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Angular Dependence of the Sun Nuclear MapCHECK 2 With New RoHS Compliant Diode Detectors

B Morris

B Morris*, J Lu , N Thorne , W Duggar , R He , A Markovich , C Yang , University of Mississippi Medical Center, Jackson, MS


SU-K-205-2 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 205

Purpose: To evaluate and quantify the impact of increased angular dependence caused by the newly modified MapCHECK2 diodes to be lead-free (RoHS-compliant) on patient specific QA.

Methods: Coupled with a MapPHAN, a number of fields were compared using one new MapCHECK2 with diodes modified and one MapCHECK2 loaner provided by Sun Nuclear with the original diode design. The full set of fields delivered and compared included AP, PA and full arc reference fields (open, 10x10 cm2) along with patient treatment fields for both 6 and 10 MV. Dose and array calibrations were created for each unit prior to beam delivery one after another on the same day. All planned doses were generated in Pinnacle v9.10. Planned and measured doses were compared with Sun Nuclear’s Patient software using 2%/2mm gamma analysis.

Results: The range of passing results for the new MapCHECK2 for all fields was 100.0 – 46.1%. The range of passing results for the unit with the original diode design was 99.8 – 82.5%. Measurement from the PA beam indicated a systematic offset compared to the calculation.

Conclusion: Overall, the original diode design shows better agreement than the new diode design and demonstrates less angular dependence. In these devices, the diode arrays are attached to two different boards stacked on top of each other. One board is oriented face up, while the other board faces down. The array calibration normalizes the angular dependency from the AP direction. But as a result, the inherent dependency is doubled for the PA direction. For PA fields, the original diode design exhibits ~2% difference between diodes that face up and those that face down, while the new diode design displays differences of ~7.5%. The magnified angular dependency does not greatly affect beams delivered from an AP orientation, but arc-based treatment field analysis is compromised.

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