Accuracy of a dose‐area product compared to an absorbed dose to water at a point in a 2 cm diameter field

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%.