Physical aspects of yttrium-90 microsphere therapy for nonresectable hepatic tumors

Administration of yttrium-90 microspheres via the hepatic artery is an attractive approach to selectively delivertherapeuticdoses of radiation to livermalignancies. This procedure allows deliveringradiation absorbed doses in excess of 100 Gy to the tumors without significant liver toxicity. The microsphere therapy involves different specialties including medicaloncology,radiationoncology, nuclear medicine, interventional radiology, medical physics, and radiation safety. We have treated 80 patients with nonresectable hepatic tumors with yttrium-90 microspheres during the past two years on an institutional study protocol. The nominal radiation absorbed dose to the tumor in this study was 150 Gy. Required activity was calculated based on the nominal radiation absorbed dose and patient’s liver volume obtained from the CT scan, assuming a uniform distribution of the microspheres within the liver. Microspheres were administered via a catheter placed into the hepatic artery. The actual radiation absorbed doses to tumors and normal liver tissue were calculated retrospectively based on the patient’s 99 m Tc-MAA study and CT scans. As expected, the activity uptake within the liver was found to be highly nonuniform and multifold tumor to nontumor uptake was observed. A partition model was used to calculate the radiation absorbed dose within each region. For a typical patient the calculated radiation absorbed doses to the tumor and liver were 402 and 118 Gy, respectively. The radiation safety procedure involves confinement of the source and proper disposal of the contaminated materials. The average exposure rates at 1 m from the patients and on contact just anterior to the liver were 6 and 135 uSv/h, respectively. The special physics and dosimetry protocol developed for this procedure is presented.