Zachary Bessette

GU Oncology Now

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&plusmn;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 (DBM &lt;-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 (DBM &lt;-img); and 2 regions of whole-body low-uptake in the skeleton (luWBS) were used to evaluate the BM self-dose (DBM &lt;-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 (DBM &lt;-organs) and the remainder of the body (DBM &lt;-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 DBM &lt;-img(L1-L4), DBM &lt;-img(femur), and DBM &lt;-img(humerus) against DBM &lt;-blood were &minus;31.6%&plusmn;43%, &minus;3.6%&plusmn;57%, and 17.6%&plusmn;70%, respectively. Researchers noted that the femur could be considered an alternative for self-dose estimates when blood samples are not present.DBM &lt;-luWBS-0.5% showed a lower variability against DBM &lt;-blood with a difference of &minus;8.3%&plusmn;37.0%, while DBM &lt;-luWBS-1% differed by &minus;23.4%&plusmn;62.0%. Additionally, they added that WBS with &lt;0.5% of WB uptake &ldquo;should be considered for BM dose calculation&rdquo; and provided more comparable results than those from the regions with small volumes.&ldquo;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,&rdquo; study authors concluded, noting that efforts to reduce the variability of imaging-based methods needs further development.

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

SNMMI 2023

A study assessed and compared bone marrow dosimetry using blood-, image-, and whole-body skeleton-based methods for patients treated with 177Lu-PSMA-617.

Comparing Bone Marrow Dosimetry for 177Lu-PSMA-617 Therapy

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