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Phys Med. 2014 Mar;30(2):234-41. doi: 10.1016/j.ejmp.2013.07.125. Epub 2013 Aug 3.

Dosimetric effect by shallow air cavities in high energy electron beams.

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Centro de Física Nuclear, Universidade de Lisboa, Lisbon, Portugal; Escola Superior de Tecnologia da Saúde de Lisboa, Lisbon, Portugal. Electronic address:
Hospital de Santa Maria - Medical Consult, Lisbon, Portugal.
Departamento de Fisiología Médica y Biofísica, Universidad de Sevilla, Sevilla, Spain.
Het Nederlands Kanker Instituut - Antoni van Leeuwenhoek, Amsterdam, Netherlands.
Escola Superior de Tecnologia da Saúde de Lisboa, Lisbon, Portugal.
Centro de Física Nuclear, Universidade de Lisboa, Lisbon, Portugal.


This study evaluates the dosimetric impact caused by an air cavity located at 2 mm depth from the top surface in a PMMA phantom irradiated by electron beams produced by a Siemens Primus linear accelerator. A systematic evaluation of the effect related to the cavity area and thickness as well as to the electron beam energy was performed by using Monte Carlo simulations (EGSnrc code), Pencil Beam algorithm and Gafchromic EBT2 films. A home-PMMA phantom with the same geometry as the simulated one was specifically constructed for the measurements. Our results indicate that the presence of the cavity causes an increase (up to 70%) of the dose maximum value as well as a shift forward of the position of the depth-dose curve, compared to the homogeneous one. Pronounced dose discontinuities in the regions close to the lateral cavity edges are observed. The shape and magnitude of these discontinuities change with the dimension of the cavity. It is also found that the cavity effect is more pronounced (6%) for the 12 MeV electron beam and the presence of cavities with large thickness and small area introduces more significant variations (up to 70%) on the depth-dose curves. Overall, the Gafchromic EBT2 film measurements were found in agreement within 3% with Monte Carlo calculations and predict well the fine details of the dosimetric change near the cavity interface. The Pencil Beam calculations underestimate the dose up to 40% compared to Monte Carlo simulations; in particular for the largest cavity thickness (2.8 cm).


Air cavity; EBT2 Gafchromic film; Electron beam; Monte Carlo algorithms; Pencil beam algorithms

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