Résumé
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. 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]. 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. Higher energy X-rays are suggested to be used for applicators of higher diameters and smaller energy X-rays for applicators having smaller diameters