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A New Beam Model for the Total Scatter Factor Stot(c,s): Application to Small Field Dosimetry in SRS and SRT

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Z Han

Z Han*, S Friesen , F Hacker , P Zygmanski , Brigham & Women's Hospital and Dana Farber Cancer Institute, Harvard Medical School, Boston, MA


SU-I-GPD-T-422 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To develop a mathematical model for total scatter factor Stot. The model can be used to determine Stot for arbitrary jaw/MLC setting down to very small field sizes such as those used in stereotactic procedures. The Stot was also measured in the presence of tertiary MLC in addition to the secondary jaws.

Methods: Stot measurement was carried out in water (5 cm depth and 95 cm SSD) for a 6 MV beam on a TrueBeam STx system dedicated for SRS and SRT procedures using an IBA CC04 ionization chamber and a PTW SRS diode (for fields <3×3 cm²). Both secondary jaws and tertiary MLC were used to collimate the radiation field. For each jaw size (c), the MLC aperture (s) varies from its minimum of 0.5×0.5 cm² up to c. Stot(c,s) was empirically modeled based on the affine transformation and an interpolation formula was derived to compute Stot for arbitrary (c,s).

Results: The simultaneous collimation using jaws and MLC gives rise to a two-argument (c,s) dependence of Stot, which presents as an s-dependent function for each given c. An empirical model has been developed for Stot(c,s) which shows good agreement with the measurements. The interpolation method, which makes the derivation of Stot for arbitrary (c,s) possible, also shows good agreement with experimental results.

Conclusion: The accuracy of the traditional treatment of collimator scatter Sc and phantom scatter Sp needs to be re-evaluated for small fields, especially with the simultaneous collimation of secondary jaws and tertiary MLC. Directly measured Stot(c,s) is recommended for monitor unit calculations without the need to separately determine Sc and Sp. We have developed an empirical model of Stot(c,s) which can be used as a practical tool in narrow field dosimetry to compute Stot for arbitrary (c,s).

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