Motion-robust intensity-modulated proton therapy for distal esophageal cancer

Purpose: To develop methods for evaluation and mitigation of dosimetric impact due to respiratory and diaphragmatic motion during free breathing in treatment of distal esophageal cancers using intensity-modulated proton therapy (IMPT). Methods: This was a retrospective study on 11 patients with distal esophageal cancer. For each patient, four-dimensional computed tomography (4D CT) data were acquired, and a nominal dose was calculated on the average phase of the 4D CT. The changes of water equivalent thickness (Delta WET) to cover the treatment volume from the peak of inspiration to the valley of expiration were calculated for a full range of beam angle rotation. Two IMPT plans were calculated: one at beam angles corresponding to small Delta WET and one at beam angles corresponding to large Delta WET. Four patients were selected for the calculation of 4D-robustness-optimized IMPT plans due to large motion-induced dose errors generated in conventional IMPT. To quantitatively evaluate motion-induced dose deviation, the authors calculated the lowest dose received by 95% (D95) of the internal clinical target volume for the nominal dose, the D95 calculated on the maximum inhale and exhale phases of 4D CT (D-CT0 and D-CT50), the 4D composite dose, and the 4D dynamic dose for a single fraction. Results: The dose deviation increased with the average Delta WET of the implemented beams, Delta WETave. When Delta WETave was less than 5 mm, the dose error was less than 1 cobalt gray equivalent based on D-CT0 and D-CT50. The dose deviation determined on the basis of D-CT0 and D-CT50 was proportionally larger than that determined on the basis of the 4D composite dose. The 4D-robustness-optimized IMPT plans notably reduced the overall dose deviation of multiple fractions and the dose deviation caused by the interplay effect in a single fraction. Conclusions: In IMPT for distal esophageal cancer, Delta WET analysis can be used to select the beam angles that are least affected by respiratory and diaphragmatic motion. To further reduce dose deviation, the 4D-robustness optimization can be implemented for IMPT planning. Calculation of D-CT0 and D-CT50 is a conservative method to estimate the motion-induced dose errors. (C) 2016 American Association of Physicists in Medicine.