Absorbed dose differences in 90Y microsphere therapy between pre-treatment 99mTc-MAA and post-treatment 90Y scans

Objectives 90Y SIR-Sphere package insert recommends liver doses <80Gy via a simple dosimetry model assuming uniform uptake. A three-compartment model (3CM) with different uptake in non-tumor (NT) and tumor (T) should provide improved dose estimates. Ambiguity in estimation of uptake and differences between planning 99mTc-MAA and post-therapy 90Y-usphere images lead to dose uncertainty. We study 3CM dose differences between planning MAA and post-therapy usphere images.

Methods 99mTc-MAA and 90Y-usphere SPECT/CTs of 23 patients undergoing SIR-Sphere therapy were used. In each exam T count densities were made using VOIs at different thresholds 50%, 75%, 90% relative to maximum. NT count density was estimated from separate VOIs. 6 values of T and NT doses were calculated for each patient (3 thresholds each for MAA & 90Y). We assessed dose changes with estimates of uptake and dose correlation between MAA and usphere.

Results With 50%, 75%, 90% thresholds on 99mTc-MAA SPECT/CT, the mean changes relative to the package insert in T doses were +203%, +231%, +244% and NT doses were -53%, -60%, -63%. With 50%, 75%, 90% thresholds on 90Y-usphere SPECT/CT, corresponding changes in T doses were +116%, +140%, +153%, and NT doses were -18%, -26%, -31%. All changes were significant (p<0.05) using a paired t-test. On average, usphere images compared to MAA yielded lower T (by ~90%) but higher NT (by ~34%) doses. T doses were well correlated between MAA and uspheres (R~0.77, p<0.01).

Conclusions 3CM using MAA or usphere images yielded higher T and lower NT doses compared to package insert. 90Y-usphere images yielded lower T but higher NT doses compared to MAA. Substantial dose differences were found between planning MAA and post-treatment usphere that has implications for planning re-treatments. Quantitative 90Y SPECT/CT will increase confidence in doses estimates