Experimental Evaluation of Depth Dose by Exit Surface Diode Dosimeters for Off-Axis Wedged Fields in Radiation Therapy

Document Type: Original Paper

Authors

1 Department of Medical Radiation Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Ph.D student, Department of Medical Physics, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran

2 Department of Radiotherapy, Oncology and Medical Physics, Tehran University of Medical Sciences, Tehran, Iran

3 Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran

4 Department of Radiation Oncology, Cancer Institute of Iran, Tehran University of Medical Sciences, Tehran, Iran

Abstract

Introduction
Evaluation of the delivered dose of externally wedged photon beams by external diode dosimeters during the treatment process requires the estimation of exit surface dose correction factors in various wedge angles and field sizes.
Materials and Methods
A system of absorbed dose evaluation, using p-type diode dosimeters placed on the exit surface of a phantom, was characterized for externally wedged photons with the maximum square field size. The values of wedge correction factor on the exit surface of the polystyrene phantom were determined for 60Co and 6 MV photons. Then, the wedge correction factors were estimated at desirable depths.
Results
Based on the findings, the deviation of off-axis wedge correction factors of the exit surface wedged fields from the central axis factor may be as large as ±10% at the evaluated depths. The results showed that the absorbed dose at each depth of patient tissue could be estimated by applying an accurate exit wedge correction factor for that particular depth, with negligible probable errors (below 1.5%).
Conclusion
In case positioning a diode dosimeter on the patient’s entrance surface of a phantom of patients is troublesome, the diode dosimeter can be placed on the exit surface in order to evaluate the absorbed dose for externally wedged photons. Based on the findings, exit dose correction factors for wedged beams cannot be discarded; in fact, these factors are variable at different directions of externally wedged beams.
 

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