Electron dosimetry in radiotherapy

The reports on electron dosimetry which have been published by ICRU and JARP have no data which are sufficient for low energy electron dosimetry ((Esub(p))0<10 MeV) by the thimble chamber. This report is concerned with practical electron dosimetry, especially in this energy range. 1) The relative percent depth ionization I(x) can be represented by the relation: I(x)=110 - 10eμx not only in the region deeper than the peak point but also in the build up region, where x is the distance from the depth at which I(x) equals 100 per cent. An experimental coefficient μ and the peak depth vary with an initial energy ((Esub(p))0), Scattering foil Skin Distance (SSD) and field size. 2) Csub(e) values for electron energies less than 10 MeV were obtained by comparing the relative depth ionizations measured using the thimble chamber with the relative depth doses measured using the Fricke dosimeter. In this experiment, the authors used the value recomended by JARP as the reference point (1 cm depth). 3) The relative depth doses measured with the thimble chamber were in agreement with values obtained by TLD within about 3%. 4) The relative surface dose of this device was measured with the Fricke dosimeter and TLD. The dose increased with the increase of the (Esub(p))0 and with the field size. (auth.)