Long Term Results of Proton Therapy in Adult Infradiaphragmatic Lymphoma

Background and purpose The role of proton radiation (PT) for infradiaphragmatic targets in lymphoma patients is unknown. We report on radiation planning details, doses achieved to organs at risk (OARs), and outcomes for this population. Patient/material and methods This is a single-institution retrospective study of patients with biopsy-proven lymphoma who received PT to infradiaphragmatic targets between 2011 and 2022. Comparison photon plans were generated for all patients. Toxicity was reported using CTCAE version 5.0. Dosimetric and clinical factors associated with toxicity and oncologic outcomes were assessed via Cox regression, while the paired t -test or Wilcoxon signed rank test was used for dosimetric analyses. Results 38 patients comprising 40 PT courses were included. Median follow-up was 48 months. Top diagnoses were DLBCL (58%) and Hodgkin lymphoma (16%). 28% of PT courses had direct overlap with prior radiation and 20% were palliative. Median dose was 30.6 Gy over 17 fractions to locations including the spine/paraspinal region (30%), retroperitoneum (28%), and pelvis (22%). There were no acute G3+ toxicities. Among patients treated with curative intent, 44% experienced progression of disease (POD), mostly distant. 5/38 patients had a second malignancy after PT. Among the photon comparison plans, PT significantly decreased doses to the kidneys (mean and V5), small/large bowel (V5, V15), and numerous other abdominopelvic organ metrics. Large bowel V30 Gy was higher with PT (12.4 vs. 7.6 cc, p = p = .007). Interpretation PT is well-tolerated and leads to excellent outcomes. PT can significantly reduce doses to key abdominopelvic OARs compared to photons.