WE‐C‐224C‐05: Tumor Brachytherapy Using Intratumoral Injection of Beta‐Emitting Therapeutic Radionuclides Carried Within Nanoparticles

The use of beta‐emitting therapeutic radionuclides carried within nanoparticles for brachytherapy potentially offers significantl advantages over sealed sources. By using an active trapping technique, both therapeutic radionuclides, ¹⁸⁶Re and ¹⁸⁸Re, and diagnostic radionuclides, ^99mTc, can be encapsulated in liposomes with high efficiency and high specific activity. ¹⁸⁶Re and ¹⁸⁸Re are beta emitters with appropriate ratio and energies of gamma emission which enable the imaging of in vivo distribution with a clinical gamma camera. The 90% absorbed dose deposit distances in soft tissue of beta ray from ¹⁸⁶Re and ¹⁸⁸Re are 1.8 mm and 4.2 mm respectively. To study the potential of using therapeutic radionuclides carried within liposomes for tumor brachytherapy, the intratumoral distribution and retention of ^99mTc‐liposomes in head and neck squamous cell carcinoma xenografts in nude rats were determined using imaging with a dedicated dual modality animal micro‐SPECT / CT scanner. Using an intratumoral administration technique developed in our lab, the volume of administered radioactivity up to over 40 % of tumor volume can be delivered to tumor with a high local retention. The ^99mTc‐liposome studies show that about 40 % of injected activity remained in tumor with a very slow clearance. The cleared activity was not retained in the nearby critical organ. Using pin‐hole collimators with a spatial resolution of 1 mm, the ^99mTc‐activity had a broad diffusion throughout the tumor even though the injection was at one point within the tumor. This study has shown the potential of using beta‐emitting therapeutic radionuclides carried within nanoparticles for tumor brachytherapy. The high intratumoral retention enables a high intratumoral radiation absorbed dose, while the penetration of beta‐particles within only a few mm decreases the toxicity to nearby critical organs, and the intratumoral diffusion makes the intratumoral administration simpler and provides for a better intratumoral absorbed dose homogeneity.