Outcomes of proton therapy to infradiaphragmatic sites in pediatric patients with Hodgkin lymphoma

Background: Proton therapy (PT) has potential advantages in pediatric Hodgkin lymphoma (pHL). However, there are limited data on PT, specifically to infradiaphragmatic targets. We report on PT planning details, doses achieved to organs at risk (OARs), and clinical and toxicity outcomes for patients with pHL who received PT to infradiaphragmatic regions. Methods: This is a retrospective study including patients treated between 2011 and 2022. Demographic and clinical factors were collected, and toxicity was reported using Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. Dosimetric and clinical factors associated with key outcomes were assessed via Cox regression. Photon plans were generated for all patients, and the paired t-tests or Wilcoxon signed rank sum tests were used for dosimetric comparisons. Results: Twenty-one patients comprising 22 PT courses were included. Median followup was 5.0 years, and mean age was 14.2 years. Median dose was 21 Gray equivalent (GyE) over 14 fractions. Top acute grade 1 (G1) toxicities included fatigue (59%) and anorexia (36%). Rates of acute G2 and G3+ toxicity were 18% and 0%, respectively. After PT, no local or marginal failures occurred. Five percent experienced disease progression, who were all successfully salvaged, and all patients were alive and disease-free at last follow-up. No secondary malignancies developed. Compared to photon radiotherapy, PT achieved significantly lower doses to the bowels, stomach, spleen, pancreatic tail, liver, kidneys, and pelvic bones. Conclusions: PT is well-tolerated and leads to excellent oncologic and toxicity outcomes with long-term follow-up. PT confers dosimetric advantages when compared to photons.