Is Lead Foil Necessary for Flattening Filter Free Beam Reference Dosimetry?
S Gao, C Nelson*, V Kathriarachchi, M Choi, R Saxena, R Kendall, P Balter, UT MD Anderson Cancer Center, Houston, TX
Presentations
MO-E115-BOP-F1-3 (Monday, 7/11/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]
Exhibit Hall | Forum 1
Purpose: To quantify the differences in beam quality conversion factor kQ between using lead foil and without lead foil for TG51 addendum protocol.
Methods: Two flattening filter free (FFF) beams, 6XFFF and 10XFFF, were calibrated on 4 Varian TrueBeams following the TG51 addendum protocol using Farmer ionization chambers, two TN 30013 (PTW) and two SNC600c (Sun Nuclear) chambers with traceable absorbed dose-to-water calibrations. In kQ determination, the values of the percentage depth-dose at 10 cm depth %dd(10) were measured with 10×10 cm2 field size at 100 cm source-to-surface distance (SSD) using a lead foil at 50 cm and without lead foil. The %dd(10)x values were calculated according to TG51, and the kQ factors were determined using the empirical fit equations in the TG-51 addendum for the PTW 30013 chambers. A similar equation was used to calculate kQ, for SNC600c chamber with the fitting parameters taken from a very recently published Monte Carlo study. The differences in the kQ factors between using lead and without lead were compared.
Results: Differences in %dd(10)x between the protocols using lead and without lead were 1.0±0.2% for 6 FFF beam and 0.6±0.1% for 10FFF beam. Differences in kQ values between using lead foil and without lead were 0.1±0.01% for 6 FFF and 0.1±0.02% for 10 FFF beams with Farmer ionization chambers.
Conclusion: Without using lead foil for FFF beams introduces an error approximately 0.1%. Since the simpler the procedure the less prone to operator error, the results demonstrated no lead foil is required for calibration of FFF beams on the TrueBeam platfrom.
Keywords
Absolute Dosimetry, Flattening Filters, Quality Assurance
Taxonomy
TH- External Beam- Photons: Calibration protocol and primary standards
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