Postlumpectomy focal brachytherapy for simultaneous treatment of surgical cavity and draining lymph nodes

Purpose: The primary objective was to investigate a novel focal brachytherapy technique using lipid nanoparticle (liposome)-carried β-emitting radionuclides (rhenium-186 [¹⁸⁶Re]/rhenium-188 [ ¹⁸⁸Re]) to simultaneously treat the postlumpectomy surgical cavity and draining lymph nodes. Methods and Materials: Cumulative activity distributions in the lumpectomy cavity and lymph nodes were extrapolated from small animal imaging and human lymphoscintigraphy data. Absorbed dose calculations were performed for lumpectomy cavities with spherical and ellipsoidal shapes and lymph nodes within human subjects by use of the dose point kernel convolution method. Results: Dose calculations showed that therapeutic dose levels within the lumpectomy cavity wall can cover 2- and 5-mm depths for ¹⁸⁶Re and ¹⁸⁸Re liposomes, respectively. The absorbed doses at 1 cm sharply decreased to only 1.3% to 3.7% of the doses at 2 mm for ¹⁸⁶Re liposomes and 5 mm for ¹⁸⁸Re liposomes. Concurrently, the draining sentinel lymph nodes would receive a high focal therapeutic absorbed dose, whereas the average dose to 1 cm of surrounding tissue received less than 1% of that within the nodes. Conclusions: Focal brachytherapy by use of ¹⁸⁶Re/¹⁸⁸Re liposomes was theoretically shown to be capable of simultaneously treating the lumpectomy cavity wall and draining sentinel lymph nodes with high absorbed doses while significantly lowering dose to surrounding healthy tissue. In turn, this allows for dose escalation to regions of higher probability of containing residual tumor cells after lumpectomy while reducing normal tissue complications.