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SU‐FF‐T‐110: Benchmarking MCNP Low‐Energy Bremsstrahlung Modeling for Electronic Brachytherapy Simulations
Author(s) -
Rivard MJ,
Axelrod S,
Rusch TW
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2241035
Subject(s) - bremsstrahlung , photon , monte carlo method , physics , electron , brachytherapy , photon energy , dosimetry , computational physics , atomic physics , nuclear physics , optics , nuclear medicine , radiation therapy , medicine , statistics , mathematics
Purpose: Electronic brachytherapy (eBx) sources have been used clinically for over a decade; however, dosimetric characterization methods using measurements or calculations are not well‐established. Monte Carlo methods for simulating electron transport, and subsequently photon production, have not been benchmarked to the same degree as for photon‐emitting HDR 192 Ir or LDR 125 I brachytherapy sources. Materials & Methods: Towards better understanding the capabilities of MCNP5 to simulate radiation transport for the Xoft Axxent eBx source, this study presents a comparison of calculated MCNP5 results obtained using coupled electron:photon transport with measured bremsstrahlung spectra from the literature. Given the electron energy and target material, MCNP5 bremsstrahlung modeling accounts for photon energy, angle, and probability based on the cross‐sections and angular distributions from NIST (Seltzer and Berger, 1985). The Axxent eBx source currently operates at 50 kV with electrons bombarding a ∼ 1 μm thick high/low Z target. Pertinent high/low Z comparisons for thin targets, defined as materials thin enough to produce negligible electron absorption in the target, were available from Motz and Placious (1958) using 50 kV on 5 nm Au and 63 nm Al, from Cosslett and Dyson (1957) using 10 kV on 25 nm Au, and from Doffin and Kuhlenkampff (1957) using 34 kV on 25 nm Al. Results: Comparisons of calculations and experimental data indicate that bremsstrahlung angular peak, relativistically shifted forward, agreed within a few degrees with measurements in the literature. However, the overall simulated distribution exhibited angularly invariant regions in the forward direction, attributed to MCNP low‐energy physics simplifications of the NIST dataset. Given that the brachytherapy target is ∼ 50 times thicker, with resultant smearing of the energy/angular distributions, the practical impact of this effect is under investigation, and complementary EGSnrc simulations are in progress. Conflict of Interest: This research was sponsored in part by Xoft, Inc.