SU-E-T-318: The Effect of Patient Positioning Errors On Target Coverage and Cochlear Dose in Stereotactic Radiosurgery Treatment of Acoustic Neuromas

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Abstract

Purpose:

Setup errors on the order of millimeters may cause under-dosing of targets and significant changes in dose to critical structures especially when planning with tight margins in stereotactic radiosurgery. This study evaluates the effects of these types of patient positioning uncertainties on planning target volume (PTV) coverage and cochlear dose for stereotactic treatments of acoustic neuromas.

Methods:

Twelve acoustic neuroma patient treatment plans were retrospectively evaluated in Brainlab iPlan RT Dose 4.1.3. All treatment beams were shaped by HDMLC from a Varian TX machine. Seven patients had planning margins of 2mm, five had 1–1.5mm. Six treatment plans were created for each patient simulating a 1mm setup error in six possible directions: anterior-posterior, lateral, and superiorinferior. The arcs and HDMLC shapes were kept the same for each plan. Change in PTV coverage and mean dose to the cochlea was evaluated for each plan.

Results:

The average change in PTV coverage for the 72 simulated plans was −1.7% (range: −5 to +1.1%). The largest average change in coverage was observed for shifts in the patient's superior direction (−2.9%). The change in mean cochlear dose was highly dependent upon the direction of the shift. Shifts in the anterior and superior direction resulted in an average increase in dose of 13.5 and 3.8%, respectively, while shifts in the posterior and inferior direction resulted in an average decrease in dose of 17.9 and 10.2%. The average change in dose to the cochlea was 13.9% (range: 1.4 to 48.6%). No difference was observed based on the size of the planning margin.

Conclusion:

This study indicates that if the positioning uncertainty is kept within 1mm the setup errors may not result in significant under-dosing of the acoustic neuroma target volumes. However, the change in mean cochlear dose is highly dependent upon the direction of the shift.

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