Pure beta emitter — no clinically useful gamma emission. The high-energy beta particles (~11 mm mean path in tissue) make it well suited for intra-arterial tumor therapy. Trace bremsstrahlung radiation (braking radiation) and the very low-abundance Y-90 → Zr-90 positron emission (0.003%) allow post-treatment SPECT/CT and PET/CT imaging to verify microsphere distribution.
Y-90 radioembolization (SIRT — selective internal radiation therapy):
- Microspheres injected via hepatic artery catheter during IR procedure
- Preferentially delivered to tumor (hypervascular) vs normal liver (dual blood supply)
- Pre-procedure Tc-99m MAA injection through the intended delivery catheter is essential to:
- Map hepatopulmonary shunting (lung dose must be calculated; >30 Gy to lungs is prohibitive)
- Identify extrahepatic flow that could irradiate bowel or other non-target tissue
- Glass microspheres (TheraSphere): fewer, higher activity per sphere, more targeted radiobiologic dose
- Resin microspheres (SIR-Spheres): more spheres, lower activity each, wider distribution
Zevalin (ibritumomab tiuxetan):
- Anti-CD20 antibody labeled with Y-90 for radioimmunotherapy of B-cell NHL
- Preceded by unlabeled rituximab (CD20 blockade of circulating B cells to improve tumor biodistribution)
- Largely supplanted by chemotherapy + rituximab and newer agents