A novel patient-specific solution for Merkel cell carcinoma in high-dose-rate brachytherapy

Superficial lesions in anatomically complex areas pose challenges for conventional high-dose-rate (HDR) brachytherapy (BT) techniques. We developed a novel 3D-printed applicator tailored for hand skin cancer, and evaluated its clinical feasibility and advantages.

A patient with Merkel cell carcinoma of the right hand, involving the palm, dorsum, and interdigital spaces, was referred for HDR-BT. After initial CT simulation, planning target volume (PTV) was delineated and applicator structure was contoured, to comply with and encompass the PTV. Catheter channels were designed, and the applicator was fabricated as palmar and dorsal components to facilitate setup, using a stereolithography 3D printer with tissue-equivalent, bio-compatible clear resin. The patient underwent a second CT simulation with the applicator and catheters in place, and treatment planning was performed. Pre-treatment dry run confirmed source travel and catheter lengths, and treatment was delivered using a Flexitron HDR-BT afterloader.

Twenty-seven catheters achieved PTV coverage of 98.6% at D_90% and 96.7% at D_95%, with a D_0.03cc of 135.6%. Air gaps were less than 5 mm. Treatment was completed in 51 minutes with a 5 Ci iridium-192 (¹⁹²Ir) source (15 minutes for setup and 36 minutes for delivery). At one-year follow-up, no recurrence or late toxicity was observed.

The patient-specific 3D-printed applicator was successfully applied for skin lesions unsuitable for conventional techniques. Reduced air gaps improved dose conformity, and the semi-rigid, translucent resin enhanced setup reproducibility. The workflow substantially optimized staff resources, offering HDR-BT solution for complex superficial lesions in clinics without in-house printing capabilities.