Validation of Monte Carlo calculated surface doses for megavoltage photon beams

Journal Article · Sat Jan 01 04:00:00 EST 2005 · Medical Physics

DOI:https://doi.org/10.1118/1.1829401· OSTI ID:20634631

Abdel-Rahman, Wamied; Seuntjens, Jan P; Verhaegen, Frank; Deblois, Francois; Podgorsak, Ervin B ^[1]

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1. Department of Medical Physics, McGill University Health Centre, 1650 Avenue Cedar, Montreal, Quebec, H3G 1A4 (Canada)

Recent work has shown that there is significant uncertainty in measuring build-up doses in megavoltage photon beams especially at high energies. In this present investigation we used a phantom-embedded extrapolation chamber (PEEC) made of Solid Water{sup TM} to validate Monte Carlo (MC)-calculated doses in the dose build-up region for 6 and 18 MV x-ray beams. The study showed that the percentage depth ionizations (PDIs) obtained from measurements are higher than the percentage depth doses (PDDs) obtained with Monte Carlo techniques. To validate the MC-calculated PDDs, the design of the PEEC was incorporated into the simulations. While the MC-calculated and measured PDIs in the dose build-up region agree with one another for the 6 MV beam, a non-negligible difference is observed for the 18 MV x-ray beam. A number of experiments and theoretical studies of various possible effects that could be the source of this discrepancy were performed. The contribution of contaminating neutrons and protons to the build-up dose region in the 18 MV x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross-section database and the NIST bremsstrahlung differential cross section do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the 18 MV. A simple incorporation of triplet production events into the MC dose calculation increases the calculated doses in the build-up region but does not fully account for the discrepancy between measurement and calculations for the 18 MV x-ray beam.

• OSTI ID: 20634631
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 1 Vol. 32; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

62 RADIOLOGY AND NUCLEAR MEDICINE
BREMSSTRAHLUNG
DEPTH DOSE DISTRIBUTIONS
DIFFERENTIAL CROSS SECTIONS
DOSIMETRY
EXTRAPOLATION CHAMBERS
IONIZATION
MONTE CARLO METHOD
NEUTRONS
PHANTOMS
PHOTON BEAMS
PROTONS
RADIATION DOSES
RADIOTHERAPY
SIMULATION
VALIDATION