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Background
Transarterial radioembolization (TARE) is a treatment for primary liver tumors and liver metastases. This technique is effective because unlike healthy liver parenchyma, which is primarily supplied by the portal venous circulation, hepatic tumors are chiefly vascularized by the hepatic artery. TARE is an arterially directed catheter-based loco-regional therapy that utilizes targeted delivery of microspheres (glass or resin) with yttrium 90 (Y90), a high-energy particle emitter with a limited depth of penetration (2.5 to 11 mm). In comparison to external radiation, this reduced depth of penetration allows higher doses of radiation to be used, while minimizing radiation toxicity to the adjacent uninvolved liver parenchyma. Importantly, TARE has less microembolic effect than other transcatheter therapies. Consequently, tumor enhancement may persist, which may mimic residual disease and confound interpretation. The understanding of anticipated and unanticipated findings following TARE are vital to future direction of the patient’s care.

Educational Goals / Teaching Points
Provide a brief and basic overview of TARE. Illustrate common imaging findings and pitfalls following TARE. Review common posttreatment response assessment algorithms, to include modified Response Evaluation Criteria in Solid Tumors (mRECIST) and Liver Imaging and Reporting Data System (LI-RADS). Discuss complications that may arise following TARE and their imaging appearances.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
In contrast to other interventional procedures, it can take several months for tumor response to become apparent following TARE. Findings suggestive of an effective response include decreasing tumor size and enhancement, typically seen by 3 - 6 months after treatment. However, there is substantial variation in imaging appearance, which makes interpretation difficult. For this reason, commonly used treatment response algorithms (such as mRECIST and LI-RADS) must be applied cautiously. To be specific, persistent arterial phase hyperenhancement (APHE) is common early after TARE, both in effectively and ineffectively treated tumors. In fact, peritumoral APHE may mimic infiltrative HCC but is actually indicative of a good response with no viable tumor. Additional findings seen following TARE include but are not limited to inflammation, peritumoral edema, hepatic fibrosis, and capsular retraction. It is imperative that the diagnostic radiologist is also familiar with possible complications from TARE. Examples of hepatic complications include development of biliary necrosis and biloma, hepatic abscess, and/or radioembolization-induced liver disease (REILD). Extrahepatic complications may include nontarget delivery (which may induce gastrointestinal ulceration and necrosis), radiation cholecystitis, and radiation pneumonitis.

Conclusion
Imaging findings following TARE are unique and susceptible to misinterpretation by the ill-informed radiologist. We aim to provide a succinct and high-yield overview of the typical and atypical post-treatment appearances, in addition to potential complications.