Dosimetric verification for intensity-modulated radiotherapy of thoracic cancers using experimental and Monte Carlo approaches

Purpose: To investigate the dosimetric accuracy of commercial treatment planning systems used in intensity-modulated radiotherapy (IMRT) for thoracic cancer. Methods and Materials: Clinical IMRT plans for lung and esophageal cancers and mesothelioma were used to investigate the accuracy of dose calculations from two commercial treatment planning systems (Pinnacle and Corvus systems). Dose distributions were measured with ion chambers and thermoluminescent dosimeters for individual IMRT fields and composite treatment plans in water phantoms and anthropomorphic phantoms. A Monte Carlo-based system was established to compute three-dimensional dose distributions to compare with the treatment planning system calculations. Results: Dose calculations from the Pinnacle system were acceptable within 5% of the local dose or a 5-mm distance-to-agreement for 80% of the points measured with ion chambers, 74% of the points measured with thermoluminescent dosimeters, and 96% of the points compared with the Monte Carlo calculations. For the Corvus system, 89% of the points agreed with the measured dose and 98% agreed with the Monte Carlo calculations. Underestimation of the dose from the treatment planning system was found in the low-dose regions (<50% of the prescribed dose), possibly caused by inadequate modeling of the multileaf collimators. Conclusion: The Pinnacle and Corvus dose calculations were acceptable for thoracic IMRT in high-dose regions. Beam modeling is likely the most critical factor for the accuracy of IMRT dose calculations.