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Lead Foil for Flattening Filter Free Reference Dosimetry: Do We Really Need It?

D Campos1*, (1) Allegheny Health Network, Pittsburgh, PA

Presentations

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

Room: AAPM ePoster Library

Purpose: Proper reference dosimetry is a vital task for both the safety and efficacy of radiotherapy treatments. In 1999, the AAPM provided a water-based reference dosimetry protocol (Almond et al’s TG-51) which simplified the task of reference dosimetry for medical physicists. For sufficiently high energy photons (10 MV and higher), the use of a 1 mm lead foil is recommended to introduce a known electron contamination. The addendum rescinds the recommendation for flattened, high-energy beams, but recommends continued use of the lead foil for flattening-filter-free (FFF) beams due to a lack of data. The purpose of this work is to assess the necessity of using the lead foil for FFF beams versus using the empirical fit equations provided in TG-51 to measure beam quality conversion factor from percent depth dose curves.

Methods: Reference dosimetry was performed on a newly-commissioned Elekta Versa HD equipped with both 6FFF and 10FFF energy photon beams. Percent depth dose curves were obtained using a PTWBeamScan 3D scanning water tank. PDD curves were evaluated with the accompanying Mephysto Analysis module. Beam quality was calculated using both the methods outlined (1 mm lead foil and empirical fit equation). This beam quality was used to calculate the beam quality conversion factor for a PTW 30013 Farmer chamber per TG-51.

Results: The difference in beam quality conversion factor was found to be 0.25% and 0.28% for 6FFF and 10FFF respectively.

Conclusion: These data suggest that because the difference is on the order of one fourth of the annual output tolerance set for accelerator QA standards. Discussion as a community may be warranted as to the continued necessity of using the lead foil at all.

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Keywords

Dosimetry Protocols, Calibration, Absolute Dosimetry

Taxonomy

TH- External Beam- Photons: Quality Assurance - Linear accelerator

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