Ion recombination and polarity corrections for small‐volume ionization chambers in high‐dose‐rate, flattening‐filter‐free pulsed photon beams. Issue 2 (2nd February 2017)

• Record Type: Journal Article
• Authors: Hyun, Megan A.; Miller, Jessica R.; Micka, John A.; DeWerd, Larry A.
• Is Part Of: Medical physics. Volume 44:Issue 2(2017)
• Page Start: 618
• Page End: 627
• Publication Date: 2017-02-02
• Subjects: flattening‐filter‐free beams -- ionization chambers -- polarity -- recombination -- reference dosimetry; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1002/mp.12053
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: To investigate ion recombination and polarity effects in scanning and microionization chambers when used with digital electrometers and high‐dose‐rate linac beams such as flattening‐filter‐free (FFF) fields, and to compare results against conventional pulsed and continuous photon beams. Methods: Saturation curves were obtained for one Farmer‐type ionization chamber and eight small‐volume chamber models with volumes ranging from 0.01 to 0.13 cm 3 using a Varian TrueBeam™ STx with FFF capability. Three beam modes (6 MV, 6 MV FFF, and 10 MV FFF) were investigated, with nominal dose‐per‐pulse values of 0.0278, 0.0648, and 0.111 cGy/pulse, respectively, at d max . Saturation curves obtained using the Theratronics T1000 60 Co unit at the UWADCL and a conventional linear accelerator (Varian Clinac iX) were used to establish baseline behavior. Jaffé plots were fitted to obtain P ion, accounting for exponential effects such as charge multiplication. These values were compared with the two‐voltage technique recommended in TG‐51, and were plotted as a function of dose‐per‐pulse to assess the ability of small‐volume chambers to meet reference‐class criteria in FFF beams. Results: Jaffé‐ and two‐voltage‐determined P ion values measured for high‐dose‐rate beams agreed within 0.1% for the Farmer‐type chamber and 1% for scanning and microionization chambers, with the exception of the CC01 which agreed within 2%. With respect to ion recombination and polarity effects, the Farmer‐type chamber, scanning chambers and the Exradin A26 microchamber exhibited reference‐class behavior in all beams investigated, with the exception of the IBA CC04 scanning chamber, which had an initial recombination correction that varied by 0.2% with polarity. All microchambers investigated, with the exception of the A26, exhibited anomalous polarity and ion recombination behaviors that make them unsuitable for reference dosimetry in conventional and high‐dose‐rate photon beams. Conclusions: The results of this work demonstrate that recombination and polarity behaviors seen in conventional pulsed and continuous photon beams trend accordingly in high‐dose‐rate FFF linac beams. Several models of small‐volume ionization chambers used with a digital electrometer have been shown to meet reference‐class requirements with respect to ion recombination and polarity, even in the high‐dose‐rate environment. For such chambers, a two‐voltage technique agreed well with more rigorous methods of determining P ion . However, the results emphasize the need for careful reference detector selection, and indicate that ionization chambers ought to be extensively tested in each beam of interest prior to their use for reference dosimetry.