Variance-covariance measurements of the dose mean lineal energy in beams for radiotherapy

An experimental arrangement to perform variance-covariance measurements, based on tissue-equivalent proportional counters, is described. Results in a narrow beam geometry for 8 and 16 MV X rays from a medical linear accelerator are given and compared with data from the literature. It is concluded that the method is applicable but the detectors have to be changed or modified before measurements in normal therapeutic beams at the isocentre of the accelerator or in phantoms can be performed for beam quality characterisation in radiotherapy dosimetry. (author)