Author: Amar K. Basavatia, Sandeepan Ganguly, Lee C. Goddard, Wolfgang A. Tomé 👨🔬
Affiliation: Montefiore Medical Center 🌍
Purpose: To investigate the dosimetric impact of WGTG51-e Report 385, recently updated CoP for clinical electron beams. This study uses a chamber specific effective point of measurement (EPOM) shift to determine the electron beam quality specifier R50 without the need for a gradient correction to calculate absorbed dose to water. We examined updates included in ICRU-90 recommendations for Mean Excitation Energy (I) of Air, Water and Graphite along with the crystalline density correction for Graphite, and their impact on a cylindrical ion chamber.
Methods: R50 was measured for electron beams (6-16 MeV) from both protocols TG –51and WGTG51-e with chamber-specific shifts, and compared with a diode. Depth ionization was measured in 3D water tank with reference-field pair detector analyzed to investigate differences in R50 and dref. Monte-Carlo calculated Consensus kQ & Stopping Power Ratio with ICRU37 & ICRU90 key data for graphite-plastic walled chamber (PTW-30013) was used to analyze surface dose, dmax & d90 between four sets of recommendations.
Results: Measured R50-values using diode, and Cylindrical Chamber with TG51, WGTG51 with ICRU37 and ICRU90 key data in cm were found to be for 6MeV (2.405,2.325,2.358,2.378), 9MeV(3.615,3.562,3.598,3.618), 12MeV(5.03,5.01,5.042,5.062) and 16MeV (6.64,6.657,6.690,6.710),respectively. Dose measured at the surface has the largest deviation (~0.3%) irrespective of energy and WGTG51-e with ICRU90 has better agreement with Diode compared to other protocols.
Conclusion: The difference between ICRU37 and ICRU90 data was negligible (<0.1%) for PTW30013 chamber, with no significant clinical impact. However, ICRU90 data did yield better agreement with Diode measured data, especially for low energies (R50<4cm). WGTG51-e simplifies the calibration process since shifting the PDI incorrectly can yield a high error in measurement. The downside being that there are only a handful of chambers made available for comparison with the protocol and the lack of incorporation of ICRU90 data was not addressed.