LET calculations based on spectral measurements of primary and scattered radiation from radiotherapeutic sources

Spectral measurements have been made on scattered radiation in a water phantom and in a perspex phantom from a kilocurie cobalt-60 unit and a 6-MeV linear accelerator using a NaI crystal spectrometer. For the 60Co radiation, measurements were made for different field sizes and depths in the water phantom. Some filtration of the scattered component with increasing depth was investigated but there seems to be only a small deviation in the photon distribution for the two materials. The measured total photon distributions in a certain depth of the phantom were used for calculations of the initial electron spectra initiated by the photons. These electron spectra were further applied to a calculation of distributions of absorbed dose in LET, using the step-wise method, and of the dose averages in LET for both materials and the fields and depths measured. The results show that 60% of the absorbed dose for 60Co gamma rays had LET values below 1.1 keV/μm for both materials, and that for the 6-MeV X-rays the LET values for the same absorbed dose were below 0.65 keV/μm and 0.8 keV/μm for perspex and water respectively. (author)