evaluating the effects of field size on beam homogeneity coefficient in the superficial radiotherapy machine using empirical method and simulation

introduction superficial x-ray therapy is one of the most important treatment methods in radiotherapy especially in the treatment of superficial skin lesions (up to 300 kvp). quality of the x-ray beam that can be expressed by half-value layer (hvl), are important indices for this type of treatment effective energy of photon and homogeneity coefficient (hc). materials and methods the hc of the superficial x-ray machine at 180 kvp was determined by an experimental method and also by simulation (egsnrc code) and the results were compared. the exposure after the first (0.5 mmcu) and second (1 mmcu) attenuation layers were measured by farmer dosimeter for three different field sizes. the hc was derived from these experimental data and was compared with corresponding results acquired by egsnrc code package. the beamnrc code was used to transport photons and electrons at exit level of the applicator, dosxyznrc code for calculation of absorbed dose in the phantom, and beamdp for drawing the output spectra. results results showed that the mean percentage difference of errors obtained by experimental data and simulation method is 3.3% and the average energy of output spectra in 180 kvp after passing through filters and attenuation layers increases to 15.3% to 25.4%. conclusion the maximum percentage difference of errors for the applied methods is higher than analogous devolves elsewhere reports which could be due to setup parameters such as ssd and field size. however, the present method has the advantage of ease of setup and matching with clinical conditions.