Gamma camera imaging characteristics of 166Ho and 99mTc used in Selective Internal Radiation Therapy

Background The administration of a 166 Ho scout dose is available as an alternative to 99m Tc particles for pre-treatment imaging in Selective Internal Radiation Therapy (SIRT). It has been reported that the 166 Ho scout dose may be more accurate for the prediction of microsphere distribution and the associated therapy planning. The aim of the current study is to compare the scintigraphic imaging characteristics of both isotopes, considering the objectives of the pre-treatment imaging using clinically geared phantoms. Methods Planar and SPECT/CT images were obtained using a NEMA image quality phantom in different phantom setups and another body-shaped phantom with several inserts. The influence of collimator type, count statistics, dead time effects, isotope properties and patient obesity on spatial resolution, contrast recovery and the detectability of small activity accumulations was investigated. Furthermore, the effects of the imaging characteristics on personalized dosimetry are discussed. Results The images with 99m Tc showed up to 3 mm better spatial resolution, up to two times higher contrast recovery and significantly lower image noise than those with 166 Ho. The contrast-to-noise ratio was up to five times higher for 99m Tc than for 166 Ho. Only when using 99m Tc all activity-filled spheres could be distinguished from the activity-filled background. The measurements mimicking an obese patient resulted in a degraded image quality for both isotopes. Conclusions Our measurements demonstrate better scintigraphic imaging properties for 99m Tc compared to 166 Ho in terms of spatial resolution, contrast recovery, image noise, and lesion detectability. While the 166 Ho scout dose promises better prediction of the microsphere distribution, it is important to consider the inferior imaging characteristics of 166 Ho, which may affect individualized treatment planning in SIRT.