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How Does the Choice of Prescription IDL Affect Dose Gradient, Target Coverage, Treatment Time and Ease of Planning for GK Radiosurgery

P Johnson

P Johnson*, M Monterroso, F Yang, University of Miami School of Medicine, Miami, FL


SU-I-GPD-T-609 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To explore how the choice of prescription isodose line affects dose gradient, target coverage, treatment time, and ease of planning for radiosurgery using the Gamma Knife Perfexion.

Methods: Data was extracted manually from the TPS using dose profiles. The distance between relative isodose lines was recorded for a variety of scenarios. The first scenario involved systematically increasing the prescription IDL while maintaining a single shot with constant collimator setting. The second scenario involved systematically changing the weights between co-located shots of varying collimator sizes while maintaining constant prescription IDL. The third scenario involved systematically separating two co-located shots with constant collimator settings and prescription IDL.

Results: The gradient for the 4, 8, and 16mm collimator settings was steepest when prescribed to the 60–70%, 50–60%, and 70–80% isodose lines, respectively. Prescribing to a higher IDL decreases the size of the prescription isodose volume. In order to cover larger targets, a lower IDL must be used or a plan must be created using a combination of different collimator sizes. The data show that there are regions of overlap in target size where either planning strategy will suffice and provide equivalent dose fall-off, typically for lesions between 2–4cc or greater than 20cc. For targets less than 1.5cc a higher IDL is necessary in order to achieve high selectivity. For cases with multiple shots, the 50% IDL provides for easier planning due to the smooth expansion of the prescription IDL as the distance between shots is increased.

Conclusion: Conventional wisdom assumes the 50% IDL provides the steepest dose gradient for GK radiosurgery. The results of this study show this is not always the case and there may be opportunities to increase the prescription IDL based on target size. Such changes could significantly impact treatment time which becomes an increasing concern as sources decay.

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