Monte Carlo calculations of the replacement correction factor, P(repl), for cylindrical chamber cavities in clinical photon and electron beams.

The purpose of this study was to calculate the replacement correction factor, P(repl) (the product P(gr)P(fl) in the AAPM's notation, or the product p(cav)p(dis) in the IAEA's notation), at a reference depth, d(ref), for cylindrical chamber cavities in clinical photon and electron beams by Monte Carlo simulation. P(repl) was calculated for cavities with a combination of various diameters and lengths. P(repl) values calculated in photon and electron beams were typically higher than those recommended by the TG-51 and TRS-398 dosimetry protocols. P(repl) values for a Farmer chamber cavity were higher by 0.3 to 0.2% and by 0.7 to 0.4%, respectively, than data of TG-51 and TRS-398, at photon energies of (60)Co to 18 MV. Similarly, the P(repl) values for electron beams were higher by 1.5 to 1.1% than data for both protocols, in a range of 6-18 MeV. The P(repl) values depended upon the cavity diameter and length, especially for lower electron energies. We found that P(repl) values of cylindrical chamber cavities for photon and electron beams were significantly different from those recommended by TG-51 and TRS-398.