Clinical Outcomes of Minibeam Radiotherapy for Superficial Skin and Soft Tissue Malignancies.

Abstract

Minibeam radiotherapy (MBRT) is a spatially fractionated technique delivering alternating submillimeter high-dose peak and low-dose valley regions using orthovoltage x-rays. Preclinical studies demonstrate normal tissue sparing and antitumor activity, but clinical data are limited. Following a prior report of commissioning and treatment in two patients, we report the first clinical series evaluating clinical outcomes with MBRT for superficial malignancies. Patients with superficial cutaneous and soft tissue malignancies were treated with MBRT at a single institution using a clinical orthovoltage unit and tungsten collimators producing 0.5-mm beams spaced 1.1 mm center-to-center. Cumulative incidence functions were used to estimate local progression. Toxicity was graded by CTCAE v5.0. Thirty-six patients with 46 lesions were treated between December 2023 and August 2025; 85% received MBRT alone. Median follow-up was 15.1 months. Median peak and valley doses per fraction were 25.9 Gy and 3.4 Gy; median number of fractions was 2. Patients were heavily pretreated, with 90% receiving ≥2 prior systemic therapies and 43% prior radiation. Symptomatic improvement occurred in 86%. Among 40 evaluable lesions, complete response occurred in 22.5%, partial response in 25%, and stable disease in 45%. Twelve-month local progression was 11% overall and 8% for MBRT alone. Documented abscopal responses occurred in 11% without systemic therapy changes. Grade 1-2 dermatitis occurred in 61%; two grade 3 adverse events occurred in patients with baseline tissue compromise. No grade 4-5 toxicities were observed. MBRT is well tolerated and yielded encouraging symptomatic benefits and local control in treatment-refractory superficial malignancies.