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 30 cm from the phantom
surface, and a value of 73.43% was obtained.
All measurements were taken with a PTW N30004 model ion chamber. The ion chamber-electrometer unit
was calibrated to measure 4.946e+7 Gy/C under labratory conditions. The temperature
in the clinic at the time of measurement was 23.1oC and the pressure was 757.8 mmHg. The
average electrometer readings at bias voltages of 150 V, -150 V, and -300 V are 14.2186 nC,
-14.2252 nC, and -14.2401 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.