrapleural pneumonectomy

The present work investigated the potential of compensator-based intensity-modulated radiation therapy (CB-IMRT) as an alternative to multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) to treat malignant pleural mesothelioma (MPM) post extrapleural pneumonectomy. Treatment plans for 4 right-sided and 1 left-sided MPM post-surgery cases were generated using a commercial treatment planning system, XIO/CMS (Computerized Medical Systems, St. Louis, MO). We used a 7-gantry-angle arrangement with 6 MV beams to generate these plans. The maximum required field size was 30 x 40 cm. We evaluated IMRT plans with brass compensators (.Decimal, Sanford, FL) by examining isodose distributions, dose-volume histograms, metrics to quantify conformal plan quality, and homogeneity. Quality assurance was performed for one of the compensator plans. Conformal dose distributions were achieved with CB-IMRT for all 5 cases, the average planning target volume (PTV) coverage being 95.1% of the PTV volume receiving the full prescription dose. The average lung V20 (volume of lung receiving 20 Gy) was 1.8%, the mean lung dose was 6.7 Gy, and the average contralateral kidney V15 was 0.6%. The average liver dose V30 was 34.0% for the right-sided cases and 10% for the left-sided case. The average monitor units (MUs) per fraction were 980 MUs for the 45-Gy prescriptions (mean: 50 Gy) and 1083 MUs for the 50-Gy prescriptions (mean: 54 Gy). Post surgery, CB-IMRT for MPM is a feasible IMRT technique for treatment with a single isocenter. Compensator plans achieved dose objectives and were safely delivered on a Siemens Oncor machine (Siemens Medical Solutions, Malvern, PA). These plans showed acceptably conformal dose distributions as confirmed by multiple measurement techniques. Not all linear accelerators can deliver large-field MLC-based IMRT, but most can deliver a maximum conformal field of 40 x 40 cm. It is possible and reasonable to deliver IMRT with compensators for fields this size with most conventional linear accelerators.