Document Type : Original Paper
Authors
1
M.Sc. Student of Medical Physics, Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran
2
Associated Professor, Medical Physics Dept., Tehran University of Medical Sciences, and Radiotherapy Physics Dept., of the Cancer Institute, Tehran, Iran
3
Physisist of the Radiotherapy Physics Department of the Cancer Institute, Tehran, Iran
4
Associate Professor, Radiotherapy and Oncology Department of the Cancer Institute, Imam Khomeini Hospital, Tehran, Iran
5
Professor, Medical Physics Dept., Tehran University of Medical Sciences, Tehran, Iran
6
M.Sc. of Medical Physics, Imam Khomeini Medical Imaging Center, Gilan University of Medical Sciences, Some’e-sara, Iran
Abstract
Introduction: 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.
Materials and methods: The entrance and exit doses of 36 pelvic-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 (0.6 cc and 0.3 cc) 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 points were calculated and the results were shown using histograms.
Results: 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 and 5.03%, 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 no 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.
Discussion and Conclusions: Some error sources are patient setup error, patient motion and dose calculation algorithm error (due to ignoring inhomogeneity and patient curvature). As no significant deviations were found in midline dose, the method used has an acceptable accuracy. In vivo dosimetry can perform a basic role in the quality control of radiotherapy departments.
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