A first step toward a new national standard for direct calibrations in clinical high energy photon beams

Purpose: Clinical reference dosimetry for high‐energy photon and electron beams is based on ion‐chamber calibrations in a Co‐60 beam for absorbed dose to water, as prescribed in the TG‐51 protocol. A complex formalism is required when converting the calibration to the beam quality of end users. Direct calibrations in clinical high‐energy x‐ray beams will eliminate the complexity and associated uncertainties, resulting in greater accuracy and measurement simplicity. Method and Materials: The current primary standard for absorbed dose is transferred through ionization chamber measurements in a Co‐60 beam and is ultimately traceable to the first‐generation water calorimeter developed at NIST by Domen. We are evaluating a new calorimeter with modern data acquisition in a Clinac 2100C photon beam at 6 MV and 18 MV. Results: To address the convection/conduction problems inherent to water calorimetry, we have devised a novel data‐collection scheme with multiple cycles of radiation‐induced temperature increase as a function of time; this has greatly improved the efficiency of measurements, thereby achieving greater precision. The temperature rise can be extracted by three approaches that yield consistent results; these will be described in detail in the presentation. The absorbed dose determined at 18 MV and 6 MV using the water calorimeter at selected depth along the beam axis are compared to the depth‐dose curves measured with a calibrated ion chamber following the TG‐51 protocol. In addition, the dependence of bridge‐excitation voltage at the higher dose rate (4 Gy/min) behaves differently from that at a lower rate (1 Gy/min) in a Co‐60 beam; this has been investigated in order to assess the influence of the thermistor power on measured results. Conclusion: We have obtained preliminary results and are moving toward a new national standard for direct calibrations in clinical beams. © 2005, American Association of Physicists in Medicine. All rights reserved.