Electron contamination and build-up doses in conformal radiotherapy fields

The dose in the build-up region depends upon the primary photon beam, backscattered radiation from the patient and contamination radiation from outside the patient. In this paper, a model based on measured data is proposed which allows the build-up dose for arbitrarily shaped treatment fields to be...

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Bibliographic Details
Published in:Physics in medicine & biology 1999-01, Vol.44 (1), p.43-55
Main Authors: Hounsell, Alan R, Wilkinson, John M
Format: Article
Language:English
Subjects:
Biological and medical sciences
Electrons
Humans
Medical sciences
Miscellaneous
Phantoms, Imaging
Photons - therapeutic use
Radiotherapy Dosage
Radiotherapy Planning, Computer-Assisted
Radiotherapy, Conformal
Radiotherapy. Instrumental treatment. Physiotherapy. Reeducation. Rehabilitation, orthophony, crenotherapy. Diet therapy and various other treatments (general aspects)
Reproducibility of Results
Scattering, Radiation
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Summary:The dose in the build-up region depends upon the primary photon beam, backscattered radiation from the patient and contamination radiation from outside the patient. In this paper, a model based on measured data is proposed which allows the build-up dose for arbitrarily shaped treatment fields to be determined. The dose in the build-up region is assumed to comprise a primary photon component and a contamination component that is a function of the field size and shape. This contamination component, for modelling purposes, is subdivided into contributions that correspond to elements of 1 cm by 1 cm cross-sectional area at the plane of the isocentre. The magnitude of these components has been obtained by fitting measured data to an exponential function. The exponent was found to vary linearly with depth for energies between 4 MV and 20 MV. The coefficient decreased linearly with depth at 4, 6 and 8 MV, but exhibited a broad build-up region at 20 MV. The primary component, in the build-up region, could be approximated by a 100 - (100 - PSD) e(-mu d) function, where PSD is the primary surface dose. The values obtained during the fitting procedure were used to calculate dose in the build-up region for arbitrarily shaped fields. Good agreement was found in each case.
ISSN:0031-9155
1361-6560
DOI:10.1088/0031-9155/44/1/005