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The effect of very small air gaps on small field dosimetry

Charles, P., Crowe, S., Kairn, T., Kenny, J., Lehmann, J, Lye, J., Dunn, L., Hill, B., Knight, R., Langton, C.M., & Trapp, J. (2012) The effect of very small air gaps on small field dosimetry. Physics in Medicine and Biology, 57, pp. 6947-6960.

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Abstract

The purpose of this study was to investigate the effect of very small air gaps (less than 1 mm) on the dosimetry of small photon fields used for stereotactic treatments. Measurements were performed with optically stimulated luminescent dosimeters (OSLDs) for 6 MV photons on a Varian 21iX linear accelerator with a Brainlab μMLC attachment for square field sizes down to 6 mm × 6 mm. Monte Carlo simulations were performed using EGSnrc C++ user code cavity. It was found that the Monte Carlo model used in this study accurately simulated the OSLD measurements on the linear accelerator. For the 6 mm field size, the 0.5 mm air gap upstream to the active area of the OSLD caused a 5.3 % dose reduction relative to a Monte Carlo simulation with no air gap. A hypothetical 0.2 mm air gap caused a dose reduction > 2 %, emphasizing the fact that even the tiniest air gaps can cause a large reduction in measured dose. The negligible effect on an 18 mm field size illustrated that the electronic disequilibrium caused by such small air gaps only affects the dosimetry of the very small fields. When performing small field dosimetry, care must be taken to avoid any air gaps, as can be often present when inserting detectors into solid phantoms. It is recommended that very small field dosimetry is performed in liquid water. When using small photon fields, sub-millimetre air gaps can also affect patient dosimetry if they cannot be spatially resolved on a CT scan. However the effect on the patient is debatable as the dose reduction caused by a 1 mm air gap, starting out at 19% in the first 0.1 mm behind the air gap, decreases to < 5 % after just 2 mm, and electronic equilibrium is fully re-established after just 5 mm.

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10 citations in Web of Science®

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ID Code: 53433
Item Type: Journal Article
Keywords: radiotherapy, stereotactic, intensity modulated, SRS, IMRT, Dosimetry, Monte Carlo, Bragg, Bragg Gray
DOI: 10.1088/0031-9155/57/21/6947
ISSN: 0031-9155
Subjects: Australian and New Zealand Standard Research Classification > PHYSICAL SCIENCES (020000) > OTHER PHYSICAL SCIENCES (029900) > Medical Physics (029903)
Divisions: Current > Schools > School of Chemistry, Physics & Mechanical Engineering
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Funding:
Copyright Owner: Copyright 2012 Institute of Physics and Engineering in Medicine
Deposited On: 03 Sep 2012 08:53
Last Modified: 11 Apr 2014 19:22

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