Calculation of k_Q factors for Farmer-type ionization chambers following the recent recommendations on new key...

To calculate by Monte Carlo simulations k_Q factors for Farmer-type ionization chambers in megavoltage photon beams using the new key dosimetry data recommended by the International Commission on Radiation Units and Measurements (ICRU) Report 90.

Monte Carlo calculations were performed with the EGSnrc code system using both the ICRU 90 and the ICRU 37 data. Farmer-type ionization chambers with graphite and plastic walls and with graphite wall and a plastic waterproofing sleeve were considered (Nuclear Enterprise NE 2571, IBA FC65-G and FC65-P). k_Q factors were calculated for photon beams in the range 6-25 MV using phase-space files as input radiation sources. The photon beam qualities in terms of TPR_20,10 and %dd(10)_x were established by simulating the depth-dose curves in water. Absorbed doses to the air cavity and to water were calculated using the egs_chamber user code with a target statistical uncertainty below 0.1%.

The update of key dosimetry data according to the ICRU report 90 had an impact of -0.2% in the absorbed dose to water and up to 0.5% in the absorbed dose to the air cavity. Nevertheless, changes partially offset each other when entering in k_Q as ratio, and the final impact on the k_Q values was below 0.3%.

The calculated values of k_Q tend to be lower than the current values in the IAEA TRS-398 protocol with differences up to about 0.5%. A slightly better agreement (within 0.3%) is observed with the Monte-Carlo calculated values provided by the addendum to the AAPM's TG-51 protocol.