Treatment and Planning

The technique, dosimetric details, and planning considerations for treatment of oropharynx cancer with IMPT at the authors’ institution have previously been described in detail [8, 27–30]. Briefly, all patients were simulated supine with a non-contrast computed tomography (CT), immobilized using a full length thermoplastic mask, bite block with or without an oral stent and a posterior customized head, neck and shoulder mold. Gross tumor plus margin was typically treated with a total prescribed dose of 66–70 Gy while elective volumes received 54 to 63 Gy, assuming a relative biological effectiveness (RBE) value of 1.1. Dose to the pharyngeal constrictors was minimized by creating an artificial avoidance structure and pushing dose as low as reasonably achievable while maintaining acceptable tumor coverage. The majority of treatments (81.4%) included the bilateral neck. Planning was performed with Eclipse proton therapy treatment planning system (version 8.9, Varian Medical Systems, Palo Alto, California). A 3 beam-arrangement was typically used (a left and right anterior oblique and single posterior beam), with multifield optimization. Daily kilovoltage image guidance was used for all patients, with verification CT simulation scans performed during treatment weeks 1 and 4.