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TH‐C‐BRB‐06: Advanced Dosimetry Techniques for Accurate Dose Measurement of Small and Nonstandard Fields
Author(s) -
Chung E,
Soisson E,
Bouchard H,
Seuntjens J
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 - 10.1118/1.3469490
Subject(s) - dosimetry , ionization chamber , imaging phantom , physics , tomotherapy , nuclear medicine , linear particle accelerator , absorbed dose , dose profile , medical physics , materials science , optics , ionization , radiation therapy , beam (structure) , medicine , radiology , ion , quantum mechanics
Purpose : To establish reference dosimetry techniques for accurate dose measurement of small and nonstandard fields and application to nonstandard field deliveries. Methods and Materials : A cylindrical PMMA phantom filled with water was constructed in the center of which reference absorbed dose to water was measured. Two candidate plan‐class specific reference (pcsr) fields for (i) linac‐based and (ii) TomoTherapy®‐based typical head and neck IMRT deliveries were created on the CT images of the phantom. The absorbed dose in each pcsr field normalized to that in a 10×10 cm 2 was measured using four reference detectors: Gafchromic® EBT films a diamond detector and an in‐house developed guarded liquid ionization chamber (GLIC‐03) for the linac‐based IMRT delivery and a PTW microLion chamber for the TomoTherapy®‐based IMRT delivery. Based on the new dosimetry formalism pcsr correction factors k Q pcsr , Q f pcsr , f ref were determined for five air‐filled ionization chambers: Exradin A12 NE2571 Exradin A1SL Exradin A14 and PinPoint® 31006. The correction factor measurements were carried out in fully‐rotated and collapsed deliveries for the linac‐based IMRT delivery and only in a fully‐rotated delivery for the TomoTherapy®‐based IMRT delivery. Results : For the linac‐based IMRT delivery the evaluated overall uncertainty in measuring k Q pcsr , Q f pcsr , f ref was 0.3 %. The k Q pcsr , Q f pcsr , f ref is chamber independent within uncertainty (0.9955–0.9986) and systematically smaller than unity in the fully‐rotated delivery. In the collapsed delivery the correction factor was more dependent on the chamber type (0.9922–1.0048). For the TomoTherapy®‐based IMRT delivery k Q pcsr , Q f pcsr , f ref was above unity (1.0037–1.0076) with a 0.3 % measurement uncertainty. However the correction factor was different by 0.33 % between the Farmer‐type chambers and the smaller ionization chambers. Conclusions : The demonstrated dosimetry techniques carried out the relative dose measurements in the pcsr fields to within 0.3 % 1 uncertainty level. These dosimetry techniques will be helpful to improve dosimetric accuracy of other nonstandard field deliveries.