Advancing the Role of Palliative Proton Therapy Through Diagnostic Scan-Based Planning

Abstract

Single-fraction palliative spine radiation is an attractive option for patients with poor prognosis because of its convenience and effectiveness in terms of pain relief. However, the associated large doses to normal tissues can lead to toxicities that negatively impact quality of life. Proton therapy offers a dosimetric solution to this problem but has not historically been used for palliative treatment because of inherent cost and time needed for planning. The purpose of this study was to assess a process for delivering cost-effective and efficient palliative proton therapy by using diagnostic scan-based planning (DSBP) and prefabricated treatment delivery devices.We designed and characterized a reusable proton aperture system capable of adjusting to multiple field sizes for spine treatment. Next, we retrospectively identified 10 patients treated with proton therapy for thoracic malignancies in 2019 who also had a diagnostic CT of the thorax within 4 months of simulation. For each case, one physician contoured T6-T9 as the target volume on both the diagnostic and simulation scans. Using the reusable proton aperture system, we generated passive-scatter proton plans on the diagnostic scans using a single PA beam with no custom range compensator to treat T6-T9 to 8 Gy x 1. The plans were then transferred to the simulation scans to compare target coverage and doses to normal tissues, followed by robustness analysis with 5mm X, Y, and Z shifts. Finally, we generated AP/PA photon plans on the simulation scans to compare doses to normal tissues between proton and photon plans. Results were compared using the Wilcoxon signed-rank test.Median D95% on the diagnostic scan plans was 101% (range, 100%-102%) of the prescription dose using the same 12L x 7W aperture for each case. Median Dmax was 107% (range, 105-108%). When transferred to simulation scans, coverage remained excellent and hot spots acceptable for all cases with a median D95% of 101% (range, 100%-102%) and a median Dmax of 106% (range, 105%-106%). Heart and esophagus mean and max doses did not vary significantly between the diagnostic scan and the simulation scan proton plans (P > 0.2 for all). Robustness analysis with 5mm X, Y, and Z shifts showed that coverage remained acceptable with D95% > 98% for all cases. Compared to the proton plans, mean heart dose was markedly higher for the AP/PA photon plans (median 4.7 Gy v. 0.4 Gy, P < 0.01), as was mean esophagus dose (median 7.1 Gy v. 4.9 Gy, P < 0.01). Heart and esophagus max doses were not significantly different between the proton and photon plans (P > 0.05).Palliative proton DSBP is technically feasible and robust with superior sparing of normal tissues compared to AP/PA photon plans. By eliminating the need for simulation and custom devices, the cost-benefit ratio for palliative proton therapy may become more acceptable, particularly in a bundled payment environment.