Dosimetric analysis on narrow 6 MV X ray beams

The absorbed dose distribution of the small beams used in stereotactic radiotherapy has been measured. In a stereotactic treatment the target position is always centred on the accelerator isocenter. Therefore, when the absolute dose of the reference field size is known, only three parameters are essential to get a complete dosimetric determination: the tissue-maximum ratio, the output factor and the off-axis ratio. In this paper are reported the 6 MV dosimetric data of the above mentioned parameters for a series of circular collimators whose diameter ranges from 10 mm to 33 mm. Each parameter was obtained varying the measurement conditions and using different dosimeters: diodes, films and ionization chambers