title: Implementation of a three-dimensional compensation system based on computed tomography generated surface contours and tissue inhomogeneities.
abstract: A computed tomography (CT) based system that compensates for patient surface contour and internal tissue inhomogeneity was implemented in our clinic. The compensators are fabricated with a mixture of tin granules and bee's wax. The tin/wax mixture was optimized for tin granule size and tin granule to wax ratio. The narrow beam attenuation coefficients were measured for 4-, 6-, 10-, and 24-MV photon beams. The compensator design and fabrication methodology were verified by measuring the dose distribution for a known surface contour irradiated with a compensated beam and for a known inhomogeneity that was submerged in a water phantom and irradiated with a compensated beam. For the surface contour, the uncompensated isodose levels varied by as much as 10% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.3%. Measured and calculated doses for this surface contour were found to differ by less than 3.4%. For the inhomogeneity, the uncompensated isodose levels varied by 27% in the compensation plane and the compensator restored the isodose level to a variation of less than 1.5%. Measured and calculated doses for the known inhomogeneity were found to differ by less than 2%. Measurements of depth-dose curves indicate that the presence of the compensator in the beam does not significantly increase the surface dose. Twenty-six compensators have now been fabricated for clinical cases. In these patients, dose variations as great as 19% occurred in the plane of compensation prior to placing the compensator in the beam.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: University of Wisconsin Hospital and Clinics, Department of Human Oncology, Madison 53792.
authors.forename: P A
authors.initials: PA
authors.lastname: Jursinic
authors.forename: M B
authors.initials: MB
authors.lastname: Podgorsak
authors.forename: B R
authors.initials: BR
authors.lastname: Paliwal
mesh_terms: D000818:Animals; D001516:Bees; D006801:Humans; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods; D014001:Tin; D014057:Tomography, X-Ray Computed; D014885:Waxes
chemical_list: D014885:Waxes; D014001:Tin