Feasibility of CT quantification of intratumoural ¹⁶⁶Ho-microspheres

Microspheres loaded with radioactive ¹⁶⁶Ho (¹⁶⁶Ho-MS) are novel particles for radioembolisation and intratumoural treatment. Because of the limited penetration of β radiation, quantitative imaging of microsphere distribution is crucial for optimal intratumoural treatment. Computed tomography (CT) may provide high-resolution and fast imaging of the distribution of these microspheres, with lower costs and widespread availability in comparison with current standard single-photon emission tomography (SPECT) and magnetic resonance imaging. This phantom study investigated the feasibility of CT quantification of ¹⁶⁶Ho-MS.

CT quantification was performed on a phantom with various concentrations of HoCl and Ho-MS to investigate the CT sensitivity and calibrate the CT recovery. ¹⁶⁶Ho-MS were injected into ex vivo tissues, in VX-2 cancer-bearing rabbits, and in patients with head-neck cancer, to demonstrate sensitivity and clinical visibility. The amount of Ho-MS was determined by CT scanning, using a density-based threshold method and compared with a validated ¹⁶⁶Ho SPECT quantification method.

In the phantom, a near perfect linearity (least squares R² > 0.99) between HU values and concentration of ¹⁶⁶Ho was found. Ex vivo tissue experiments showed an excellent correlation (r = 0.99, p < 0.01) between the dose calibrator, SPECT, and CT imaging. CT recovery was on average 86.4% ex vivo, 76.0% in rabbits, and 99.1% in humans.

This study showed that CT-based quantification of Ho microspheres is feasible and is a high-resolution alternative to SPECT-based determination of their local distribution.