The application of the approximation method of equivalent square for magavoltage photon beams
Osman, G.E.; Tsung, P.
Journal of Medicine 11(2-3): 117-125
1980
ISSN/ISBN: 0025-7850 PMID: 6774038 Document Number: 161353
In treatment planning of radiation fields, knowledge of the dose received at various tissue levels in the irradiated volume is essential. For a certain radiation quality and source to skin distance, radiation dose at maximum build-up or at a depth in tissue is a function of the size and shape of the radiation field. Equilibrium dose and the depth dose along the central axis of the beam for a rectangular field can be determined by applying the equivalent square technique. The approximation method of area/perimeter of different portal sizes used in practice for finding the equivalent squares of a rectangular field was tested on a 35 MV Clinac Linear Accelerator for treatment planning. Data were taken for doses of 8 and 25 MV X-rays at the depths of maximum build-up and at 10 cm below the surface of an adequate water phantom. Maximum variation of dose calculation from the above method was less than 1.6% for routine clinical usage and less than 3.2% for extreme rectangular field sizes. Dose variation was improved as the depth in phantom was increased. [Applications to therapy of tumor patients are suggested.].