Selective Internal Radiation Therapy With Yttrium-90 Glass Microspheres: Biases and Uncertainties in Absorbed Dose Calculations Between Clinical Dosimetry Models.

Purpose: To quantify differences that exist between dosimetry models used for Y selective internal radiation therapy (SIRT).

Methods And Materials: Retrospectively, 37 tumors were delineated on 19 post-therapy quantitative Y single photon emission computed tomography/computed tomography scans. Using matched volumes of interest (VOIs), absorbed doses were reported using 3 dosimetry models: glass microsphere package insert standard model (SM), partition model (PM), and Monte Carlo (MC). Univariate linear regressions were performed to predict mean MC from SM and PM. Analysis was performed for 2 subsets: cases with a single tumor delineated (best case for PM), and cases with multiple tumors delineated (typical clinical scenario). Variability in PM from the ad hoc placement of a single spherical VOI to estimate the entire normal liver activity concentration for tumor (T) to nontumoral liver (NL) ratios (TNR) was investigated. We interpreted the slope of the resulting regression as bias and the 95% prediction interval (95%PI) as uncertainty. MC represents MC absorbed doses to the NL for the single tumor patient subset; other combinations of calculations follow a similar naming convention.

Results: SM was unable to predict MC or MC (p>.12, 95%PI >±177 Gy). However, SM was able to predict (p<.012) MC, albeit with large uncertainties; SM and SM yielded biases of 0.62 and 0.71, and 95%PI of ±40 and ± 32 Gy, respectively. PM and PM predicted (p<2E-6) MC and MC with biases of 0.52 and 0.54, and 95%PI of ±38 and ± 111 Gy, respectively. The TNR variability in PM increased the 95%PI for predicting MC (bias = 0.46 and 95%PI = ±103 Gy). The TNR variability in PM modified the bias when predicting MC (bias = 0.32 and 95%PI = ±110 Gy).

Conclusions: The SM is unable to predict mean MC tumor absorbed dose. The PM is statistically correlated with mean MC, but the resulting uncertainties in predicted MC are large. Large differences observed between dosimetry models for Y SIRT warrant caution when interpreting published SIRT absorbed doses. To reduce uncertainty, we suggest the entire NL VOI be used for TNR estimates when using PM.