Author: Stephen M Corner, Daiki Hara, Jiasen Ma, Robert W Mutter, Mark D. Pepin, Nicholas B. Remmes, Hok Seum Wan Chan Tseung 👨🔬
Affiliation: Mayo Clinic - Rochester 🌍
Purpose: Proton minibeam radiotherapy (pMBRT) is a form of spatially fractionated radiation therapy leveraging submillimeter beams to improve the therapeutic ratio. We report on collimator design, commissioning, treatment planning, and workflow for single-field pMBRT irradiation with spatially fractionated target dose.
Methods: A set of 4cm-thick pMBRT collimators were constructed with 0.5mm-wide slits and center-to-center distances (CTC) of 2.1, 3.6, 6.9 mm. The divergent slit design uses tilted brass plates held by machined aluminum sides. The nozzle attachment device supports stackable aperture/collimator combinations for treating targets up to ~8cm wide. Collimator and aperture geometries were simulated with two independent Monte Carlo (MC) simulations: TOPAS and GPU-based software. These were commissioned using a Bragg Peak Chamber (BPC) and radiochromic film measurements on square SOBP fields with ~10cm range. Treatment plans were first generated in Eclipse without collimators, then GPU MC-calculated at high resolution with devices inserted. A breast case is given as example. QA consists of: (1) secondary dose calculations (with/without respiratory motion), (2) pre-treatment delivery to phantom, (3) pre-treatment collimator alignment checks.
Results: Brass plate thicknesses in the MC were tuned (within machining tolerance) to match the BPC measurements, both superficially and in the homogeneous dose region. Simulated profiles at different depths were then validated with calibrated film measurements, showing valley doses within 5%. Both MCs were in excellent agreement. With the 3.6mm CTC collimator, for a 4cm-deep, ~4cm-diameter breast target, peak-to-valley dose ratios (PVDR) of 15 and 4 are achieved at 1cm and 4cm depths, respectively. With 6.9mm CTC, PVDR significantly improves under respiratory motion, at the cost of reduced number of peaks within the target. Treatment delivery times are ~15-20 mins for targets up to 8cm wide, with valley doses of ~2Gy.
Conclusion: Equipment, planning techniques and workflows were established for pMBRT, a challenging and novel proton therapy modality.