Dosimetry for 4 MV narrow X-ray beams

In order to concentrate radiation beams to a small target definitely, a new method of convergent radiotherapy with such narrow X-ray beams, less than 20 x 20 mm2 in field size (F.S.), was developed by using 4 MeV linear accelerator. It is essential to know characteristics of the narrow X-ray beams used in this method. Field factor, effects of penumbra and scattered radiation were studied by a film method. Field factor was 0.409, when a 5 x 5 mm2 F.S. of the beam was used. Size of penumbra is determined geometrically, where the penumbra becomes relative large size when the smaller field is used. As the field size is decreased, contribution of the scattered radiation for the beam decreases. (author)