Impact of field number and monitor units per segment on magnetic resonance-guided hypofractionated stereotactic radiotherapy for brain metastases: plan quality, deliverability, and robustness trade-offs

Background: Magnetic resonance-guided linear accelerators (MR-Linacs) have recently been introduced for radiotherapy of brain metastases (BMs), including hypofractionated stereotactic radiotherapy (HSRT). However, optimal strategies for planning HSRT within MR-guided adaptive workflows remain to be established. This study aimed to evaluate the influence of field number and minimum monitor unit per segment (MU/segment) on plan quality, and robustness of dose calculations in MR-guided HSRT for patients with solitary medium-sized BMs.

Methods: This retrospective study included 20 patients who underwent HSRT for solitary medium-sized BMs, receiving a prescription dose of 30 Gy in three fractions. Four intensity-modulated radiotherapy (IMRT) plans were systematically generated for each patient by varying the number of fields (9 vs. 15) and minimum MU/segment (15 vs. 5): 9FL-IMRT (9 fields, 15 MU/segment), 9FS-IMRT (9 fields, 5 MU/segment), 15FL-IMRT (15 fields, 15 MU/segment), and 15FS-IMRT (15 fields, 5 MU/segment). Plan quality, treatment efficiency, and delivery accuracy were assessed based on dose distributions optimized using structure-based bulk electron density assignment, with patient-specific quality assurance (QA) evaluated via global gamma passing rate (GPR) using 3%/2 mm, 2%/2 mm, and 2%/1 mm criteria. To assess robustness against density-related spatial uncertainties, each plan was reoptimized after simulating skull misalignment with random translational offsets within ±2 mm. These reoptimized plans were recalculated on original computed tomography (CT) datasets using voxel-based electron density as the reference standard. Robustness was quantified by comparing dose distributions between bulk density-based and voxel-based recalculations through GPR analysis using 3%/2 mm, 2%/2 mm, 2%/1 mm, and 1%/1 mm criteria.

Results: All four IMRT configurations provided clinically acceptable plans with similar target coverage and no significant differences in conformity, gradient, or homogeneity indices (all P>0.05). Although some dose parameters for normal brain tissue reached statistical significance (overall P<0.001), absolute differences were minor (≤1 cm³) and clinically irrelevant. The 9FL-IMRT configuration yielded the lowest MU (2,037±174) and shortest beam-on time (BOT; 6.69±0.92 min), while the 15FL-IMRT configuration required only a minimal increase in BOT (7.59±1.18 min) and maintained fewer segments (33±7). Delivery accuracy was consistently high across all techniques, with mean GPR values ≥95% at 3%/2 mm, showing no significant inter-technique differences (P=0.67). However, under conditions of simulated skull displacement, notable robustness differences emerged under stricter criteria, with 15FL-IMRT consistently showing the highest GPR at 2%/1 mm (96.02%±2.93%, overall P=0.004) and 1%/1 mm (91.87%±3.55%, overall P<0.001), significantly outperforming 9FL-IMRT and 9FS-IMRT, indicating enhanced tolerance to density-related spatial uncertainties.

Conclusions: The 15FL-IMRT configuration provided an optimal balance among plan quality, deliverability, and robustness of dose calculation, supporting its adoption as the preferred planning approach for MR-guided HSRT in patients with solitary medium-sized BMs.