Determination of field size limitations in stereotactic and intensity modulated radiotherapy

For dose measurement in small photon fields, different detectors are currently in use: TLD, semiconductor, diamond-detector, film, etc. But for absolute dosimetry, ionization chambers show the most advantages. To meet the basic dosimetrical requirements for lateral electron equilibrium the field size F must not remain under specified values: i.e. 5.2 x 5.2 cm2 for 15 MeV X-bremsstrahlung. As well as increasing the focus-chamber-distance, changing the physical density of the build-up material in the close vicinity of the chamber will be helpful to determine the output factor OF for smaller fields. By means of a correction factor, k(mat), this is taken into account. For a 6 MeV X and a 15 MeV X-bremsstrahlung of linear accelerators the lower limit of the field size F is determined: F > or = 0.8 cm. This value is mainly dependent on the diameter of the focal-spot (phi = 3 mm) of the treatment unit including design characteristics of the treatment head. Beside the dosimetrical aspects, some geometrical parameters have to be considered, when accuracy of dose application should remain on the same level as in medium and large field treatment (4 cm < or = F < or = F(max)). To keep dose-volume errors as low as +/- 10 % (diameter of PTV: 20 mm), the mean total error delta of CT-scanning (delta(p)), planning (delta(pl)), patient positioning (delta(x)), and treatment unit instabilities (delta(m)) should not exceed +/- 0.8 mm.