Accuracy of a dose‐area product compared to an absorbed dose to water at a point in a 2 cm diameter field. Issue 7 (10th June 2016)

• Record Type: Journal Article
• Authors: Dufreneix, S.; Ostrowsky, A.; Rapp, B.; Daures, J.; Bordy, J. M.
• Is Part Of: Medical physics. Volume 43:Issue 7(2016)
• Page Start: 4085
• Page End: 4092
• Publication Date: 2016-06-10
• Subjects: biomedical measurement -- calibration -- calorimeters -- diamond -- dosimeters -- dosimetry -- graphite -- thin film devices -- thin films; Dose‐volume analysis -- Biomedical instrumentation and transducers, including micro‐electro‐mechanical systems (MEMS) -- Standards and calibration -- Calorimeters -- Dosimetry/exposure assessment; Measuring for diagnostic purposes; Identification of persons -- Testing or calibrating of apparatus or arrangements provided for in groups G01D1/00 to G01D15/00 -- Measuring temperature; Measuring quantity of heat; Thermally‐sensitive elements not otherwise provided for -- Biological material, e.g. blood, urine; Haemocytometers -- Dosimeters -- Calibrating of instruments or apparatus -- Scintigraphy; small fields -- dose‐area product -- EBT3 -- dosimetric references; Dosimetry -- Calorimeters -- Calibration -- Graphite -- Diamond -- Collimators -- Ionization chambers -- Photons -- Elemental semiconductors; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4953207
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: Graphite calorimeters with a core diameter larger than the beam can be used to establish dosimetric references in small fields. The dose‐area product (DAP) measured can theoretically be linked to an absorbed dose at a point by the determination of a profile correction. This study aims at comparing the DAP‐based protocol to the usual absorbed dose at a point protocol in a 2 cm diameter field for which both references exist. Methods: Two calorimeters were used, respectively, with a sensitive volume of 0.6 cm (for the absorbed dose at a point measurement) and 3 cm diameter (for the DAP measurement). Profile correction was calculated from a 2D dose mapping using three detectors: a PinPoint chamber, a synthetic diamond, and EBT3 films. A specific protocol to read EBT3 films was implemented and the dose‐rate and energy dependences were studied to assure a precise measurement, especially in the penumbra and out‐of‐field regions. Results: EBT3 films were found independent on dose rates over the range studied but showed a strong under‐response (18%) at low energies. Depending on the dosimeter used for calculating the profile correction, a deviation of 0.8% (PinPoint chamber), 0.9% (diamond), or 1.9% (EBT3 films) was observed between the calibration coefficient derived from DAP measurements and the one directly established in terms of absorbed dose to water at a point. Conclusions: The DAP method can currently be linked to the classical dosimetric reference system based in an absorbed dose at a point only with a confidence interval of 95% ( k = 2). None of the detectors studied can be used to determine an absorbed dose to water at a point from a DAP measurement with an uncertainty smaller than 1.2%.