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Title: SU-E-T-72: Commissioning of a Standardized SRS Cone Set: Determination of the Bolus Gap Factors in a Passively Scattered Proton Beam

Purpose: To determine the proton output factors for an SRS cone set using standardized apertures and varied range compensators (bolus blanks); specifically, to determine the best method for modeling the bolus gap factor (BGF) and eliminate the need for patient specific calibrations. Methods: A Standard Imaging A-16 chamber was placed in a Plastic Water phantom to measure the change in dose/MU with different treatment combinations for a proton SRS cone, using standardized apertures and range compensators. Measurements were made with all apertures in the SRS cone set, with four different range compensator thicknesses and five different air gaps between the end of the SRS cone and the surface of the phantom. The chamber was located at iso-center and maintained at a constant depth at the center of modulation for all measurements. Each aperture was placed in the cone to measure the change in MU needed to maintain constant dose at the chamber, as the air gap was increased with different thicknesses of bolus. Results: The dose/MU varied significantly with decreasing aperture size, increasing bolus thickness, or increasing air gap. The measured data was fitted with the lowest order polynomials that accurately described the data, to create a model for determiningmore » the change in output for any potential combination of devices used to treat a patient. For a given standardized aperture, the BGF could be described by its constituent factors: the bolus thickness factor (BTF) and the nozzle extension factor (NEF). Conclusion: The methods used to model the dose at the calibration point could be used to accurately predict the change in output for SRS proton beams due to the BGF, eliminating the need for patient specific calibrations. This method for modeling SRS treatments could also be applied to model other treatments using passively scattered proton beams.« less
Authors:
; ; ; ; ; ; ;  [1]
  1. Loma Linda University Medical Center, Loma Linda, CA (United States)
Publication Date:
OSTI Identifier:
22339839
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 41; Journal Issue: 6; Other Information: (c) 2014 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; 60 APPLIED LIFE SCIENCES; APERTURES; MODULATION; PHANTOMS; POLYNOMIALS; PROTON BEAMS; RADIATION DOSES; RADIOTHERAPY; THICKNESS