B. N. Gray

Dose distribution following selective internal radiation therapy

Selective Internal Radiation Therapy is the intrahepatic arterial injection of microspheres labelled with 90Y. The microspheres lodge in the precapillary circulation of tumor resulting in internal radiation therapy. The activity of the 90Y injected is managed by successive administrations of labelled microspheres and after each injection probing the liver with a calibrated beta probe to assess the dose to the superficial layers of normal tissue. Predicted doses of 75 Gy have been delivered without subsequent evidence of radiation damage to normal cells. This contrasts with the complications resulting from doses in excess of 30 Gy delivered from external beam radiotherapy. Detailed analysis of microsphere distribution in a cubic centimeter of normal liver and the calculation of dose to a 3-dimensional fine grid has shown that the radiation distribution created by the finite size and distribution of the microspheres results in an highly heterogeneous dose pattern. It has been shown that a third of normal liver will receive less than 33.7% of the dose predicted by assuming an homogeneous distribution of 90Y.

Tolerance of the liver to the effects of yttrium-90 radiation

There are no reliable data documenting the tolerance of the human liver to ionizing radiation from a continuous Yttrium-90 source. As Yttrium-90 incorporated into microspheres is being used to treat patients with liver cancer, it is imperative that the tolerance of the human liver to this form of radiation damage be determined. Four patients with metastatic liver cancer were treated with Yttrium-90 to deliver radiation doses above that considered tolerable when given by conventional external sources. Patients were monitored with serial estimations of liver function tests and between 7 and 9 months after treatment liver biopsies were performed. Histological examination of the liver biopsies confirmed only minimal changes in the normal liver parenchyma. These data indicate that the human liver may tolerate relatively large radiation doses when delivered by Yttrium-90 microspheres embedded in the liver parenchyma as a number of discrete point sources.