Calculation of kQ factors for Farmer-type ionization chambers following the recent recommendations on new key dosimetry data. (January 2019)

• Record Type: Journal Article
• Authors: Pimpinella, Maria; Silvi, Luca; Pinto, Massimo
• Is Part Of: Physica medica. Volume 57(2019)
• Page Start: 221
• Page End: 230
• Publication Date: 2019-01
• Subjects: Beam quality correction factor -- kQ -- ICRU 90 -- Monte-Carlo calculated kQ -- Radiotherapy dosimetry; Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2018.12.006
• Languages: English
• ISSNs: 1120-1797

Abstract

Highlights: kQ factors are calculated by Monte Carlo using up-to-date basic data. The influence of Monte Carlo simulation modality on kQ values is investigated. Impact of ICRU 90 recommendations on kQ calculation is evaluated. Up-to-date MC kQ values are compared with values provided by dosimetry protocols. Abstract: Purpose: To calculate by Monte Carlo simulations kQ 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). kQ 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 kQ as ratio, and the final impact on the kQ values was below 0.3%. Conclusions: The calculated values of kQ 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.