Strategies for motion robust proton therapy with pencil beam scanning of esophageal cancer.

Abstract Introduction Proton therapy of esophageal cancer is superior to photon radiotherapy in terms of normal tissue sparing. However, respiratory motion and anatomical changes may compromise target dose coverage due to density changes, geometric misses and interplay effects. Here, we investigate the combined impact on CTV coverage and compare to IMRT. Material and methods This study includes twenty-six esophageal cancer patients previously treated with IMRT planned on a 4D-CT(pCT). For each patient, seven proton pencil beam scanning (PBS) plans were created with different field configurations and optimization strategies. The impact of respiration was investigated by calculating the phase doses, 4D dose and 4D dynamic dose (including interplay effects). The impact of anatomical changes was investigated by re-calculating all plans on all phases of a 4D-CT surveillance scan(sCT). Results The most robust PBS plans were achieved by use of two posterior beams and requiring coverage of PTV and simultaneously using robust optimization (RO) of CTV (2PAPTVRO) resulting in only one patient showing V95%CTV below 97% in one or more phases of the pCT. For the least robust PBS plans obtained by use of lateral+posterior beams and only CTV-RO, but not requiring PTV coverage (2LPRO), ten patients showed underdosage. For IMRT, two patients showed underdosage. Interplay effects reduced V95%CTV significantly when delivering only one fraction, but the effects generally averaged out after ten fractions. The impact of interplay was significantly larger for RO-only plans compared to plans optimized with RO combined with PTV coverage. Combining the impact of anatomical changes and respiration on the sCT resulted in V95%CTV below 97% for three (2PAPTVRO), 16 (2LPRO), and eight (IMRT) patients. Conclusion PBS using posterior beam angles was more robust towards anatomical changes and respiration than IMRT. The impact of respiration was enhanced when anatomical changes were present. Single fraction interplay effects deteriorated the dose distribution, but were averaged out after ten fractions.