Planning Dosimetry for 90 Y Radioembolization with Glass Microspheres: Evaluating the Fidelity of 99m Tc-MAA and Partition Model Predictions.
PURPOSE
99m Tc-MAA-SPECT/CT may be used in 90 Y glass microsphere radioembolization treatment planning to assess perfused liver volumes and absorbed dose distributions. The partition model (PM) offers a more detailed planning dosimetry option beyond the single compartment MIRD model more traditionally used in 90 Y radioembolization. As 90 Y radioembolization treatments shift toward activities and doses that aim to achieve tumor eradication, accurate and reliable treatment planning dosimetry for both tumors and normal liver (NL) becomes more critical. In this work, we explore the accuracy and precision of 90 Y dosimetry predictions from pre-therapy 99m Tc-MAA and PM.


METHODS
Both PM and voxel dosimetry models were used to calculate tumor and NL mean doses using both planning 99m Tc-MAA- and verification 90 Y-SPECT/CT in this retrospective analysis of hepatocellular carcinoma cases treated with glass microspheres (NCT01900002, N=32). Linear regression models were developed to at first assess, and then later correct, the estimates by 1) 99m Tc-MAA for 90 Y voxel dosimetry and 2) 99m Tc-MAA PM for voxel dosimetry, separately for both tumors and NL. Bland-Altman analysis was then used to evaluate the accuracy and precision of the regression model predictions with the mean bias and 95% prediction intervals (PI, ±1.96σ). Two categories of cases were stratified (catheter-matched versus catheter-unmatched) by establishing the level of 99m Tc-MAA and 90 Y catheter position alignment. Only catheter-matched cases were included in the 99m Tc-MAA vs 90 Y voxel dosimetry comparison, while all cases were used to compare dosimetry models (PM vs. voxel).


RESULTS
Half (16/32) of cases were deemed catheter-matched. 99m Tc-MAA could reliably predict NL doses in catheter-matched cases after application of the linear model, with mean bias (PI) of -1% (±31%). PM was equivalent to voxel dosimetry for NL doses with mean bias (PI) of 0% (±1%). Even among catheter-matched cases, 99m Tc-MAA planning for 90 Y tumor voxel doses was poor, overestimating dose by an average of nearly 40%. Upon application of the linear model, 99m Tc-MAA predictions for 90 Y tumor voxel dose were only minimally biased (-4%) but possessed very large PI (±104%). PM predictions for tumor voxel dose using the linear model also showed small bias (-6%) but maintained similarly high PI of ±90%. Cases with tumors representing a large majority (>80%) of the total tumor volume demonstrated the best scenarios for 99m Tc-MAA and PM tumor dose predictions, with mean biases (PI) of -3% (±53%) and -4% (±21%), respectively.


CONCLUSION
The unconditional use of 99m Tc-MAA to predict 90 Y dosimetry across all cases is not recommended due to: 1) demonstrated risk of unmatched catheter positions between procedures, and 2) large bias and uncertainty in 99m Tc-MAA predictions in cases with matched catheter locations. However, NL voxel dose predictions with 99m Tc-MAA are clinically viable and either PM or voxel dosimetry can be used to produce equivalent predictions. Both 99m Tc-MAA and PM can provide tumor dose predictions with potential clinical utility, but only in catheter-matched cases and with tumors comprising a clear majority (>80%) of the total tumor volume. These findings stratify the predictive fidelity of 99m Tc-MAA- and PM-based treatment planning for 90 Y dosimetry in improving treatment outcomes.