The thermal characteristics of different diodes on in vivo patient dosimetry

Diode sensitivity variations with temperature (SVWT) have been reported to vary from small negative values up to 0.6% per °C. Thus it is possible for diode calibration factors established at room temperature (~20 °C) to yield errors in the range of -1% to +9% when diodes are placed on a patientʹs skin (~30 °C) for in vivo entrance dose measurements. In this study we simulated several skin temperatures using a temperature-controlled aluminum surface in contact with a section of Bolus. The internal temperatures of several diodes with different buildup thickness were monitored as a function of time when placed in contact with the heated bolus. Our results indicate that for different combinations of room temperature (18 °C-23 °C) and patient skin temperature (28 °C-34 °C) diodes reached 90% of their equilibrium temperature within 3-5 min. In addition, the range of typical skin temperatures was determined by measurements performed on a number of actual patients under clinical conditions. Based on the results of our experiments a protocol was developed to minimize the temperature based errors for in vivo dosimetry.