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Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields
Author(s) -
Chung Eunah,
Bouchard Hugo,
Seuntjens Jan
Publication year - 2010
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 - 10.1118/1.3392247
Subject(s) - dosimetry , imaging phantom , ionization chamber , absorbed dose , physics , dose profile , detector , nuclear medicine , percentage depth dose curve , materials science , optics , ionization , radiation , medicine , ion , quantum mechanics
Purpose To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields. Methods A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [Alfonso et al. , Med. Phys. 35, 5179–5186 (2008)], a candidate plan‐class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10 × 10cm 2field was measured using three radiation detectors: Gafchromic ® EBT films, a diamond detector, and a guarded liquid‐filled ionization chamber developed in‐house (GLIC‐03). Pcsr correction factorsk Q pcsr , Qf pcsr , f refwere determined for five different types of air‐filled ionization chambers (Exradin A12, NE2571, Exradin A1SL, Exradin A14, and PinPoint ® 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery). Results The combined standard uncertainty in measuring the correction factork Q pcsr , Qf pcsr , f refusing the three dosimetry techniques was 0.3%. For all the air‐filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than ±0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955–0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922–1.0048. In the collapsed delivery, the Farmer‐type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber‐type dependence. Conclusions The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factorsk Q pcsr , Qf pcsr , f reffor air‐filled ionization chambers at the 0.3% 1 σ uncertainty level. These techniques can be used to determine criteria for the selection of plan‐class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.