TG-51 Practice Problems
A medical physicist is performing absolute dose calibration for a 15 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 76.92% was obtained.
All measurements were taken with a NE2561 model ion chamber. The ion chamber-electrometer unit
was calibrated to measure 5.094e+7 Gy/C under labratory conditions. The temperature
in the clinic at the time of measurement was 21.3oC and the pressure was 752.5 mmHg. The
average electrometer readings at bias voltages of 150 V, -150 V, and -300 V are 17.3064 nC,
-17.2970 nC, and -17.3343 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.