Plan Quality and Sensitivity Analysis of Conformal Arc Informed Volumetric Modulated Arc Therapy (CAVMAT)

Purpose: Linac based radiosurgery to multiple metastases is commonly planned with VMAT as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT may struggle to block between targets, which may lead to highly modulated and/or nonconformal MLC trajectories. This phenomenon is particularly apparent in multi-target geometries as targets often share the same MLC leaf pair, creating an MLC opening and yielding insufficient inter-target collimation. Given the complex geometries, multi-target SRS necessitates high degrees of dosimetric accuracy. Dosimetric accuracy may be impacted by beam commissioning as a single dosimetric leaf gap (DLG) must be selected and used for all targets, geometries, and MLCs. Conformal Arc Informed VMAT (CAVMAT) aims to reduce healthy tissue dose by utilizing simplified MLC trajectories and by producing more conformal dose distributions. It is hypothesized that the simplified leaf motion and reduced complexity of CAVMAT may reduce sensitivity to commissioning and treatment delivery uncertainty.

Materials & Methods: CAVMAT is a hybrid treatment planning technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has three main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were re-planned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5 Gy, 6 Gy, 12 Gy, and 16 Gy, conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. Of the 20 VMAT plans, 10 were selected and replanned with CAVMAT, at DLG values of 0.4 mm, 0.8 mm, and 1.2 mm and the change in V6Gy [cc], V12Gy [cc], V16Gy [cc], and target dose was quantified. The 10 VMAT and CAVMAT plans were delivered to a Delta4 QA phantom and dose agreement was quantified using gamma index with 3%/1mm, 2%/1mm, and 1%/1mm criteria. Trajectory log files were collected and analyzed to quantify MLC positioning errors during delivery. 16 targets were selected, with at least one target from each plan, and were delivered to an SRS Mapcheck QA phantom to evaluate dose difference per DLG.

Results: After replanning the 20 VMAT cases, CAVMAT reduced the average V2.5Gy[cc] by 25.25±19.23%, V6Gy[cc] by 13.68±18.97%, V12Gy[cc] by 11.40±19.44%, and V16Gy[cc] by 6.38±19.11%. CAVMAT improved conformity by 3.81±7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35±24.66%, leading

to an increased treatment time of ~2 minutes. For the DLG analysis, the 10 VMAT plans

demonstrated an average sensitivity to variation of V6Gy [cc], V12Gy [cc], V16Gy [cc] of 35.83 ± 9.48%/mm,

34.12 ± 6.61%/mm, and 39.22 ± 8.41%/mm, respectively, compared to 23.18 ± 4.53 %/mm, 22.45± 4.28 %/mm, and 24.88 ± 4.91 %/mm for CAVMAT. VMAT was found to be roughly twice as sensitive as CAVMAT to changes in target doses for a varying DLG. For the plans delivered to the Delta4 CAVMAT demonstrated an improved dose agreement, with the strictest criteria of 1%/1mm resulting in a passing rate of 94.53 ± 4.42% for VMAT compared to 99.28 ± 1.74% for CAVMAT. Log file analysis demonstrated CAVMAT’s improved resistance to treatment delivery uncertainties, though the difference compared to VMAT is not expected to be substantial.

Conclusions: The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. CAVMAT is more robust to dosimetric and treatment delivery uncertainties and better maintains target dose and healthy tissue sparing. The reduced complexity, inherent tissue sparing, improved conformity, and reduced sensitivity to uncertainties indicates CAVMAT to be a promising method to treat brain metastases.