Dosimetric impact of tumor position displacements between photon and proton stereotactic body radiation therapy for lung cancer

Purpose: To investigate the impact of tumor position displacements (TPDs) on tumor dose coverage in photon and proton stereotactic body radiation therapy (SBRT) treatments for lung cancer patients. Methods: From our institutional database of 2877 fractions from 770 lung cancer patients treated with photon SBRT in 2017-2021, 163 fractions from 88 patients with recorded iso-center shifts of >1.5 cm in any direction under kV-cone-beam CT guidance were identified. By double registrations with bony and tumor alignments, the difference between the iso-center shifts of these two alignments was categorized as TPDs. One fraction from each of 15 patients who had TPD magnitudes >3 mm were selected for this study. For each patient, one proton plan using intensity modulated proton therapy (IMPT) with robust optimization was generated retrospectively. All photon plans had V-100%RX>99% of GTVs and V-100%RX>98% of ITVs. Proton plans were evaluated with two worse-case scenario (voxelwise worst and worst scenario) using 5mm and 3.5% uncertainty to achieve the same planning goals as the corresponding photon plans. These two evaluation proton plans were named proton-1st and proton-2nd plans. The dosimetric effect of TPD was simulated by shifting tumor contours with the corresponding shift on patient specific planning CT and by recalculating the dose of the original plan. Results: The range of magnitude of TPDs was 3.58-28.71 mm. In photon plans, TPDs did not impact tumor dose coverage, still achieving V-100%RX of the GTV >= 99% and V-100%RX of the ITV >= 98%. In proton plans for patients with TPDs>10 mm, inadequate target dose coverage was observed. More specifically, 8 fractions of proton-1st plans and 4 fractions of proton-2nd had V-100% RX of the GTV<99% and V-100%RX of the ITV<98%. Conclusions: Adequate tumor dose coverage was achieved in photon SBRT for magnitude of TPDs up to 20 mm. TPDs had greater impact in proton SBRT and adaptive planning was needed when the magnitude of TPDs>10 mm to provide adequate tumor dose coverage.