Comparing Blood-, Image-, and Whole-Body Skeleton-Based Bone Marrow Dosimetry in 177Lu-PSMA-617 Therapy

A study presented at the Society of Nuclear Medicine and Molecular Imaging Annual Meeting assessed and compared bone marrow (BM) dosimetry using blood-based, image-based, and whole-body skeleton (WBS)-based methods for patients treated with ¹⁷⁷Lu-PSMA-617.

BM is the main organ at risk for ¹⁷⁷Lu-PSMA radiopharmaceutical therapies for prostate cancer, and patient-specific active BM distribution is challenging to image. Subsequently, dosimetry for BM is a difficult process due to complicated quantitative measurements that include various contributors to the absorbed dose.

A group of Canadian researchers sampled 19 patients, including 4 without bone metastases (b-mets) and 6 with a high burden of b-mets. The injected activity was 7.40±0.74 GBq for each treatment cycle, and single photon emission computed tomography/computed tomography (SPECT/CT) scans were performed at 24 and 48 hours postinjection to estimate the organ biodistribution.

Researchers collected 3 blood samples at 15 minutes postinjection, as well as 15 minutes before each of the SPECT/CT scans, to evaluate the BM self-dose using the blood-based method (D_{BM <-blood}). The self-dose was also calculated using 2 methods: small volumes of interest (1 cm diameter) located at patient lumbar vertebrae (L1-L4), femora and humeri, while trying to avoid b-mets, were drawn to estimate the BM self-dose (D_{BM <-img}); and 2 regions of whole-body low-uptake in the skeleton (luWBS) were used to evaluate the BM self-dose (D_{BM <-luWBS}). Researchers noted that those luWBS regions were defined based on the WBS with less than 1% (luWBS-1%) and less than 0.5% (luWBS-0.5%) of the whole-body activity uptake.

Furthermore, cross-dose contributions from high-uptake organs (D_{BM <-organs}) and the remainder of the body (D_{BM <-rob}) were further estimated based on whole-organ and whole-body segmentation. The total BM dose was calculated by summing the self-dose and cross-dose contributions.

Results of the analysis showed that BM self-dose from the image-based method depended largely on the localization of the different volumes of interest. The differences of D_{BM <-img(L1-L4)}, D_{BM <-img(femur)}, and D_{BM <-img(humerus)} against D_{BM <-blood} were −31.6%±43%, −3.6%±57%, and 17.6%±70%, respectively. Researchers noted that the femur could be considered an alternative for self-dose estimates when blood samples are not present.

D_{BM <-luWBS-0.5%} showed a lower variability against D_{BM <-blood} with a difference of −8.3%±37.0%, while D_{BM <-luWBS-1%} differed by −23.4%±62.0%. Additionally, they added that WBS with <0.5% of WB uptake “should be considered for BM dose calculation” and provided more comparable results than those from the regions with small volumes.

“Our results suggest that using volumes of interest in the femur or taking the BM luWBS-0.5% lead to small deviations when compared with blood-based methods,” study authors concluded, noting that efforts to reduce the variability of imaging-based methods needs further development.