Small field in‐air output factors: The role of miniphantom design and dosimeter type. Issue 2 (24th January 2014)

• Record Type: Journal Article
• Authors: Warrener, Kirbie; Hug, Benjamin; Liu, Paul; Ralston, Anna; Ebert, Martin A.; McKenzie, David R.; Suchowerska, Natalka
• Is Part Of: Medical physics. Volume 41:Issue 2(2014)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2014-01-24
• Subjects: Dosimetry/exposure assessment -- Biomedical instrumentation and transducers, including micro‐electro‐mechanical systems (MEMS) -- Therapeutic applications -- Monte Carlo simulations -- Fiber‐optic instruments -- Other fiber‐optical devices; biomedical equipment -- collimators -- dosimeters -- linear accelerators -- Monte Carlo methods -- optical fibres -- phantoms -- radiation therapy; fibre optic dosimeter -- diode -- film -- miniphantom -- Sc -- in‐air output ratio; Radiation therapy -- Dosimeters -- Using diaphragms, collimators -- Linear accelerators -- Scintigraphy; Field size -- Photons -- Scintillation detectors -- Monte Carlo methods -- Dosimetry -- Particle beam detectors -- Ionization chambers -- Alloys -- Multileaf collimators -- Radiosurgery; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4861710
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: : The commissioning of treatment planning systems and beam modeling requires measured input parameters. The measurement of relative output in‐air, Sc is particularly difficult for small fields. The purpose of this study was to investigate the influence of miniphantom design and detector selection on measured Sc values for small fields and to validate the measurements against Monte Carlo simulations. Methods: : Measurements were performed using brass caps (with sidewalls) or tops (no sidewalls) of varying heights and widths. The performance of two unshielded diodes (60012 and SFD), EBT2 radiochromic film, and a fiber optic dosimeter (FOD) were compared for fields defined by MLCs (5–100 mm) and SRS cones (4–30 mm) on a Varian Novalis linear accelerator. Monte Carlo simulations were performed to theoretically predict Sc as measured by the FOD. Results: : For all detectors, Sc agreed to within 1% for fields larger than 10 mm and to within 2.3% for smaller fields. Monte Carlo simulation matched the FOD measurements for all size of cone defined fields to within 0.5%. Conclusions: : Miniphantom design is the most important variable for reproducible and accurate measurements of the in‐air output ratio, Sc, in small photon fields (less than 30 mm). Sidewalls are not required for fields ≤ 30 mm and tops are therefore preferred over the larger caps. Unlike output measurements in water, Scp, the selection of detector type for Sc is not critical, provided the active dosimeter volume is small relative to the field size.