Calculation of kQ factors for Farmer-type ionization chambers following the recent recommendations on new key dosimetry data

Purpose: 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. Methods: 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 TPR20,10 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%. Results: 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%. Conclusions: 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.