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Effect of ICRU report 90 recommendations on Monte Carlo calculated k Q for ionization chambers listed in the Addendum to AAPM’s TG‐51 protocol
Author(s) -
Kawachi Toru,
Saitoh Hidetoshi,
Katayose Tetsurou,
Tohyama Naoki,
Miyasaka Ryohei,
Cho Sang Yong,
Iwase Tsutomu,
Hara Ryusuke
Publication year - 2019
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.13743
Subject(s) - addendum , monte carlo method , dosimetry , nuclear medicine , physics , medical physics , statistical physics , nuclear physics , medicine , mathematics , statistics , political science , law
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, 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 60 Co 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 60 Co 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 60 Co 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 60 Co 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.