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Monte Carlo Calculations of Cerrobend to Brass Conversions for Extended Source-To-Surface Distance Total-Body Irradiation Treatments

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A Li

A Li1*, Ke Sheng2, YM Yang2, (1) University of California Los Angeles, Los Angeles, CA (2) UCLA Department of Radiation Oncology, Los Angeles, CA


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

Purpose: Lung block attenuators utilized in total body irradiation (TBI) treatments are often made of Cerrobend, a lead-containing solder with a low melting point. Cerrobend may be recycled and reused with minimal equipment, but comes with increased hazardous materials risks. Recently, copper and brass patient-specific blocks have become available from commercial sources, with turnaround times suitable for regular use in the clinic. Additionally, many institutions implement TBI treatments with an extended source-to-surface distance (SSD) configuration, for which commercial dose calculation algorithms are ill-suited. In our investigation, we utilize Monte Carlo to determine the appropriate conversions from Cerrobend to brass in extended-SSD TBI treatments.

Methods: The investigation was performed within the Geant4 v10.3.0 particle transport toolkit. The geometry consisted of a human-sized water phantom of size 42cm × 175cm × 24cm, located at 500 cm SSD. For adequate statistics at extended SSD, the phantom was divided into large planar voxels of dimensions 140 mm x 350 mm x 2 mm, representative of general anatomical regions (e.g. head, chest, umbilicus, etc.). Slabs of Cerrobend or brass were simulated 20 cm from surface of the phantom in varying thickness between 2-5 cm. 6MV Clinac and Truebeam source models were simulated, using the phase-space files provided by the Varian VirtuaLinac project. Non-linear least-squares fitting was used to determine the semi-empirical transmission curve parameters α,β, and γ.

Results: At the time of this submission, a full set of transmission factors is being generated for two different Cerrobend compositions, as well as the brass composition offered by one commercial entity specializing in radiation therapy blocks and cutouts.

Conclusion: The application of Monte Carlo methods and commercial phase-space files provide an accurate, reliable conversion for institutions transitioning from Cerrobend to safer alternatives.

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