Evaluation of depth dose characteristics of superficial X-rays machine using different kVp and applicators diameter

Background: radiotherapy treatment planning requires different dosimetric quantities as input in order to calculate a desired dose distribution. this study has been focused to evaluate the depth dose characteristics of superficial x-rays being used for radiotherapy treatment. materials and methods: computerized 3-d water phantom of multi-data system was used. the measurements were made through ptw (physikalirsch-technische werkstalten) farmer type nt-30006 waterproof ionization chamber of 0.6cc, and ptw electrometer for digital dose rate reading in gy/min using five different diameter applicators and filters at five different values of accelerating potentials (kvps). results: the dose rate at various kvp x-ray beams was observed to decrease significantly with increasing depth in water phantom for all applicator diameters from 98% (at 0.1cm depth) down to 43% (at 2cm i.e. reference condition). the dose rate increases by increasing the value of kvp with a maximum at 150 kvp (1.6 and 0.93 gy/ min for respective applicator diameters 2.5cm and 10cm). applicator with 2.5cm diameter demonstrates better dose rate at 85kvp at different depths. pdd decreases lower than 50% for all combination of applicators and kvps at/or above 2cm depth so these measurements should not be considered for treatment planning. conclusion: higher energy x-rays are suggested to be used for applicators of higher diameters and smaller energy x-rays for applicators having smaller diameters.