Partition model for estimating radiation doses from yttrium-90 microspheres in treating hepatic tumours

Abstract

A uniform distribution of yttrium-90 (90Y) microspheres throughout the entire liver has always been assumed for dose calculation in treating hepatic tumours. A simple mathematical model was formulated which allows estimation of the activities of a therapeutic dose of 90Y microspheres partitioned between the lungs, the tumour and the normal liver, and hence the radiation doses to them. The doses to the tumour and normal liver were verified by intra-operative direct beta-probing. The percentage of activity shunted to the lung and the tumour-to-normal tissue ratio (T/N) were obtained from gamma scintigraphy using technetium-99m-labelled macroaggregated albumin (MAA) which simulates the 90Y microspheres used in subsequent treatment. The intrahepatic activity was 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. The estimated radiation doses were correlated with the doses directly measured using a calibrated beta-probe at laparotomy by linear regression. The radiation doses to the tumour and the normal liver, estimated using the partition model, were close to that measured directly with coefficients of correlation for linear regression: 0.862 for the tumours and 0.804 for the normal liver compartment (P<0.001). The partition model permits a distinction between the radiation doses received by the tumour and the normal liver to be made and the doses thus estimated are close to the actual doses received. The optimal doses to the tumour and normal liver and hence the required quantity of 90Y microspheres to be administered can be easily predetermined.

General Theory of Predictive Dosimetry for Yttrium-90 Radioembolization to Sites Other than the Liver

“…R is a ratio of activities, not of mean radioconcentrations and, therefore, is conceptually different from the ''tumor-to-normal liver ratio'' defined by the original partition model [8].…”

“…1); (4) vascular stasis and microsphere reflux has not occurred at the conclusion of 90 Y radioembolization; (5) implanted tissue activity is proportional to scintigraphic counts, and its quantification is accurate; (6) there is no postimplantation redistribution of microspheres nor leeching of 90 Y activity; (7) there are no arteriovenous shunts from nontumorous tissue contributing to lung shunting of microspheres. If the user suspects that arteriovenous shunting may be occurring within nontumorous tissue (e.g., congenital vascular malformations, previous surgery, or trauma), the entire predictive dosimetry will revert in principle to the original ''partition model'' [8] as was developed for 90 Y radioembolization to the liver. This is because the original partition model accounts for the possibility of arteriovenous shunts occurring from nontumorous cirrhotic liver parenchyma.…”

“…The TLSF assumes no lung shunt contribution from nontumorous tissue, and is therefore conceptually different from the ''liver-to-lung shunt fraction'' defined by the original partition model [8].…”

“…Technology and innovations that may improve the safety and effectiveness of 90 Y radioembolization to sites other than the liver are: (1) hybrid tomographic scintigraphy with integrated CT, i.e., SPECT/CT [7,8] or PET/CT [10]; (2) antireflux catheters to minimize reflux of 90 Y microspheres into nontarget tissue [11]; (3) Voxel dosimetry and dose-volume histograms to overcome inherent limitations of dosimetric planning based on mean absorbed doses [12]; (4) multi-isotope surrogates to qualitatively and quantitatively assess microsphere biodistribution within Fig. 3 For multiple, small or ill-defined tumor lesions, the entire target arterial territory may be conceptually regarded as ''tumor'' and treated by a single intended mean absorbed dose Fig.…”

Semi-Quantitative Analysis of Post-Transarterial Radioembolization 90Y Microsphere Positron Emission Tomography Combined with Computed Tomography (PET/CT) Images in Advanced Liver Malignancy: Comparison With 99mTc Macroaggregated Albumin (MAA) Single Photon Emission Computed Tomography (SPECT)

“…The intrahepatic tracer distribution and tumor-to-normal uptake ratio (TNR) are important for dose calculation especially when using a partition model. Many reports have suggested the correlation of 99m Tc MAA dosimetry and intraoperative estimation of radiation doses [6][7][8][9][10][11]. After the injection of the microsphere, postembolization evaluation for the distribution of 90 Y microspheres is essential to predict possible complications and treatment response [12].…”