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Medical Physics / Volume 25 / Issue 9

Med. Phys. 25, 1692 (1998); http://dx.doi.org/10.1118/1.598350 (10 pages)

A generalized solution for the calculation of in-air output factors in irregular fields

Siyong Kim, Jatinder R. Palta, and Timothy C. Zhu

Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385

(Received 17 July 1997; accepted 18 June 1998)

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Three major contributors of scatter radiation to the in-air output of a medical linear accelerator are the flattening filter, wedge, and tertiary collimator. These were considered separately in the development of an algorithm to be used to set up an in-air output factor calculation formalism for open and wedge fields of irregular shape. A detector’s eye view (DEV) field defined at the source plane was used to account for the effects of collimator exchange and the partial blockage of the flattening filter by the tertiary collimator in the determination of head scatter. An irregular field determined at the source plane by a DEV was segmented and mapped back into the detector plane by a field-mapping method. Field mapping was performed by using a geometric conversion factor and equivalent field relationships for head scatter. The scatter contribution of each segmented equivalent field at the detector plane was summed by Clarkson integration. The same methodology was applied for determining both tertiary collimator and wedge scatter contribution. However, the field size that determined the amount of scatter contribution was not the same for each component. For tertiary collimator scatter and external wedge scatter, a field projected to the detector plane was used directly. Comparisons of calculated and measured values for in-air output factors showed good agreement for both open and external wedge fields. This algorithm can also be used for multileaf collimator (MLC) fields irrespective of the position of the MLC (i.e., whether the MLC replaces one secondary collimator or is used as a tertiary collimator). The measurement and parameterization of tertiary collimator scatter is necessary to account for its contribution to the in-air output. Because a source-plane field is mapped into the detector plane, no additional dosimetric data acquisition is necessary for the calculation of head scatter. © 1998 American Association of Physicists in Medicine.

© 1998 American Association of Physicists in Medicine

Digital Object Identifier
http://dx.doi.org/10.1118/1.598350

KEYWORDS and PACS

Keywords

radiation therapy, linear accelerators, dosimetry, in-air output factor, head scatter, wedge scatter, irregular field, MLC, detector’s eye view, field mapping

PACS

  • 87.53.Bn

    Dosimetry/exposure assessment

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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