Diode calibration for dose determination in total body irradiation

Background: Total Body Irradiation (TBI) is different from standard radiotherapy in many aspects, so it is not easy to predict the delivered dose to the patient under TBI treatment. Diode dosimetry procedures for surface dose reading can help to define variations of the actually delivered dose from the prescribed one. The aim of this study was to describe the measurements made to calibrate diodes in order to implement as a dosimeter for TBI tratment. An algorithm was also proposed based on diode dosimetry in order to determine the midplane dose. Materials and Methods: In this study, four p-type diodes connected to a MULTIDOSE electrometer were implemented as dosimeter. For diode calibration a water phantom with dimension of 30×30×32cm3 along with a 0.6 cc Farmer ion chamber were used. Directional dependence of diodes, the effects of thickness correction factor and complete backscatter factor were studied. Three algorithms, arithmetic, geometric mean and proposed algorithm were used to investigate midplane dose determination in TBI condition. Results: It was found by measurements that the effect of angle incident on diode response was significant and should have been taken into account. Variation in thickness correction factor was found about 0.7%. The accuracy in midplane dose determination in the arithmetic, geometric mean and proposed algorithm was about 3.8, 12.5 and 3.3%, respectively. Conclusion: Diode dosimetry is very useful as a check of midplane dose delivered to patients under TBI treatment. When the calibration and correction factors are carefully determined, high precision can be obtained. The proposed algorithm by this study seems to be useful in order to midplane dose determination in TBI condition. Iran. J. Radiat. Res., 2008 6 (1): 43-50