Application of intensity modulated photon fields in radiation oncology (in German)
Intensity-modulated radiation therapy(IMRT) with a maximum photon energy near 10 MeV can produce highly conformal dose distributions to minimize the dose to normal tissues while delivering a high dose to the planning target volume. IMRT is especially useful for obtaining optimal dose distributions in irregularly shaped tumors which surround radiation-sensitive normal tissues. This thesis describes fixed beam modifiers for IMRT developed for routine clinical use. They consist of individually shaped negative forms, cut into Styrofoam with a computer controlled milling machine, that are then filled with steel powder. The individually designed beam attenuator is mounted on a perspex plate, covered with another perspex plate, and secured in the accessory holder of any medical accelerator. The steel powder does not absorb enough radiation for proper shielding of irregularly shaped radiation fields. The addition of a custom designed cerrobend block permits intensity-modulated photon fields even with older linear accelerators lacking multi-leaf collimation. Several verification procedures for intensity-modulated photon fields are described using a treatment plan for a virtual tumor in an anthropomorphic Alderson–Rando phantom. Beam fluence maps are checked with a video camera based megavoltage imaging system (“BIS 710,” Wellhofer Dosimetrie). Ionization chambers, films and an amorphous silicon prototype megavoltage imaging system (“ADAS,” Wellhofer Dosimetrie) are used to verify the dose distribution delivered by each field. The doses measured at 200 points in the Alderson–Rando phantom with thermoluminescent dosimeters were compared with doses calculated with two treatment planning systems (“KonRad” and “VOXELPLAN,” Cancer Research Centre DKFZ in Heidelberg, Germany). Other dose measuring tools and methods are described.