[Determination of the equivalent field size of asymmetric fields in photon beam therapy analytically based on physical theories]

The aim of this study was to obtain relatively theoretical methods based on physical rules in interaction of radiation with matters which can be sensitively, rapidly and easily used in different radiotherapy centers. In this study with the aid of the exponential attenuation rules of photon beams and inverse square distance rule, in 6MV and 18MV energy of 2100C Varian accelerator X-ray and 1.25MeV energy of Cobalt-60 gamma-ray in Ahvaz Golestan Hospital and considering the scatter radiation of therapeutic field and irregular field to be equal, the side of equivalent square field were calculated. In this method, since the calculation was done taking into account the situation of the field, energy and position of the shield, a very good agreement with dose measurements showed [with error, less than 2%]. Not all of these considerations are taken into account in previous methods. In addition, the calculations can be done on a computer for each therapeutic irregular field.

Calculation of analytical expressions for measured percentage depth dose data in megavoltage photon therapy

In external radiation therapy, the percentage depth dose [PDD] is an important factor for estimating patient dose and dose distribution in target volume; therefore, its exact measurement or calculation is important. The aim of this study was to evaluate analytically the dose received by different points in water phantom and to compare it with dosimetry measurement data. To find the dose distribution throughout the tumor volume, first, the mathematical approach was performed for derivation of percentage depth dose photon beams of 6MV and 18MV Varian accelerator. Second, by dosimetry for different fields in different depths of water phantom, one can parameterize the obtained formula for percentage depth dose. By comparing the mathematical and dosimetry results, the parameters of PDD-expression were computed in terms of the dimension of equivalent square field in different depths. From this formula, one can find the PDD for any fields in different depths, surface skin dose, and depth of build-up region of dose distribution, which are in agreement with empirical results with R2 > 0.995, showing a good agreement with the experimental data. So one can measure the surface skin-dose, the depth of build-up region, and their variations in terms of square field size exactly; the measurement of these quantities have some technical problems in radiation dosimetry