抄録
The narrow beam dosimetry for stereotactic irradiation contains several factors of uncertainty due to geometric errors such as inadequate size of dosimeter, rapid change in dose distribution and so on. These errors cannot easily be minimized under the usual process of dosimetry. However, with the Monte Carlo simulation, it is possible to estimate the dosimetric parameters in a region where conventional dosimetry is quite difficult to put into practice and to keep these geometrical errors to a minimum.
In this study, we attempted to calculate the dose distribution at several depths and transverse directions in tissue and the field factor by the Monte Carlo simulation.
The dose distribution indicated that the tissue peak ratio is almost independent of the size of detection volume in the horizontal direction (detection size), and the ratio varies greatly with position in the irradiation field. The transverse dose distribution indicated rapid change on the off center ratio. Thus, it was considered necessary to determine the central axis to a geometric accuracy of 1 mm or less. We calculated the ideal absorbed dose at the central axis for the detection size of 0 cm φ and obtained the field size and the detection size with which secondary electron equilibrium is established. It was also shown that a method with a reference point at a maximum dose is better than are having a reference depth fixed by incidence photon energy to obtain the field factor.
The fundamental characteristics of a narrow beam extremely much variable compared with those of a broad beam, and for this reason it is emphasized that these characteristics shoud be defined precisely at the beginning of dosimetric the process.
