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Evaluation of Three Dosimeters On Output Factor Measurement of the New Aktina SRS Cone System

L Zhuang1*, M Gao2, M Pankuch2, M Xu3, G Pixton1, C Breit1, (1) Northwestern Lake Forest Hospital, Lake Forest, IL, (2) Northwestern Medicine Chicago Proton Center, Warrenville, IL, (3) Northwestern Medicine Delnor hospital, Geneva, IL

Presentations

PO-GePV-T-340 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To evaluate the performance of three dosimeters on output factor(OF) measurement of the newest Aktina SRS Cone system attached to Elekta VersaHD LINAC using 6FFF beam.

Methods: The Aktina SRS cone system is equipped with cone sizes of 5mm, 7mm, 9mm, 11mm, 13mm and 15mm. OF for each cone defined as the cone output at 10 cm depth divided by the output of open field of size 10cm at 90cm SSD was measured. A daisy chain correction factor was applied to calculate the final OF value using field size of 3cm. Dosimeters evaluated in this study include EBT3 Gafchromic film, EDGE detector, and Exradin A16 Micro chamber. EDGE & A16 measurements were performed in water tank, while EBT3 film was conducted in solid water. EDGE & A16 OF were also corrected based on published small field output correction factors for comparison purpose. In addition, IROC SRS OF phantom was utilized to independently verify the OF for cone size of 15mm.

Results: The OFs measured with EBT3 were 0.502, 0.609, 0.664, 0.720, 0.761 and 0.783 respectively. OFs measured by EDGE were 0.550, 0.645, 0.705, 0.746, 0.773 and 0.795 respectively which were consistently higher (mean diff= 2.9%) than EBT3 measurements. A16 measurements were consistently lower than EBT3 measurements (mean diff= -2.4%) which were 0.455, 0.574, 0.651, 0.703, 0.743 and 0.770 respectively. EBT3 OFs, corrected EDGE OFs and corrected A16 OFs on average were within ±1% of each other. IROC results showed a satisfactory independent check of OF for cone size of 15mm (TLD/Inst =0.96 at 1.5cm & TLD/Inst= 0.97 at 7.5cm).

Conclusion: Overall, EDGE over-responded to small field measurement and A16 Chamber under-responded to small field measurement due to volume averaging. OFs measured by EBT3 were found to be in good agreement with corrected EDGE and A16 OFs.

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