Technical Note: Correction for intra‐chamber dose gradients in reference dosimetry of flattening‐filter‐free MV photon beams. Issue 8 (21st July 2016)

• Record Type: Journal Article
• Authors: Ruggieri, Ruggero; Naccarato, Stefania; Stavrev, Pavel; Stavreva, Nadejda; Pasetto, Stefano; Salamone, Ignazio; Alongi, Filippo
• Is Part Of: Medical physics. Volume 43:Issue 8(2016)Part 1
• Page Start: 4729
• Page End: 4733
• Publication Date: 2016-07-21
• Subjects: dosimetry -- ionisation chambers; Dose‐volume analysis; Scintigraphy; reference dosimetry -- Farmer ion‐chamber -- volume averaging -- flattening‐filter‐free; Dosimetry -- Photons -- Linear accelerators -- Integrated circuits -- Ion beams -- Hydrological cycles -- Error analysis -- Electric measurements -- Addendum -- Ionization chambers; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4958960
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: To estimate and correct for the volume averaging effect which results from the intra‐chamber dose gradients when a Farmer ionization chamber (IC) is used for reference dosimetry in flattening‐filter‐free (FFF) MV photon beams. Methods: An intra‐chamber dose gradients correction factor ( k icdg ) to the charge reading of a Farmer IC is estimated by comparison to a small volume IC (∼0.1 cm 3 ), in the FFF beams of a TrueBeam™ (Varian, Inc.) linear accelerators. An independent estimate of the correction for the volume averaging effect ( p vol ) is deduced using the ratio of the active length ( L ) of the Farmer IC to the integral of a high‐resolution FFF radial dose profile over this same length. Results: Mean (sd) values for k icdg equal to 1.0025 (0.0025) for 6 MV‐FFF, and equal to 1.0057 (0.0025) for 10 MV‐FFF, were estimated based on four dosimetry sessions, performed in a time interval of six months. Similarly, p vol Farmer equal to 1.0030 (0.0003) for 6 MV‐FFF, and equal to 1.0063 (0.0005) for 10 MV‐FFF, were computed. Conclusions: The systematic bias which results from intra‐chamber dose gradients when a Farmer IC is used for reference dosimetry in FFF MV photon beams is estimated to be −0.6% for 10 MV‐FFF, and −0.3% for 6 MV‐FFF, based on the obtained values of the factor k icdg . This bias can be corrected, within 0.1%, by the simple measure of p vol at the beginning of the dosimetry session.