The Impact of Diferent 99mTc Activities on Dual Isotope Imaging for 166Holmium Scout Radioembolization Procedure

Abstract

Aim/Introduction: Partition modelling allows a personalized activity calculation for holmium-166 (166Ho) radioembolization. However, it requires the defnition of tumor and non-tumorous liver, typically by segmentation and registration of a separately acquired contrast enhanced CT, which is time-consuming and prone to error. A dual-isotope protocol including 166Hoscout, for treatment simulation, and technetium-99m (99mTc) stannous phytate (accumulating in the healthy liver) for automated, robust healthy liver delineation was proposed12. This phantom study investigates the efect of diferent 99mTc activities on the 166Ho-scout images and on the usability of the 99mTc images for liver segmentation. Materials and Methods: The liver compartment, including two tumors, of an anthropomorphic phantom was flled with 250 MBq of 166Hochloride, with a tumor to non-tumorous activity concentration ratio of 10:1. Multiple SPECT/CT scans were acquired, ranging up to 125 MBq of 99mTc activities in the non-tumorous liver. Images were reconstructed using a commercially available protocol incorporating partial volume correction, attenuation and window-based scatter correction by applying a single combined k-factor previously estimated34. The resulting contrast recovery coefcient (CRC) of hot and cold spheres for 166Ho and 99mTc respectively was measured in the tumors. Automatic segmentation of the healthy liver and the tumor volumes using 99mTc images was evaluated by recovered volume and Sørensen-Dice Index. Results: 166Ho scout scattercorrected image quality was not hampered by the diferent 99mTc. CRC for partial volume corrected cold spheres were approximately 100% independently of the 99mTc activity. The applied 99mTc activities were all suitable for automatic healthy liver segmentation using phantom data. Conclusion: It is possible to inject 99mTc activities up to at least 125 MBq without compromising the 166Ho image quality, provided that a proper scatter correction is applied. 99mTc images acquired in the activity range from 25 MBq up to 125 MBq all performed equally well for the purpose of segmentation of the healthy liver.