3400 - Implementing Proton Beam Therapy for Mediastinal Lymphoma

Materials/

Methods: A retrospective evaluation of mediastinal lymphoma, or other lymphoproliferative disorders, treated with PBT at our institution between December 2022 and March 2024 was performed. Treatment techniques, dosimetry and clinical data were captured from electronic records and patient information planning systems. Descriptive statistics are reported.

Results: In total, 19 patients aged over 18y with lymphoproliferative disorders have been treated with PBT. Mediastinal site was involved in 7 patients: 3 mediastinum only, 4 with neck involvement (2 bilateral, 2 unilateral). Median age was 23y (range 19 – 29y). Diagnoses include Hodgkin Lymphoma (2), Primary mediastinal B cell lymphoma (3), Diffuse large B cell Lymphoma (1), Castleman’s disease (1).

Head & neck shell immobilization was used for disease extending into neck. Arms up position for mediastinal only disease. Additional customized head & neck supports, grip rings and kneefix were used. Planning imaging incorporated 4DCT and MRI.

Dose delivered was a median 36Gy RBE (range 30 - 45Gy RBE), 1.8-2Gy RBE daily fractions. Median Internal Target Volume (ITV) was 336.1cm³ (range 158.9 - 1513.8cm³); median ITV craniocaudal length was 17.8cm (range 15.4 - 30.4cm). A median of 5mm optimization target volume (OTV) (range 5 - 7mm) was used for planning with 3.5-5% range uncertainties. Motion effects were assessed on 4DCT. 2- 3 anterior fields and single field optimization were used in mediastinal only cases. 4 patients required 90-degree couch rotation to maximize breast sparing. Neck targets required additional 1-2 fields and multi-field optimization. Range shifters (median 3cm; range 3 - 5cm) were employed for shallow dose coverage and to help mitigate motion effects.

Median ITV of 97.56% received 95% of the planned dose (range 97.30 – 100.0%). Table 1 shows OAR dosimetry. On treatment replanning was required in 2 patients for poor ITV coverage caused by breathing pattern changes. No RTOG toxicity >2 reported.

Conclusion: Our initial experience has demonstrated successful implementation of PBT for mediastinal lymphomas and allowed development of a class solution for planning. To fully realize the benefits of PBT for these patients, we aim to explore enhanced motion mitigation to further reduce critical OAR doses in the future.

Abstract 3400 – Table 1: OAR dosimetry