PARTITION MODEL FOR ESTIMATING RADIATION-DOSES FROM Y-90 MICROSPHERESIN TREATING HEPATIC-TUMORS

A uniform distribution of yttrium-90 (Y-90) microspheres throughout th e entire liver has always been assumed for dose calculation in treatin g hepatic tumours. A simple mathematical model was formulated which al lows estimation of the activities of a therapeutic dose of Y-90 micros pheres partitioned between the lungs, the tumour and the normal liver, and hence the radiation doses to them. The doses to the tumour and no rmal liver were verified by intra-operative direct beta-probing. The p ercentage of activity shunted to the lung and the tumour-to-normal tis sue ratio (T/N) were obtained from gamma scintigraphy using technetium -99m-labelled macroaggregated albumin (MAA) which simulates the Y-90 m icrospheres used in subsequent treatment. The intrahepatic activity wa s partitioned between the tumour and the normal liver based on the T/N and their masses determined from computerized tomography slices, The corresponding radiation doses were computed using the MIRD formula. Th e estimated radiation doses were correlated with the doses directly me asured using a calibrated beta-probe at laparotomy by linear regressio n, The radiation doses to the tumour and the normal liver, estimated u sing the partition model, were close to that measured directly with co efficients of correlation for linear regression: 0.862 for the rumours and 0.804 for the normal liver compartment (P<0.001). The partition m odel permits a distinction between the radiation doses received by the tumour and the normal liver to be made and the doses thus estimated a re close to the actual doses received, The optimal doses to the tumour and normal liver and hence the required quantity of Y-90 microspheres to be administered can be easily predetermined.