VARIANCE-COVARIANCE MEASUREMENTS IN THERAPEUTIC AND DIAGNOSTIC RADIATION BEAMS

Cylindrical tissue-equivalent twin ionisation chambers have been devel oped based on experience with proportional counters for application in microdosimetric variance-covariance measurements of the dose mean lin eal energy, y(D)BAR, in therapeutic and diagnostic radiation beam qual ities. It is concluded that the method is applicable but the size of t he detector signals has to be optimised very carefully in respect to n oise reduction and the total relative variance. It is proposed that re sults from the proportional mode and the ionisation mode are compared to obtain reliable values for the microdosimetric beam quality charact erisation necessary to assess clinically relevant differences.