Original Article
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World J Radiol. Nov 28, 2010; 2(11): 434-439
Published online Nov 28, 2010. doi: 10.4329/wjr.v2.i11.434
Peripheral dose measurement in high-energy photon radiotherapy with the implementation of MOSFET
Vassiliki Vlachopoulou, Georgia Malatara, Harry Delis, Kiki Theodorou, Dimitrios Kardamakis, George Panayiotakis
Vassiliki Vlachopoulou, Georgia Malatara, Harry Delis, George Panayiotakis, Department of Medical Physics, School of Medicine, University of Patras, 265 00 Patras, Greece
Kiki Theodorou, Department of Medical Physics, School of Medicine, University of Thessaly, 411 00 Larissa, Greece
Dimitrios Kardamakis, Department of Radiotherapy, School of Medicine, University of Patras, 265 00 Patras, Greece
Author contributions: Vlachopoulou V, Malatara G, Kardamakis D and Panayiotakis G contributed to the initial study concept and design; Vlachopoulou V, Malatara G, Delis H and Theodorou K performed the data acquisition and analyses; Vlachopoulou V, Delis H and Panayiotakis G drafted and revised the manuscript; all authors approved the final version of the submitted manuscript.
Supported by The Greek Central Council of Health (110Κ/93)
Correspondence to: George Panayiotakis, PhD, Professor, Department of Medical Physics, School of Medicine, University of Patras, 265 00 Patras, Greece. panayiot@upatras.gr
Telephone: +30-2610-969131 Fax: +30-2610-996113
Received: March 22, 2010
Revised: July 20, 2010
Accepted: July 27, 2010
Published online: November 28, 2010

AIM: To study the peripheral dose (PD) from high-energy photon beams in radiotherapy using the metal oxide semiconductor field effect transistor (MOSFET) dose verification system.

METHODS: The radiation dose absorbed by the MOSFET detector was calculated taking into account the manufacturer’s Correction Factor, the Calibration Factor and the threshold voltage shift. PD measurements were carried out for three different field sizes (5 cm × 5 cm, 10 cm × 10 cm and 15 cm × 15 cm) and for various depths with the source to surface distance set at 100 cm. Dose measurements were realized on the central axis and then at distances (1 to 18 cm) parallel to the edge of the field, and were expressed as the percentage PD (% PD) with respect to the maximum dose (dmax). The accuracy of the results was evaluated with respect to a calibrated 0.3 cm3 ionization chamber. The reproducibility was expressed in terms of standard deviation (s) and coefficient of variation.

RESULTS: % PD is higher near the phantom surface and drops to a minimum at the depth of dmax, and then tends to become constant with depth. Internal scatter radiation is the predominant source of PD and the depth dependence is determined by the attenuation of the primary photons. Closer to the field edge, where internal scatter from the phantom dominates, the % PD increases with depth because the ratio of the scatter to primary increases with depth. A few centimeters away from the field, where collimator scatter and leakage dominate, the % PD decreases with depth, due to attenuation by the water. The % PD decreases almost exponentially with the increase of distance from the field edge. The decrease of the % PD is more than 60% and can reach up to 90% as the measurement point departs from the edge of the field. For a given distance, the % PD is significantly higher for larger field sizes, due to the increase of the scattering volume. Finally, the measured PD obtained with MOSFET is higher than that obtained with an ionization chamber with percentage differences being from 0.6% to 34.0%. However, when normalized to the central dmax this difference is less than 1%. The MOSFET system, in the early stage of its life, has a dose measurement reproducibility of within 1.8%, 2.7%, 8.9% and 13.6% for 22.8, 11.3, 3.5 and 1.3 cGy dose assessments, respectively. In the late stage of MOSFET life the corresponding values change to 1.5%, 4.8%, 11.1% and 29.9% for 21.8, 2.9, 1.6 and 1.0 cGy, respectively.

CONCLUSION: Comparative results acquired with the MOSFET and with an ionization chamber show fair agreement, supporting the suitability of this measurement for clinical in vivo dosimetry.

Keywords: Radiotherapy, Peripheral dose, Metal oxide semiconductor field effect transistor, Dosimeter