Proton Beam Therapy for Testicular Seminoma: Clinical Outcomes and Toxicity From the Proton Collaborative Group Prospective Registry.

INTRODUCTION

Testicular seminoma predominantly affects young men, and while photon-based radiotherapy achieves excellent disease control, concerns about late effects, including secondary malignancies, have driven interest in proton beam therapy (PBT) as a way to improve dose conformality and spare normal tissue. Prospective clinical data for PBT in seminoma, however, remain scarce.

METHODS

We reviewed 19 consecutive patients with histologically confirmed testicular germ cell tumors treated with PBT between 2010 and 2024 from the Proton Collaborative Group (PCG) multi-institutional prospective registry. Primary endpoints included overall survival (OS), progression-free survival (PFS), local control, and toxicity graded per Common Terminology Criteria for Adverse Events (CTCAE) version 5.0.

RESULTS

All 19 patients (19/19, 100%) completed their prescribed radiation course without interruption (median dose 32.8 GyRBE; median 17 fractions). Among 10 patients (10/19, 53%) with evaluable follow-up (median 23.9 months), two-year OS and PFS were both 100% in non-metastatic patients (n=9), with 100% local control (9/9) across the evaluable cohort. One death (1/2 metastatic patients) occurred in a patient with metastatic disease at presentation. Acute toxicities were predominantly grade 1-2 (fatigue, nausea, radiation dermatitis), and all resolved without treatment interruption. No treatment-related grade ≥3 adverse events (0/19) or secondary malignancies were observed. Late toxicities were observed in two of 19 patients (2/19, 11%), with only one late event (grade 1 urinary urgency) attributable to radiation in the non-metastatic cohort. No secondary malignancies were observed during 28.8 person-years of follow-up, comparing favorably to predicted excess rates of two to eight per 100 person-years reported for photon-based approaches.

CONCLUSIONS

These results support PBT as a feasible and well-tolerated option for testicular seminoma. A longer follow-up is needed to assess secondary malignancy risk reduction.