3360 - Proton Pencil Beam Scanning Facilitates Safe Treatment of Extended Radiation Targets for Hodgkin Lymphoma: A Report from the Proton Collaborative Group Registry

Purpose/Objective(s): In 2018, the International Lymphoma Radiation Oncology Group guideline highlighted 3 patient populations that may benefit most from proton beam therapy (PBT): 1) those with lower mediastinal disease; 2) young females; and 3) those who have been heavily pretreated. The role of PBT in lymphoma patients that require RT to both axillary and mediastinal disease, defined here as extended field RT (EFRT), is less clear. Using the prospective Proton Collaborative Group registry, we evaluated acute toxicities with EFRT and dosimetric differences between delivered PBT plans and plans replanned with VMAT.

Materials/

Methods: Eligible patients included HL patients treated with PBT to both mediastinum and axilla, in which DICOM-RT was available. For comparison plans, VMAT butterfly technique was used, using a total of 5-8 arcs with anterior beam arcs (300°?30°) at both couch 0 deg and 90 deg with a posterior arc from 150°?210° at couch at 0 deg. Photon dose coverages were matched to the delivered proton plan metrics or to a minimum of CTV/ITV D99>98%, PTV D98>95%, whichever was lower. VMAT dose was prioritized to decrease dose to the lungs, followed by heart and breast in this order of importance.

Results: Among 363 HL patients in the registry, 12 received proton EFRT between 2016-2021. Patients were young (median age 26 years, range 17-54). Most (n = 9) were male, had stage 2 disease (n = 8), and were treated at initial diagnosis (n = 9). Seven (58%) were treated to unilateral axilla and 5 (42%) to both axillae. Eight (67%) patients also received radiation to the neck. The median dose was 30.6 Gy(RBE) (range 21-36). Pencil beam scanning was used in all patients. PBT was associated with greater target coverage, better conformity, and lower heterogeneity, while achieving lower dose to organs at risk (Table 1). No acute grade 3+ toxicities (CTCAEv5) were reported. One patient developed pneumonitis 11 months after PBT that was attributed to immunotherapy.

Conclusion: Proton EFRT can be safely delivered and is associated with improved sparing of the heart, lungs, and breast tissue compared to modern photon techniques. In particular, lung doses with photons may be prohibitively high to safely deliver in some cases. Patients requiring both mediastinal and axillary treatment may be another subgroup of lymphoma patients who may benefit from PBT.

Abstract 3360 - Table 1: Dosimetry comparison between photon and proton plans

All values are median (range)

^*Treated volume enclosed by 100% dose / Vol in PTV receiving 100% dose