All currently employed dosimetry protocols provide methods for determining the dose-to-water using exposure-calibrated ionization chambers. Because of the nature of ionization chambers, and the complex and not-completely-understood process of converting the ionization of the air in the chamber to dose-to-water, the accuracy of radiation-therapy accelerator calibrations is no better than +or-3%. This level of inaccuracy could have serious implications for RBE determinations for high- versus low-energy photons, and for clinical trials comparing high- and low -LET radiations. The water calorimeter offers the possibility of determining the dose-to-water in absolute terms, i.e., without recourse to radiation-dependent parameters such as G-values, stopping-power ratios, energy to produce an ion pair, etc. At the present time it suffers from two distinct problems: the temperature change per unit absorbed dose is about 3% higher than predicted (an exothermic heat defect) and convection currents are caused by temperature gradients (dose gradients) across the calorimeter. (This latter phenomenon was first observed using the flexible, temperature-regulated water calorimeter developed in this laboratory.) The exothermic reaction is due to as yet undefined radiochemical reactions. Using an all-glass version of our present calorimeter, we propose to study the response of highly-purified water containing accurately-known concentrations of dissolved gases such as nitrogen, oxygen, and hydrogen in order to determine the source of exothermicity. The effect of radical scavengers such as nitrous oxide and iodine will also be investigated as well as the effect of elevated and reduced temperatures. Should it persist, the extent of exothermicity will be accurately documented by comparison with a graphite calorimeter. Convection currents will be studied using high-dose-gradient electron beams. In principle, convection currents should not occur when the water is at 3.98 C, and operation of the calorimeter at this temperature will be evaluated. The use of thin baffles to prevent convection currents will also be tried. Finally, a reference-grade water calorimeter will be constructed utilizing the data obtained from the above studies. This device will be compared with a graphite calorimeter for the calibration of therapy accelerators directly in terms of dose-to-water. It will also be used for the calibration of ionization chambers in terms of dose-to-water for a wide range of x-ray and electron beam energies used in radiation therapy.
| Schulz, R J; Verhey, L J; Huq, M S et al. (1992) Water calorimeter dosimetry for 160 MeV protons. Phys Med Biol 37:947-53 |
| Schulz, R J; Huq, M S; Venkataramanan, N et al. (1991) A comparison of ionization-chamber and water-calorimeter dosimetry for high-energy x rays. Med Phys 18:1229-33 |
| Schulz, R J; Venkataramanan, N; Huq, M S (1990) The thermal defect of A-150 plastic and graphite for low-energy protons. Phys Med Biol 35:1563-74 |
