title: [Measurement of peak correction factor of Farmer chamber for calibration of flattening filter free (FFF) clinical photon beams].
abstract: Farmer-type ionization chambers are considered the most reliable detectors and for this reason they are most frequently used for the calibration of photon beams of medical linear accelerators. Flattening filter free (FFF) photon beams of linear accelerators have recently started to be used in radiotherapy. The dose profile of FFF beams is peaked in the center of the field and the dose distribution will be inhomogeneous along the axis of the 2.3 cm long measuring volume of the Farmer chamber. The peaked radiation field will result in volume averaging effects in the large Farmer chamber, therefore this chamber will underestimate the true central axis dose. Our objective was to determine the value of the peak correction factor (Kp) of Farmer-type chamber with measurements to avoid the underestimation of the central axis dose during the calibration of FFF radiation fields. Measurements were made with 6 MV and 10 MV flattened (6X and 10X) and FFF beams (6XFFF and 10XFFF) of a Varian TrueBeam medical linear accelerator in a solid water phantom at 10 cm depth. The source surface distance (SSD) was 100 cm, the field size was 10×10 cm and the dose rate was always 400 MU/min during the measurements. We delivered 100 MU in each measurement and the absorbed dose to water was calculated according to the IAEA TRS-398 dosimetry protocol. The measured signals of the ionization chambers were always corrected for the ion recombination loss. The ion recombination correction factors (Kr) were determined with the two-voltage method separately for the used ion chambers and for flattened and unflattened beams. First, we measured the dose to water with PTW TM30012 Farmer chamber in 6XFFF and 6X beams, then calculated the ratio of doses of 6XFFF and 6X beams (R6,Farmer). Immediately after this we repeated the above measurements with PTW TM31010 Semiflex chamber and determined the ratio of doses of 6XFFF and 6X beams again (R6,Semiflex). The length of the sensitive volume of the Semiflex chamber is only 6.5 mm. According to our dose profile measurements the peak correction factor of this chamber equals to unity for both photon energies. As a consequence R6,Semiflex is larger than R6,Farmer and Kp6XFFF = R6,Semiflex / R6,Farmer, where Kp6XFFF is the peak correction factor of the Farmer chamber in 6XFFF beam. The advantage of this method is that we have to calculate ratio of doses, so it is not necessary to know the calibration factors of the chambers. Repeating the above measurements with 10X and 10XFFF beams we determined the peak correction factor of Farmer chamber for 10XFFF beam, too (Kp10XFFF). According to our measurements Kp6XFFF = 1.0025 and Kp10XFFF = 1.009. The bigger peak correction factor for 10XFFF beam is in accordance with the fact that the peak of dose profile is steeper for higher photon energy. The above described method for the determination of Kp can be used for other photon energies and other large volume ionization chambers.
authors.affiliation: Sugárterápiás Központ, Országos Onkológiai Intézet, Budapest, Hungary. kontra@oncol.hu.
authors.forename: Gábor
authors.initials: G
authors.lastname: Kontra
authors.affiliation: Sugárterápiás Központ, Országos Onkológiai Intézet, Budapest, Hungary. kontra@oncol.hu.
authors.forename: Tibor
authors.initials: T
authors.lastname: Major
authors.affiliation: Sugárterápiás Központ, Országos Onkológiai Intézet, Budapest, Hungary. kontra@oncol.hu.
authors.forename: Csaba
authors.initials: C
authors.lastname: Polgár
mesh_terms: D002138:Calibration / Q000592:standards*; D006814:Hungary; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014867:Water
chemical_list: D014867:Water