A carbon ion minibeam treatment planning method with scissor beams

Minibeam radiation therapy (MBRT) employs a highly modulated spatial dose distribution characterized by the peak-to-valley dose ratio (PVDR). Carbon minibeam radiation therapy (cMBRT) offers higher PVDR and relative biological effectiveness (RBE) compared to proton MBRT. However, achieving uniform target dose (UTD) coverage while maintaining a high PVDR in organs at risk (OAR) remains challenging.

To address this challenge and optimize the balance between PVDR in OAR and target dose homogeneity, we introduce the scissor-beam (SB) approach for cMBRT.

The SB method introduces scissor beam (SB) splitting, where each original beam is divided into a primary beam and a complementary beam. The primary beam maintains the same angle as the original beam, while the complementary beam is rotated by a small degree. This rotation angle is determined based on the center-to-center distance and the relative positions of the OAR and the target. Monte Carlo simulation using GATE/GEANT4 were performed for dose calculations. The effectiveness of SB was evaluated in comparison to conventional cMBRT method (MB) and crossfire (CF) method in terms of target dose uniformity, OAR sparing, and PVDR in OAR across three clinical cases: lung, pancreas, and head-and-neck (HN) cancers.

Compared to MB (2 mm center to center distance(d_ctc)), SB increased OAR PVDR by 150% and matched the PVDR of MB (4 mm d_ctc) with ≤5% difference across lung, pancreas, and head and neck (HN) cases. SB improved target conformity (CI) by 118%-167% over MB (4 mm d_ctc), reducing lung D_mean by 12%-27%, liver D_mean by 18%, and brainstem/spinal cord D_max by 42%-54%. Relative to CF, SB maintained similar PVDR (≤4% difference) while enhancing OAR sparing: 33% lower left lung D_mean, 71% reduced kidney D_mean, and complete spinal cord sparing (pancreas) and HN cases saw 37% lower brainstem D_max with SB. These results highlight the effectiveness of the SB method in achieving better target dose uniformity and OAR sparing while maintaining comparable PVDR values.

We have proposed a novel SB method for cMBRT. SB provides a better balance between UTD and PVDR in OAR compared to MB. Additionally, SB demonstrates superior OAR protection compared to CF. This innovative approach holds significant potential to enhance the therapeutic ratio of cMBRT, offering improved treatment outcomes and reduced risks for patients.