Accuracy of a dose-area product compared to an absorbed dose to water at a point in a 2 cm diameter field
Résumé
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%. (C) 2016 American Association of Physicists Medicine.
Mots clés
small fields
dose-area product
EBT3 films
dosimetric references
RADIOCHROMIC FILM DOSIMETRY
Monte Carlo
DOSIMETRY
RELATIVE DOSIMETRY
DIAMOND DOSIMETER
BEAMS
EBT
RADIOTHERAPY
instrumentation
nuclear instrumentation
radioactivity
ionizing radiation
metrology
calorimeter
diamond
PinPoint chamber
out-of-field dose
calibration
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