Purpose: SRS treatment planning for single-isocenter multi-target (SIMT) cases involves balancing the competing factors of dose conformity, target coverage and inter-target leaf bridging, especially when a large number of targets are involved. This study tests the hypothesis that adding a VMAT arc helps improve the plan quality and reduce leave bridging.
Methods: Eight SRS cases previously treated with SIMT technique were selected for this study. The number of targets in each case ranged from 10 – 15 and the target volumes ranged from 0.07cc-15.25cc. To further improve plan quality and increase each target’s exposure to the beams, the arc at zero couch angle was duplicated and the plan was re-optimized using the same set of constraints as the clinical plan. Dosimetric parameters and inter-target leave bridging were compared with those of the original clinical plan.
Results: The test plans with an additional arc exhibited equivalent or moderately better dose conformity (mean conformity index = 1.25 vs 1.27) and target coverage (mean V100% = 99.81 vs 99.74). Even with an additional arc, the total MU for most of the test plans was slightly lower than the corresponding clinical plan (mean total MU = 3327 vs 3433). A qualitative assessment of leaf travel showed a reduction in the amount of bridging between targets. Treatment plan optimization for the test plans required one to four iterations to achieve equal or better plan quality, significantly improving treatment planning efficiency. However, the treatment time may increase slightly due to the extra arc field.
Conclusion: Adding an additional arc to clinical SIMT SRS plans has been shown to improve target dose conformity and coverage using comparable total MU. The additional arc helped to reduce inter-target leaf bridging and improve treatment planning efficiency at the cost of slightly increased treatment time.
Treatment Planning, Stereotactic Radiosurgery