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Gamma Knife Perfexion Output Factor Measurements Using PRESAGE 3D Optical Dosimeters

S Klawikowski

S Klawikowski1*, J Yang1, J Adamovics2, G Ibbott1, (1) UT MD Anderson Cancer Center, Unit 94, Houston, TX, (2) Heuris Pharma LLC, Skillman, NJ,

SU-E-T-125 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To measure the 4mm and 8mm collimator output factors for a Gamma Knife Perfexion radiotherapy machine (GK) using PRESAGEâ„¢ 3D dosimetry.

Methods:A polystyrene sleeve and keying mechanism were machined to hold 60mm diameter x 70mm tall PRESAGE cylindrical dosimeters within an anthropomorphic head phantom. We mounted a GK treatment head frame onto the phantom for GK planning and irradiation. We irradiated two PRESAGE dosimeters with single 4mm and 16mm shots spaced apart by 40mm in order to measure the 4mm output factor relative to that of 16mm. Similarly we irradiated one PRESAGE dosimeter with an 8mm and 16mm shot in order to calculate the relative output for the 8mm collimator. The PRESAGE dose images were scanned on the Radiological Physics Center DMOS optical CT scanner (Duke University, Durham, NC). We found and computed the center-of-mass (optical density) for each of the near-spherical dose shots. We then measured the mean optical density over spheres of varying radii centered on the individual shot centers. This effectively allowed us to simulate measuring the dose response over a spectrum of different detector volumes and therefore extrapolate the distribution back to a theoretical zero-volume detector to eliminate volume averaging effects.

Results:We measured the 4mm output factor to be: 0.827 [0.805-0.849] (95% confidence limits) and 0.815 [0.769-0.861] in separate experiments. We measured the 8mm collimator output factor to be: 0.906 [0.895-0.917]. Our measurements fell within 1.5%, 0.12%, and 0.70% of the manufacturer accepted Monte Carlo derived results.

Conclusion:This work indicates potential for PRESAGE dosimeters to accurately measure the notoriously difficult to measure 4mm GK output factor. 3D dosimetry offers a solution to both the volume averaging problems of ion chamber measurements and negates the requirement for precise mechanical alignment of the detector to the GK ISO center.

Funding Support, Disclosures, and Conflict of Interest: This work is partially funded by NIH Grant: 5R01CA100835

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