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Probabilistic Approach to Plan Evaluation in Linac-Based Stereotactic Radiosurgery: A Patient Specific Uncertainty Analysis

P Park

P Park*, J Roper , T Fox , E Schreibmann , E Elder , M Tejani , W Curran , I Crocker , A Dhabaan , Radiation Oncology and Winship Cancer Institute, Emory University, Atlanta, GA


SU-E-T-513 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Acoustic neuroma is treated using highly conformal, framed-based radiosurgery. The gross target volume (GTV) is located near the radiosensitive brainstem. Currently, plan evaluation methods that consider GTV dose conformity and dose to organs at risk (OAR) do not account for setup uncertainty, thereby potentially giving a false sense of target coverage and OAR sparing. More realistic plan evaluations should consider delivery uncertainties. In this work, we introduce a new plan evaluation method based on probability and investigate the effect of translational and rotational setup errors.

Methods:Clinically approved 18 acoustic neuroma treatment plans were studied retrospectively. For each treatment plan, 200 new dose distributions were re-calculated while introducing both translational (in all cardinal directions) and rotational errors (in all 3 orthogonal rotation angles). The magnitude of errors for each simulation was randomly generated from the Gaussian distribution with one standard deviation of 0.5 mm and 0.5 degree respectively. From the simulated dose distributions, an approximated probability density function of dose objective for both GTV and brainstem were calculated. The probability density function was then used to compute the probability of delivering prescribed dose to 90% of GTV for each patient. For brainstem, the probability of limiting maximum dose to 11Gy was calculated. These probabilistic metrics were then analyzed with respect to GTV volume, conformity index, and min and max dose to the GTV and brainstem.

Results:Across all cases, the average probability of delivering prescribed dose to the 90% of GTV volume was 85±14%. Four patients showed > 50% risk of exceeding 11Gy limit to the brainstem.

Conclusion:In highly conformal, single fraction radiosurgery, a small setup error can be critical. We found that the probabilistic assessment of plan objectives can be used to quantitatively assess true target coverage better than the conventional plan assessment metrics.

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