Document Type : Original Paper
Authors
1
M.Sc. in Medical Physics, Medical Imaging Center, Imam Hosein Hospital, Shahrood College of Medical Sciences, Shahrood, Iran.
2
Assistant Professor, Medical Physics Dept., Cancer Institute, Imam Khomeini Hospital, Tehran University of Medical Sciences, Tehran, Iran.
3
M.Sc. in Medical Physics, Cancer Institute, Imam Khomeini Hospital, Tehran University of Medical Sciences, Tehran, Iran.
4
Associate Professor, Radiation Therapy Dept., Cancer Institute, Imam Khomeini Hospital, Tehran University of Medical Sciences, Tehran, Iran.
Abstract
Introduction: During radiotherapy, high accuracy in the dose delivery is required because there is a strong
relationship between the absorbed dose, local tumor control and particularly the normal tissue damage. In
many institutions, in vivo dosimetry using diodes is performed to check the actual dose delivered. In general,
the uncertainty in the dose delivered should fall within ± 5% of the prescribed dose as recommended by the
International Commission on Radiation Units and Measurements (ICRU).
Materials and Methods: The combined entrance and exit dose measurements have been performed for brain
tumors by diode detectors. In vivo detectors used in this study were P-type semiconductor diodes used for
determination of absorbed dose and exit transmission (T ex ). A Perspex water phantom (30×30 cm3 area and
thickness ranging from 5 to 30 cm) and a farmer type ionization chamber (0.6 cm3) were used for the
measurements. The calibration and correction factor are calculated and the relevant curves have been
obtained. The SSD correction factor (SSD = 80 cm for all set-up), directional dependence and temperature
dependence (<1%) were ignored in the determination of the absorbed dose.
Results: Errors more than 5% between the measured and the calculated entrance, exit and midline doses were
detected. The measured entrance, exit and midline doses were compared with the calculated ones, and no
significant difference (P>0.1) was observed.
Discussion and Conclusion: In vivo measurements have been shown to be very useful as a check of the dose
delivered to a given patient.
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