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TU‐D‐BRB‐02: Advanced Techniques to Determine Plan‐Class Specific Reference Field Correction Factors for Accurate Dosimetry of Nonstandard Beams
Author(s) -
Chung E,
Bouchard H,
Sutherland J,
Seuntjens J
Publication year - 2009
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 - 10.1118/1.3182368
Subject(s) - dosimetry , dosimeter , imaging phantom , ionization chamber , materials science , physics , absorbed dose , dose profile , nuclear medicine , optics , medical physics , ionization , medicine , ion , quantum mechanics
Purpose: To establish experimental reference dosimetry techniques for measuring plan‐class specific reference field correction factors k Q pcsr, Qf pcsr, f reffor the proposed new formalism for reference dosimetry in nonstandard fields. Methods and Materials: A Lucite cylindrical phantom filled with water was constructed in the center of which reference absorbed dose to water for an IMRT delivery was measured. A plan‐class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on CT images of the phantom. The absorbed dose in the pcsr field normalized to that in a 10×10 cm 2 field was measured using three reference dosimeters: Gafchromic® EBT films, a diamond detector, and an in‐house developed guarded liquid ionization chamber (GLIC‐03). Pcsr correction factors k Q pcsr, Qf pcsr, f refwere determined for five air‐filled ionization chambers (Exradin A1SL, Exradin A12, Exradin A14, PinPoint® 31006, and NE2571) in a fully‐rotated delivery and in a delivery from a single angle (collapsed delivery). Results: The relative dose measurement accuracy of the three dosimeters was 0.56%, 0.10%, and 0.29% for the film, diamond detector, and GLIC‐03, respectively. The combined relative standard uncertainty in measuring k Q pcsr, Qf pcsr, f refusing the three techniques was 0.3%. For all chambers and the pcsr field selected, k Q pcsr, Qf pcsr, f refwas unity to within ±1% and in the range of 0.990–0.993 and 0.990–1.003 in the fully‐rotated and collapsed deliveries, respectively. The correction factors were the same for the chambers in the fully‐rotated delivery. In the collapsed delivery, the Farmer‐type chambers (Exradin A12 and NE2571) had a larger but consistent correction factor (0.990 and 0.991, respectively). The correction factors for the smaller chambers were close to unity but showed chamber type dependence. Conclusions: Our techniques provide a potential to improve the dosimetric accuracy in suitable plan‐class specific reference fields. The techniques of determining the correction factor k Q pcsr, Qf pcsr, f refwill be extremely valuable for other nonstandard field deliveries.