title: [Bicentric stereotaxic convergent-beam irradiation].
abstract: OBJECTIVE
  Circular collimated X-ray beams rotated in various non-coplanar planes as commonly used in stereotactic linac-based radiotherapy are generating spherical dose distributions with typically steep dose gradients. In case of irregular, especially elongated lesions, this technique fails to conform the shape of the corresponding planning target volume, and a collimator aperture has to be selected which allows for an acceptable dose distribution only on the expense of an excess dose to a larger amount of normal tissue. Therefore we have investigated a bicentric irradiation technique with sequentially treating an elongated target volume over two separated isocenters.
  
  
  METHODS
  For the computer simulation studies we used the planning system of the stereotactic unit SRS-200 by Philips. Selected combinations of different collimator aperture were examined systematically by varying the distance between the two isocenters. Dose distributions for a specially developed head-phantom were calculated and analysed.
  
  
  RESULTS
  Optimization tables were generated taking into account the geometric quality criteria as well as the corresponding dose maxima. For a given size of the target volume, the appropriate parameters for the bicentric radiation treatment can be chosen. The calculated dose distributions turned out to be independent of the positioning of the two isocenters within the phantom.
  
  
  CONCLUSIONS
  The optimization tables serve to select a set of starting values for the interactive planning process of an elongated target volume resulting in a significant decrease of the overall time for treatment planning of irregular shaped lesions.
authors.affiliation: Abteilung für Medizinische Physik, Radiologische Universitätsklinik Tübingen.
authors.forename: M
authors.initials: M
authors.lastname: Bischof
authors.forename: J
authors.initials: J
authors.lastname: Major
authors.forename: G
authors.initials: G
authors.lastname: Christ
authors.forename: G
authors.initials: G
authors.lastname: Becker
authors.forename: B
authors.initials: B
authors.lastname: Schlattinger
authors.forename: F
authors.initials: F
authors.lastname: Nüsslin
mesh_terms: D005373:Film Dosimetry; D006257:Head; D006801:Humans; D008961:Models, Structural; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D012984:Software; D013997:Time Factors