Comparing Proton Beam to Intensity Modulated Radiation Therapy Planning in Esophageal Cancer

Abstract

Purpose: To compare passive-scatter proton beam therapy (PBT) versus intensity-modulated radiation therapy (IMRT) for reducing heart/lung dose in esophageal cancer, and to identify anatomy and treatment planning parameters that can lead to suboptimal proton plans. Patients and Methods: Passive-scatter PBT versus IMRT mean doses and coverage to the lung/heart were evaluated for 55 patients with esophageal cancer from 2007 to 2010. Geometric relationships between the tumor (distance from planning target volume to carina, and percentage of uninvolved heart) and doses to the lung/heart, respectively, were used to predict expected doses to these normal tissues. Cross-comparisons of heart versus lung dose and proton versus photon mean lung/heart dose were used to identify suboptimal proton dosimetry. Results: Dose–volumetric analysis showed that, for the lung and heart, although protons resulted in smaller volumes receiving lower doses (5–30 Gy), photons achieved smaller volumes receiving higher doses (40–50 Gy). Both distance from planning target volume to carina and percentage of uninvolved heart were linearly associated with IMRT and PBT mean lung/heart doses and can, therefore, be used to estimate expected mean heart/lung doses. Compared with IMRT, initial PBT plans had lower mean lung doses in 100% of cases but only lower mean heart doses in 92% of cases. Reasons for initial suboptimal protons plans with unexpectedly high mean heart doses included poor beam arrangements (ie, AP/PA) and weighting and/or unique patient anatomy. Reoptimization of these initially suboptimal proton plans using a left lateral/PA beam with 1:2 weighting resulted in significantly improved heart doses in most cases, except with patients who had unique anatomies. Nevertheless, after reoptimization, PBT was consistently superior to IMRT in lowering mean lung/heart doses in 100% of the cases, despite suboptimal anatomy. Conclusion: Compared with IMRT, passive-scatter PBT using a left lateral/PA beam approach with 1:2 weighting can further reduce mean lung/heart dose in esophageal cancer.