Radioembolization with 90Y-labeled glass microspheres: Analytical methods for patient-personalized voxel-based dosimetry

Abstract

Radioembolization (RE) is a radiation treatment based on intra-arterial injection of Yttrium-90 (90Y) labeled microspheres, recognized as an efficient clinical practice for liver neoplasms either primary or metastatic. As a current practice, the RE procedure follows the guidelines of the microsphere manufacturers. At the Champalimaud Centre for the Unknown (CCU), Champalimaud Foundation (CF), we have been using 90Y-labeled glass microspheres (Therasphere®) in our patients referred for hepatic RE treatments. Following the Therasphere® guidelines, the calculation of 90Y activity to be injected through the hepatic artery, is based on the formalism of the MIRD (Medical Internal Radiation Dose) committee of the Society of Nuclear Medicine and Molecular Imaging (SNMMI). The calculation model is based on the prescribed absorbed dose and the liver volume (total, lobe or segment), not considering any patient-specific characteristics that can influence the microspheres deposition. Accurate absorbed dose calculation in the liver is crucial for treatment planning and establishing the dose-effect correlation between tumor control and normal tissue complications. Therefore, our research group has been developing patient-personalized voxel-based dosimetry analysis of Single Photon Emission Computer Tomography (SPECT) images obtained from the patient injected with Technetium-99 m Macroaggregated Albumin (99mTc-MAA) in a pre-treatment phase. The use of 99mTc-MAA as a 90Y microsphere surrogate is one of the biggest challenges in hepatic RE due to the intrinsic differences between particles (MAA vs glass microspheres) and consequent distribution in the liver parenchyma. Therefore, we capture post-treatment 90Y Positron Emission Tomography PET images to investigate the predictive power of the pre-treatment MAA dose distributions in comparison with the post-treatment 90Y dose maps. Two analytical methods have been used in our analysis: the gamma-index (γ-index) test, as a numerical measure of the agreement between two image datasets, and the Dose-Volume Histograms (DVH), to quantify the absorbed dose in each volume of interest (VOI) previously defined, i.e., the total liver, the normal functioning liver tissue and the planning target volume (PTV). We can conclude so far, that patients with multiple PTV undergoing hepatic RE (total or lobar) can be simulated by the MAA. For these patients, optimized pre-treatment voxel-based dosimetry seems to be suitable. Our hypothesis (ongoing work) is that patients with few PTV may also benefit from personalized pre-treatment dosimetry (segment or tumor) and optimization based on MAA.