Effect of ICRU report 90 recommendations on Monte Carlo calculated kQ for ionization chambers listed in the Addendum to AAPM's TG-51 protocol

Purpose The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (s(w, air) and s(g, air)) and the individual perturbation correction factors P-i was calculated. The effect on the beam quality conversion factors k(Q) for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol. Methods The s(w, air), s(g, air), individual P-i,P- and k(Q) were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the P-i and k(Q) were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one Co-60 beam and ten photon beams with nominal energy between 4 MV and 25 MV. Then, the change in k(Q) for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of Co-60 beam and 24 MV photon beam were used. Results The significant changes (p < 0.05) were observed for s(w, air), s(g, air), the wall correction factor P-wall, and the waterproofing sleeve correction factor P-sleeve. The decrease in s(w, air) varied from -0.57% for a Co-60 beam to -0.36% for the highest beam quality. The decrease in s(g, air) varied from -0.72% to -1.12% in the same range. The changes in P-wall and P-sleeve were up to 0.41% and 0.14% and those maximum changes were observed for the Co-60 beam. All changes in the central electrode correction factor P-cel, the stem correction factor P-stem, and the replacement correction factor P-repl were from -0.02% to 0.12%. Those changes were statistically insignificant (p = 0.07 or more) and were independent of photon energy. The change in k(Q) was mainly characterized by the change in s(w, air), P-wall, and P-sleeve. The relationship between the change in k(Q) and the beam quality index was linear approximately. The changes in k(Q) of the simple-models were agreed with those of the precise-models within 0.08%. Conclusion The effects of ICRU-90 recommendations on k(Q) for the ionization chambers listed in the Addendum to AAPM's TG-51 protocol were from -0.15% to 0.30%. To remove the known systematic effect on the clinical reference dosimetry, the k(Q) based on ICRU-37 should be updated to the k(Q) based on ICRU-90.