Journal of Medical Physics
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ORIGINAL ARTICLE
Year : 2012  |  Volume : 37  |  Issue : 1  |  Page : 32-39

Evaluation of off-axis wedge correction factor using diode dosimeters for estimation of delivered dose in external radiotherapy


1 Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran
2 Department of Medical Radiation Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran
3 Radiotherapeutic Oncology Department of Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
4 Department of Radiotherapy, Tehran University of Medical Sciences, Tehran, Iran

Correspondence Address:
Mahmoud Allahverdi
Department of Medical Physics, Tehran university of Medical Sciences, Tehran
Iran
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Source of Support: Tehran University of Medical Sciences and Health, Conflict of Interest: None


DOI: 10.4103/0971-6203.92718

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An in vivo dosimetry system, using p-type diode dosimeters, was characterized for clinical applications of treatment machines ranging in megavoltage energies. This paper investigates two different models of diodes for externally wedged beams and explains a new algorithm for the calculation of the target dose at various tissue depths in external radiotherapy. The values of off-axis wedge correction factors were determined at two different positions in the wedged (toward the thick and thin edges) and in the non-wedged directions on entrance and exit surfaces of a polystyrene phantom in 60 Co and 6 MV photon beams. Depth transmission was defined on the entrance and exit surfaces to obtain the off-axis wedge correction factor at any depth. As the sensitivity of the diodes depends on physical characteristics [field size, source-skin distance (SSD), thickness, backscatter], correction factors were applied to the diode reading when measuring conditions different from calibration situations . The results indicate that needful correction factors for 60 Co wedged photons are usually larger than those for 6 MV wedged photon beams. In vivo dosimetry performed with the proposed algorithms at externally wedged beams has negligible probable errors (less than 0.5%) and is a reliable method for patient dose control.


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