Running Title: ANALYTICAL DERIVATION OF GAMMA PUTTY PARAMETERS

Radiation Oncology - HSHS St Vincent Hospital, Wisconsin, USA

^*Corresponding Author

Aime M Gloi

Radiation Oncology

HSHS St Vincent Hospital

835 South Van Buren Street

Green Bay, WI 54301

USA

Email: agloi@sbcglobal.net

Abstract

Background: Several descriptors are used to characterize the dosimetric parameters of a photon beam through an attenuator. This study evaluates these descriptors analytically through various thicknesses of Gamma Putty. Specifically, we measure percent ionization depth doses as a function of depth and field size, and fit three models to the data. Materials and Methods: Measurements of percent ionization at various depths along the central axis were generated from 6 MV and 18 MV photon beams at a source-axis distance of 100 cm with an ionization in solid water phantom. We fit analytic models to the data to determine linear attenuation, beam hardening, beam quality, and electron contamination. Results: We report best-fit parameters for all the analytical models. All models yielded a root mean square (RMS) error of less than 1% with respect to the data. At depths below 5cm in the phantom, the largest attenuation coefficients (μ) were observed for the 6 MV beam, regardless of Gamma Putty thickness. Also, the smallest field sizes (4x4 and 5x5 cm²) have the largest attenuation coefficients at these depths, for both beam energies. At a depth of 10 cm, the variation in μ was negligible for both beam energies. Conclusions: By fitting parametric models to axial ionization profiles, it is possible to characterize the dosimetric parameters of any attenuator as a function of thickness and field size, without knowing the precise spectral distribution of the beam. Parameters such as attenuation coefficients, beam hardening, and electron contamination can then be calculated accurately for any combination of field size and attenuator thickness.

Keywords: Analytic, beam hardening, electron contamination, Gamma Putty, beam quality index.

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