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Significant Differences in Small-Field Output Factors Measured with Various Dosimeters

H Soysal*, C Yang, University of Mississippi Medical Center, Pearl, MS


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To demonstrate the discrepancies in small field output factors using various dosimeters and to obtain correction factors for the Razor Nano Chamber for small field dosimetry.

Methods: IBA CC01 and Nano ion chambers and a stereotactic diode were irradiated with a 6 MV photon beam from an Elekta Versa HD LINAC in a large water tank at SSD 100 cm with the effective point of measurement at 10 cm depth for various small field sizes (4x4, 3x3, 2x2, 1x1, 0.5x0.5) to obtain output factors. Gafchromic film (EBT3) was irradiated under the same conditions as above, but with solid water phantom. Field output correction factors from the IAEA TRS-483 report were applied to the CC01 and diode. The film measurements were used as the ground truth values to which other dosimeters were compared. The corrected output factors from the multiple dosimeters were then used to obtain a loose small field output correction factor for the Nano chamber for use in our own institution.

Results: Our results showed that for the two smallest field sizes, the difference of the corrected output factors between all dosimeters was significant, differing as much as about 5% for the 1x1. However, for the larger field sizes up to 4x4, the corrected output factors were in much closer agreement. The loose Nano chamber field output correction factors were only reliable for these larger small field sizes.

Conclusion: This work shows significant discrepancy in small field output factors determined by different dosimeters based on the correction factors from the IAEA TRS-483 report. We strongly recommend using multiple dosimeters in determination of small field output factors used for commissioning a treatment planning system.


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