(P 355) PROTON RADIOTHERAPY FOR DESMOID TUMORS: A SINGLE-CENTER EXPERIENCE

Objective: Desmoid tumors (or aggressive fibromatosis) are rare benign soft tissue tumors that can present significant treatment challenges due to their high recurrence rates following radical surgery. When tumors are asymptomatic or slow-growing, a 'watch and wait' approach is often chosen. However, active treatment is sometimes necessary for locally aggressive cases. Treatment options include surgical resection, systemic therapy, and radiotherapy. Proton radiotherapy represents a promising option for managing desmoid tumors due to its ability to deliver precise, high-dose radiation to the tumor while minimizing damage to surrounding healthy tissues.

Methods: We evaluated nine patients with desmoid tumors treated with proton radiotherapy at the Proton Therapy Center in Prague, Czech Republic, between 2014 and 2022. The median age of patients at the time of treatment was 32 years. Prior treatment was highly heterogenous including various systemic therapies and surgical resections. Proton radiotherapy was used at the time of disease progression with macroscopic tumor in 8 out of 9 patients. In one case, radiotherapy was administered following a non-radical surgical resection. The prescribed radiation dose for eight patients was 60 CGE, delivered in 2 CGE per fraction. The exception was a patient with a desmoid tumor of the mediastinum and chest wall, who received a dose of 70 CGE. The median planning target volume was 1051 mL, with the largest being 3017 mL.

Results: The median follow-up period after radiotherapy was 42 months. Toxicity, when present, was mainly grade 1 fibrosis and hyperpigmentation. The only severe treatment-related toxicity was observed in a patient with familial adenomatous polyposis-associated abdominal desmoid tumor, who developed tumor necrosis three months post-radiotherapy and required resection of the ileum. Complete response was achieved in 3 patients (33%), partial response in 2 patients (22%) and stable disease in 2 patients (22%). During the follow-up period, disease progression occurred in 2 patients (22%), with a median progression-free survival of 26 months.

Conclusion: Proton radiotherapy is an effective treatment option for desmoid tumors. Based on our experience, this approach offers a favorable toxicity profile and can complement, or in some cases, replace, systemic therapy. Radiotherapy can also be a suitable option for patients where radical surgery would result in high morbidity.