In vivo diode dosimetry for total marrow irradiation.

Purpose: To describe a method and evaluate the efficacy of using a p-type silicon diode as an alternative to thermoluminescent dosimeters for verifying the accuracy of total marrow irradiation setups and calculations.

Methods And Materials: A calibration factor has been measured for a 6 MV photon beam incident horizontally onto a polystyrene phantom inside an in-house built total marrow irradiation stand. Signal responses due to positioning and orientation of the diode with respect to the source were compared to a 0.6 cc cylindrical ionization chamber inside a polystyrene phantom. Procedures for predicting the diode reading and taking entrance measurements have been developed and action levels established to determine causes for discrepancies when ratios between predicted and actual values fell outside a +/- 5% tolerance range. Measurements were taken at the skin surface over the umbilicus calculation point for alternating 1.5 Gy anterior and posterior fractions given bidaily over a 3-day period.

Results: A total of 137 measurements taken from January to September 1994 for 23 patients were analyzed using a frequency histogram. The histogram indicated a mean reading of 1.002 +/- 2.6, and that three of the measurements fell outside the 5% tolerance. Investigation into the cause of the discrepancies showed that the diode had been improperly placed one time and that further patient immobilization needs to be considered.

Conclusion: It is possible to use a diode as an in vivo dosimeter for a total marrow irradiation technique. The ease of implementation and immediate readouts make a diode system preferable to a thermoluminescent system for identifying systematic errors and verifying treatment configurations and monitor unit calculations.