Heterogeneous Distribution and Absorbed Dose of Radiolabeled Nanoparticles and Colloids in Sentinel Lymph Nodes.

Background: For breast cancer staging, radiolabeled colloids and superparamagnetic iron oxide nanoparticles (SPIONs) are used for sentinel lymph node (SLN) imaging. This study characterized the intranodal activity distribution and absorbed dose distribution. Material and Methods: Six patients diagnosed with primary breast cancer were intradermally injected with ^99mTc-Nanocoll. The SLNs were resected, weighed, and measured for activity. Three groups of six rats were subcutaneously injected into the hind paw with either ^99mTc-Nanocoll, ^99mTc-SPIONs, or ⁶⁸Ga-SPIONs. Macro- and small-scale dosimetry calculations were performed using autoradiography images of cryosections of SLNs from patients and animals. Results: The mean absorbed dose in patient SLNs was 0.5 ± 0.3 mGy/MBq for ^99mTc-Nanocoll and 3.4 ± 1.8 mGy/MBq, assuming a ^99mTc-Nanocoll-based distribution of ⁶⁸Ga-SPIONs. Due to different decay characteristics, the heterogeneity in the absorbed dose differed between ^99mTc-SPIONs and ⁶⁸Ga-SPIONs with a maximum to mean absorbed dose ratio of 2.7 ± 0.3 and 1.6 ± 0.2, respectively. Conclusions: This study shows that ^99mTc- and ⁶⁸Ga-SPIONs and ^99mTc-nanocolloids have similar activity distribution in human and animal lymph nodes. Small-scale dosimetry models combined with clinical patient biokinetics may serve as a bridge between organ and tissue dosimetry and the interpretation of intrinsic geometric variation and its uncertainties in absorbed dose.