Comparison of three radiation dosimetry models for Yttrium-90 microsphere radioembolotherapy

Objectives Quantify differences of 3 dosimetry models for treatment of hepatic tumors with ⁹⁰Y-microsphere radioembolotherapy (RE).

Methods The ⁹⁰Y microsphere (SIR-Spheres, Sirtex Medical, Australia) package insert recommends a cumulative liver dose <80Gy via a simple dosimetry model that assumes uniform ⁹⁰Y distribution in tumor and liver & organ masses based on MIRD Standard Man (SM): total body 73.7kg, liver 1.91kg, lung 1kg. Two 3-compartment (liver, tumor, lung) partition models were investigated: CT-based (CTB) & weight-based (WTB). The main difference between them is in estimation of liver mass: in CTB it was estimated from CT image segmentation; while in WTB the SM liver mass was scaled by patient weight/sex. SM lung mass was scaled by patient weight/sex for CTB & WTB. Tumor fraction was estimated from CT & tumor uptake was estimated from pre-therapy ^99mTc-MAA SPECT/CT. Model differences in estimated lung, liver, tumor doses were analyzed for 11 patients.

Results The mean absolute liver mass difference was 0.64kg between CTB & SM and 0.42kg between CTB & WTB. Lung doses (3–12Gy) were similar between all 3 models with mean difference 0.1±1.5Gy. WTB & CTB non-tumor liver doses (4–31Gy) were similar with mean difference 0.7±5.5Gy but lower than SM by ~28Gy. WTB & CTB tumor doses (63–227Gy) were similar with mean difference 10±35Gy but higher than SM by ~65Gy.

Conclusions The more realistic WTB or CTB models, that incorporate patient anatomical and functional data, yielded lower normal liver but similar lung doses than SM, and therefore should be the preferred methodology for ⁹⁰Y RE treatments.