Feasibility and Patient Tolerance of Proton Based Stereotactic Body Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> Proton-based stereotactic body radiotherapy (SBRT) may provide a dosimetric benefit vs photon SBRT in certain cases, seeking to reduce toxicity risks or facilitate reirradiation, though data are limited. We assessed the feasibility and patient tolerance of proton SBRT for palliation and ablation. <h3>Materials/Methods</h3> From Sep. 2019 to Dec. 2020, 58 patients were simulated for proton SBRT to 68 lesions. Acute toxicities were prospectively recorded using CTCAE grading without attribution, and patient-reported outcomes were assessed prior to and at completion of treatment using the MD Anderson Symptom Inventory (MDASI) and EQ-5D5L visual analogue score (VAS). <h3>Results</h3> 52 of 58 patients (90%) received SBRT treatment. Treatment was not initiated in 6 patients due insurance denial (n = 1), change in treatment plan (n = 1), interval disease progression (n = 1), transition to hospice (n = 2), or death (n = 1). Fractionation varied by indication and disease site with a median of 5 fractions and a median fractional dose of 8 Gy. Proton range uncertainty (RU) was ± 3.5% in soft tissue and ± 5% in lung. For bone or immobile lesions, a target equivalent to a standard photon SBRT PTV was defined and optimized with RU. For mobile targets, 4D CT was used to define an ITV for robust optimization, typically with 5 mm positional uncertainty + RU, using a PTV Evaluation for assessment of comparable SBRT plan metrics. Among 27 patients with 4D CT assessment of target motion, 20 (74%) were treated with beam-gated breath hold, 4 (15%) with abdominal compression, and 3 (11%) free breathing. Cone beam CT (CBCT) was used for image guidance at each fraction. Average conformity index was 1.06 and average number of treatment fields per target was 4 (range of 2-8). The median treatment time per fraction was 41 minutes (range 18-171 mins). There was a significant difference in treatment time for patients requiring breath hold (67 vs 47 mins, <i>P</i> = 0.006). Five patients (10%) required adaptive replanning due to set up uncertainties (n = 3) or tumor growth (n = 2). 11 patients (21%) had concurrent systemic therapy. 21 patients (40%) had no observable acute toxicity associated with treatment. Maximum recorded toxicities, related or unrelated to SBRT treatment, were grade 1 in 25 patients (48%), grade 2 (fatigue, depression, anorexia, proctitis) in 5 patients (10%), and grade 3 (dysphagia and hoarseness) in 1 patient (2%). Comparing pretreatment to end of treatment timepoints, there was a significant improvement in the mean VAS (68 to 76, <i>P</i> = 0.027, n = 33), with no significant change in the mean MDASI symptom (1.8, 1.8) or interference (2.2, 2.3) scores. <h3>Conclusion</h3> Proton-based SBRT was feasible and well-tolerated with no decrement in patient reported outcomes and a mean 8 point improvement in VAS at the conclusion of SBRT. Further follow-up is necessary for tumor control and late effects analysis.