TG-51 Practice Problems
A medical physicist is performing absolute dose calibration for a 10 MV photon beam. The percent depth dose at 10 cm depth, %dd(10)Pb, for a 10 x 10 cm2
field size at 100 cm SSD was measured using a lead foil placed about 50 cm from the phantom
surface, and a value of 72.33% was obtained.
All measurements were taken with a PTW 31003 model ion chamber. The ion chamber-electrometer unit
was calibrated to measure 5.246e+7 Gy/C under labratory conditions. The temperature
in the clinic at the time of measurement was 22.1oC and the pressure was 765.0 mmHg. The
average electrometer readings at bias voltages of 150 V, -150 V, and -300 V are 16.1507 nC,
-16.1311 nC, and -16.1703 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 SSD for this beam at clinical setup is 100 cm)
**All values, unless stated within the problem, should be obtained using the TG-51 report.