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Medical Physics / Volume 13 / Issue 3

Med. Phys. 13, 409 (1986); http://dx.doi.org/10.1118/1.595886 (4 pages)

Off‐center ratios for three‐dimensional dose calculations

Chen‐Shou Chui and Radhe Mohan

Department of Medical Physics, Memorial Sloan–Kettering Cancer Center, New York, New York 10021

(Received 26 September 1985; accepted 21 February 1986)

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A new method is proposed for computing the off‐center ratios (OCR’s) in three‐dimensional dose calculations. For an open field, the OCR at a point is computed as the product of the primary OCR (POCR) and the boundary factors (BF’s). The POCR describes the beam profile for an infinite field, that is, without the effect of the collimators. It is defined as the ratio of the dose at a point off the central ray to the dose at the point on the central ray at the same depth for an infinite field. The POCR is a function of radial distance from the beam central ray and depth. The BF describes the shape of the beam in the neighborhood of the field boundary defined by the collimators. It is defined as the ratio of the OCR at a point for a finite field to the OCR at the same point for an infinite field. The BF is a function of distance from the field boundary, depth, and field size. For a wedged field, we assume that the boundary factors remain the same as for open fields but the POCR’s are altered. The changes in beam profiles are described by a factor called the wedge profile factor (WPF), defined as the ratio of the dose at a point for the largest wedged field to the dose at the same point for an open field of the same field size. The WPF is a function of lateral distance from the beam central plane and depth. Calculated OCR’s using this new method are in agreement with the measured data along both the transverse and the diagonal directions of the field.
Digital Object Identifier
http://dx.doi.org/10.1118/1.595886

KEYWORDS and PACS

Keywords

MATHEMATICAL MODELS, RADIATION DOSES, RADIOTHERAPY, PLANNING, BEAM PROFILES, DOSIMETRY

PACS

  • 87.53.Bn

    Dosimetry/exposure assessment

PUBLICATION DATA

ISSN

0094-2405 (print)  

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