Purpose To determine the dosimetric effects of rotational errors on target coverage using volumetric modulated arc therapy (VMAT) for multi-target stereotactic radiosurgery (SRS). Methods and Materials This retrospective study includes 50 SRS cases, each with 2 intracranial planning target volumes (PTVs). Both PTVs were planned for simultaneous treatment to 21 Gy using a single-isocenter, non-coplanar VMAT SRS technique. Rotational errors of 0.5°, 1.0°, and 2.0° were simulated about all axes. The dose to 95% of the PTV (D95) and the volume covered by 95% of the prescribed dose (V95) were evaluated using multivariate analysis to determine how PTV coverage is related to PTV volume, PTV separation, and rotational error. Results At 0.5° rotational error, D95 values and V95 coverage rates were ≥ 95% in all cases. For rotational errors of 1.0°, 7% of targets had D95 and V95 values below 95%. Coverage worsened substantially when the rotational error increased to 2.0°: D95 and V95 values were > 95% for only 63% of the targets. Multivariate analysis showed that PTV volume and distance to isocenter were strong predictors of target coverage. Conclusions The effects of rotational errors on target coverage were studied across a broad range of SRS cases. In general, the risk of compromised coverage increases with decreasing target volume, increasing rotational error and increasing distance between targets. Multivariate regression models from this study may be used to quantify the dosimetric effects of rotational errors on target coverage given patient-specific input parameters of PTV volume and distance to isocenter.

Purpose: Rotational IMRT has been adopted by many clinics for its promise to deliver treatments in a shorter amount of time than other conventional IMRT techniques. In this paper, the authors investigate whether RapidArc is more susceptible to delivery uncertainties than dynamic IMRT using fixed fields. Methods: Dosimetric effects of delivery uncertainties in dose rate, gantry angle, and MLC leaf positions were evaluated by incorporating these uncertainties into RapidArc and sliding window IMRT (SW IMRT) treatment plans for five head-and-neck and five prostate cases. Dose distributions and dose-volume histograms of original and modified plans were recalculated and compared using Gamma analysis and dose indices of planned treatment volumes (PTV) and organs at risk (OAR). Results of Gamma analyses using passing criteria ranging from 1%-1 mm up to 5%-3 mm were reported. Results: Systematic shifts in MLC leaf bank positions of SW-IMRT cases resulted in 2-4 times higher average percent differences than RapidArc cases. Uniformly distributed random variations of 2 mm for active MLC leaves had a negligible effect on all dose distributions. Sliding window cases were much more sensitive to systematic shifts in gantry angle. Dose rate variations during RapidArc must be much larger than typical machine tolerances to affect dose distributions significantly; dynamic IMRT is inherently not susceptible to such variations. Conclusions: RapidArc deliveries were found to be more tolerant to variations in gantry position and MLC leaf position than SW IMRT. This may be attributed to the fact that the average segmental field size or MLC leaf opening is much larger for RapidArc. Clinically acceptable treatments may be delivered successfully using RapidArc despite large fluctuations in dose rate and gantry position.

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Purpose: To characterize the risks of compromised coverage in single‐isocenter multiple‐lesion VMAT SRS. Methods: Fifty patients were selected retrospectively from our SRS program. Each patient had two lesions treated to ≥ 21 Gy. Single‐isocenter VMAT SRS plans were created in Eclipse. PTV volumes and distances from isocenter were recorded. PTV coverage (D95 and V95) was evaluated across rotational setup errors of 0.5, 1.0, or 2° applied to three axes. Coverage rates were analyzed versus volume, distance, and rotation. For a rotational error of 2°, lesion size and separation distance were compared across coverage rate levels using ANOVA. A multivariate logistic regression model was fit using generalized estimating equations (GEE), modeling the probability of a 95% V95/D95 rate or higher given lesion size and separation distance while accounting for intra‐patient correlation. The estimated probabilities from the GEE model were used to capture the operating conditions in a receiver operating characteristic (ROC) curve; area under the curve (AUC) was estimated. Results: Mean lesion volume and distance to isocenter are 0.96±1.25cc and 3.53±1.61cm. V95/D95 proportions above 95% range from 92‐100% when rotational errors are ≤1°. At 2.0° rotation, V95/D95 are >95% in only 62–64% of cases; V95 falls to 75% for <0.3cc lesions at 4cm yet remains >90% up to 6cm for lesions >0.9cc. V95 is <40% in an extreme case. The logistic regression analysis shows that lesion volume and distance to isocenter are independent predictors (p< 0.001) of V95/D95 rates exceeding 95%. The ROC derived from a GEE multivariate model has an AUC of 0.87. Conclusion: PTV coverage can be compromised substantially by rotational setup errors of 2°, in particular for <0.3cc lesions at distances >4cm from isocenter. Statistical analysis suggests that lesion volume and distance to isocenter could be used to select patients who are good candidates for single‐isocenter multiple‐lesion SRS.