A Comparison of Microdosimetric Measurements with Spherical Proportional Counters and Solid-State Detectors

Microdosimetric spectra for range-modulated negative pion beams have been measured by two methods. Data were obtained with a Rossi-type tissue-equivalent spherical proportional counter of 1.3 cm radius and at an equivalent diameter of 2 ..mu..m in unit density tissue. Corresponding measurements were performed with a 7-..mu..m-thick, 0.1-cm-radius lithium-drifted silicon detector. A comparison is warranted since both techniques are being used in clinical trials to characterize the radiation quality of the beams. A comparison of the results shows significant differences in both the absolute magnitudes and shapes of the lineal energy spectra. Monte Carlo calculations indicate that these differences result primarily from the different diameters of the two detectors as well as from their different thicknesses. Attempts to unfold LET distributions from the silicon detector data using conventional algorithms prove unsatisfactory because of the large fraction of events from particles which start or stop in the detector. The relative merits of the two techniques are discussed in relation to the present use of both methods in connection with therapeutic programs.