HO-166-MICROSPHERE LIVER RADIOTHERAPY - A PRECLINICAL SPECT DOSIMETRYSTUDY IN THE PIG

Liver metastases cause the majority of deaths from colorectal cancer a nd response to chemotherapy is poor. Intrahepatic arterial Y-90-micros pheres may induce tumour regression but the beta-radiation dose is var iable and cannot be determined in patients. The 81 keV gamma emission of holmium-166 (Ho-166) was used to determine, by single photon emissi on computed tomographic (SPECT) imaging, the beta-radiation absorbed d ose to normal liver in pigs following intrahepatic arterial administra tion of Ho-166-microspheres. The SPECT system was calibrated with anth ropomorphic liver phantoms containing known activity concentrations of Ho-166-chloride. The relationship of SPECT counts to phantom activity concentration was linear with a correlation coefficient of r = 0.996. The SPECT pattern of liver distribution following successive administ rations of tracer activities of Ho-166-microspheres was similar. The r atio of initial to total SPECT estimates of mean activity concentratio n in regions of interest, from which anatomically matched biopsy sampl es were later obtained and counted in an ionization chamber, showed go od correlation (r = 0.924). Prospective SPECT dosimetry performed on a tracer activity of Ho-166-microspheres predicted the total administer ed activity required to deliver a prescribed radiation absorbed dose o f 25 Gy to the liver within an error of +/- 8%. This study demonstrate s the feasibility of prospective control of the absorbed radiation dos e to the critical normal organ by SPECT dosimetry on a tracer dose of Ho-166-microspheres prior to administration of a therapy dose.