Journal of Medical Physics
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Year : 1986  |  Volume : 11  |  Issue : 3  |  Page : 12

A General Formulation For Depth Dose Computation In Photon Beam Dosimetry


Correspondence Address:
H B Tripathi


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This paper describes a generalised formulation for calculation of depth dose distribution at any point in the irradiated medium. Primary (P) and Scattered (S) components of the total dose is calculated at the required point using the following equations: Where is the dose to small of the tissue just large enough to provide electronic equilibrium and located at (f + dm) in free space, is the linear attenuation coefficient for the incident beam energy in water medium, d is depth of the point of calculation along the central axis, dm is the depth of maximum ionisation, f is the source to surface distance, Sm is the scatter part of peak scatter factor. de defines the effective height of the cylindrical volume contributing scattered dose to the axial point. This cylindrical volume is bounded by the beam area at dm. Off-axis dose computation is done with the help of off-axis dose computation is done with the help of off-axis ratio (OAR). OAR is a function of machine and beam area at dm. Off-axis dose computation is done with the help of off-axis ratio (OAR). OAR is a function of machine and beam parameters and is evaluated from the measured transverse beam profiles at 5, 10 and 15 cm computations. These simple relations have been used to generate treatment planning soft wares for Co-60 and Nagavoltage X-rays beam therapy. These softwares can be incorporated into any small computer for computations of complete dose distributions in different planes. Beam weighting factors, wedge filters, body curvature, oblique beam incidence and body heterogeneities are taken into calculations, wherever applicable, before contributions from each beams are summated to yield total dose at the point of interest. Details of the various steps involved and results obtained will be presented during this presentation.


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