IMPLEMENTATION OF A 3-DIMENSIONAL COMPENSATION SYSTEM BASED ON COMPUTED-TOMOGRAPHY GENERATED SURFACE CONTOURS AND TISSUE INHOMOGENEITIES

A computed tomography (CT) based system that compensates for patient s urface contour an internal tissue inhomogeneity was implemented in our clinic. The compensators are fabricated with a mixture of tin granule s and bee's wax. The tin/wax mixture was optimized for tin granule siz e and tin granule to wax ratio. The narrow beam attenuation coefficien ts were measured for 4-, 6-, 10-, and 24-MV photon beams. The compensa tor design and fabrication methodology were verified by measuring the dose distribution for a known surface contour irradiated with a compen sated beam and for a known inhomogeneity that was submerged in a water phantom and irradiated with a compensated beam. For the surface conto ur, the uncompensated isodose levels varied by as much as 10% in the c ompensation plane and the compensator restored the isodose level to a variation of less than 1.3%. Measured and calculated doses for this su rface contour were found to differ by less than 3.4%. For the inhomoge neity, the uncompensated isodose levels varied by 27% in the compensat ion plane and the compensator restored the isodose level to a variatio n of less than 1.5%. Measured and calculated doses for the known inhom ogeneity were found to differ by less than 2%. Measurements of depth-d ose curves indicate that the presence of the compensator in the beam d oes not significantly increase the surface dose. Twenty-six compensato rs have now been fabricated for clinical cases. In these patients, dos e variations as great as 19% occurred in the plane of compensation pri or to placing the compensator in the beam. Measured and calculated dos e profiles with the compensators in place have been found to agree wit hin 2.3%.