Three-dimensional tumor dosimetry for hepatic yttrium-90-microsphere therapy.

The nonuniformity of dose deposition for hepatic 90Y-microsphere therapy is believed to play an important role in the relative sparing of normal liver tissues. To help study this issue, three-dimensional dose calculations have been performed for the VX2 tumor model in the rabbit treated with hepatic arterial administration of 90Y-glass microspheres (90Y-MS). Colored, nonactivated spheres of similar size to 90Y-MS were injected into the hepatic artery to mimic the treatment deposition of 90Y-MS. Sample blocks of treated liver were serially sectioned (200 microns thickness), fixed and photographed showing the position of the colored microspheres. The microsphere positions were digitized into a three-dimensional treatment planning system, and three-dimensional dose calculations were performed. A 2-mm diameter liver tumor nodule receiving 15 times more microspheres than nearby normal liver resulted in tumor-to-normal-tissue (TNT) calculated dose ratios of 2.6 (average dose) and 1.9 (minimum dose). The nonuniform microsphere distribution resulted in a dose gradient over the nodule with a minimum value which was less than one half the average dose. The relative dose deposition in the vicinity of the tumor nodule does not fully reconcile the known liver tolerance dose derived from uniform irradiation with the large calculated average doses tolerated with this type of therapy.