ABSTRACT
Delivering maximum dose to tumor and minimum dose to normal tissues is the most important goal in radiotherapy. According to ICRU, the maximum acceptable uncertainty in the delivered dose compared to the prescribed dose should be lower than 5% and this is because of the relationship between absorbed dose, tumor control and normal tissue damage. Absorbed dose accuracy is investigated by an in vivo dosimetry method. In this paper, we compared absorbed dose in the tumors of the breast and pelvic region against the calculated dose. The amount of deviations and the factors that cause this deviation in dose delivery to patients and some methods for decreasing them were evaluated. The entrance and exit doses of 36 pelvi-region cancer patients and 38 breast cancer patients who were treated by cobalt-60 teletherapy were measured using p-type diodes. It should be noted that the transmission method was used to assess the dose at isocenter. Two ionization chambers [0.6cc and 0.3cc] were used for calibration and determination of the correction coefficients in water and slab phantoms. Deviations between calculated and measured doses of entrance, exit and midline point were calculated and the results were shown using histograms. The average and standard deviation for entrance, exit and midline points for pelvis cancer were assessed to be about 0.10%, -1.86% and -1.35% for mean deviation an d5.03%, 7.32% and 5.86% for standard deviation, respectively. The corresponding data for breast cancer were 0.78%, 5.29% and 3.59% for mean deviation and 5.97%, 10.23% and 9.86%, respectively. There was not significant difference between the calculated and measured doses [p>0.1], except exit dose in breast cancer [p<0.05]. The temperature and angle of incidence correction factors were neglected due to their less than 1% deviations. Some error sources are patient setup error, patient motion and dose calculation algorithm error [due to ignoring in homogeneity and patient curvature]. As no significant deviations were found in midline dose, the method used has an acceptable accuracy. In vivo dosminetry can perform a basic role in the quality control of radiotherapy departments