TG-51 Practice Problems
A medical physicist is performing absolute dose calibration for a 21 MeV electron beam. The depth of 50% absorbed dose, R50, was measured and a value
of 8.77 cm was obtained.
All measurements were taken with a PTW N30002 cylindrical model ion chamber. The gradient correction factor was measured and a value of 0.9954 was obtained. The ion chamber-electrometer unit
was calibrated to measure 4.863e+7 Gy/C under labratory conditions. The temperature
in the clinic at the time of measurement was 22.8oC and the pressure was 768.5 mmHg. The
average electrometer readings at bias voltages of 150 V, -150 V, and -300 V are 15.7535 nC,
-15.7295 nC, and -15.7877 nC, respectively. Each measurement was taken after 100 machine monitor
units (MU) had been delivered. What is the value of the absorbed dose to water at dmax for an SSD
setup, in cGy/MU? (The PDD for this beam at clinical setup is 94.76%)
**All values, unless stated within the problem, should be obtained using the TG-51 report.