Planning dosimetry for 90Y radioembolization with glass microspheres: Evaluating the fidelity of 99mTc‐MAA and partition model predictions
Authors: Thomas M. Allan , Mahvash Armeen , Abdelsalam Mohamed , Kaseb Ahmed O. , Kappadath S. Cheenu 
Published: August 2020

Purpose

99mTc‐MAA‐SPECT/CT may be used in 90Y‐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 model more traditionally used in 90Y radioembolization. As 90Y radioembolization treatments shift toward activities and doses that aim to achieve tumor control, 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 90Y dosimetry predictions from pretherapy 99mTc‐MAA and PM.

Methods

Both PM and voxel dosimetry models were used to calculate tumor and NL mean doses using both planning 99mTc‐MAA and verification 90Y‐SPECT/CT in this retrospective analysis of hepatocellular carcinoma cases treated with glass microspheres (NCT01900002, n = 32). Linear regression models were developed at first access, and then later correct, the estimates by (a) 99mTc‐MAA for 90Y voxel dosimetry and (b) 99mTc‐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 vs catheter unmatched) by establishing the level of 99mTc‐MAA and 90Y catheter position alignment. Only catheter‐matched cases were included in the 99mTc‐MAA vs 90Y 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. 99mTc‐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, 99mTc‐MAA planning for 90Y tumor voxel doses was poor, overestimating dose by an average of nearly 40%. Upon application of the linear model, 99mTc‐MAA predictions for 90Y 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 99mTc‐MAA and PM tumor dose predictions, with mean biases (PI) of −3% (±53%) and −4% (±21%), respectively.

Conclusion

The unconditional use of 99mTc‐MAA to predict 90Y dosimetry across all cases is not recommended due to: (a) demonstrated the risk of unmatched catheter positions between procedures, and (b) large bias and uncertainty in 99mTc‐MAA predictions in cases with matched catheter locations. However, NL voxel dose predictions with 99mTc‐MAA are clinically viable and either PM or voxel dosimetry can be used to produce equivalent predictions. Both 99mTc‐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 99mTc‐MAA‐ and PM‐based treatment planning for 90Y dosimetry in improving treatment outcomes.