en Radiation Biology Radiation Biology تحقيق بديع Original Research <span style="font-weight: bold">Background:</span> 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. <span style="font-weight: bold">Materials and Methods:</span> 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). <span style="font-weight: bold">Results:</span> 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. <span style="font-weight: bold">Conclusion:</span> Higher energy X-rays are suggested to be used for applicators of higher diameters and smaller energy X-rays for applicators having smaller diameters. <span style="font-weight: bold">Iran. J. Radiat. Res., 2011 9(3): 159-166 </span> Radiotherapy treatment, Pantak Therapax SXT-150, depth dose, applicators diameter, kVp, superficial X-ray machine 159 166 http://ijrr.com/browse.php?a_code=A-10-1-397&slc_lang=en&sid=1 M. Ismail ismaeel_malik@yahoo.com 790031947532846002392 790031947532846002392 Yes M. Afzal 790031947532846002393 790031947532846002393 No M. Nadeem 790031947532846002394 790031947532846002394 No A.M. Rana 790031947532846002395 790031947532846002395 No S. Amjad 790031947532846002396 790031947532846002396 No S.A. Buzdar 790031947532846002397 790031947532846002397 No