Strategies for enhancing delivery efficiency on MR-Linac: A dosimetric study and historical plan review

The Elekta Unity MR-Linac enables online adaptive radiotherapy (oART) with integrated 1.5T MRI. Given its relatively low dose rate, identifying optimal IMRT planning parameters is essential to achieve efficient delivery. This study investigates parameter optimization strategies that enhance delivery efficiency while maintaining high dosimetric quality.

A dosimetric study was designed using ten prostate simultaneous integrated boost (SIB) patients (45 Gy/40 Gy in five fractions) and 10 liver SBRT patients (42-54 Gy in 3-5 fractions). Each patient has three plans: (1) Clinical, (2) Refa (segment-limited, ≤120 segments), and (3) Refb (MU-modulated plan with larger minimum segment width (0.7 cm or 1 cm) and higher fluence map smoothing level (medium or high)). A linear equation for estimating delivery time was established and validated against 1173 retrospective Unity plans based on total segments and Monitor Units (MUs). All the study plans were evaluated by comparing delivery efficiency related metrics (total number of segments, MUs, estimated delivery time), and plan quality using dose-volume histogram (DVH) metrics for targets and organs-at-risk (OARs). Additionally, an online adaptive study was performed on representative cases: the three reference plans were adapted to a daily image, and the resulting adaptive plans were assessed for optimization time and plan quality against clinical criteria.

Using the derived delivery time estimation equation on 173 unseen test cases, the mean absolute error (MAE) between estimated and recorded times was 0.9 min, with 84% of cases with a prediction error within ±1 min, and with 95% of cases falling within ±2 min. Both strategies reduced delivery complexity compared with clinical plans. For prostate cases, Refa and Refb lowered segment counts to <120 (vs. original 137-178) and reduced MUs from 3022 to 2899 (Refa) and 2497 (Refb), respectively. Delivery times decreased on average by 2.6 min with Refa (up to 5.4) and 4.0 min with Refb (up to 6.7). For liver cases, segment counts dropped from (149-198) to <120, with mean MUs reduced from 4233 to 3528 (Refa) and 3309 (Refb). Delivery times were shortened from 24.4 min to 18.9 (Refa) and 18.3 (Refb), up to 7.6 min saving for Refa and up to 8.9 min for Refb. Across both cohorts, PTV coverage and OAR sparing were maintained, with no statistically significant differences (p > 0.05). All adaptive plans from Refa and Refb were completed within 5-13 min, consistent with standard clinical online optimization times. After normalization to the clinical adaptive plan, all plans met the dosimetric goals. A similar time-saving trend was observed across the adaptive plans.

Systematic adjustment of Unity IMRT planning parameters-specifically limiting total segment number, reasonable minimum segment width, and fluence smoothing-can markedly improve delivery efficiency while maintaining clinically acceptable dosimetric quality. These findings provide practical, evidence-based guidelines for parameter selection in Unity planning, supporting reduced treatment times, improved patient throughput, and broader clinical feasibility of MR-guided online adaptive radiotherapy.