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. Iran. J. Radiat. Res., 2011 9(3): 159-166