99m-Tc MAA Simulation for Yttrium-90 Microsphere Treatment of Non-resectable Liver Neoplasm

• The patient is brought to the angiography suite.
• Angiography is performed and arterial supply to the tumor is determined, with special attention to variant hepatic artery anatomy.
• The arterial supply is carefully evaluated to ensure that
  • The majority of microspheres can be reliably delivered to the tumor, and
  • Small arteries leading from the main hepatic arteries to other organs can be avoided during implantion of the microspheres to prevent unintentional irradiation of these organs. Embolization of these vessels should be performed if it is felt that they cannot be reliably avoided during implantion of the microspheres.
• Once it is determined that a safe route exists for implantation of the microspheres, the patient is injected with 4 mCi ^99mTc macro-aggregated albumin (MAA) through an arterial catheter.
• The catheter must be placed in a similar manner to that which will be used to deliver the Yttrium-90 Microspheres later.
• The patient is brought to the nuclear medicine department.
• The liver is imaged in anterior and posterior projections for a set number of counts or a set duration duration (700k counts or 5 minutes at our institution).
• The lung is imaged in anterior and posterior projections for the same number of counts or duration as the anterior liver image.
• SPECT imaging of the liver is performed if the partition method (vs. empiric method) is to be used for calculation of dose/activity of the microspheres.
• Total counts for the lung and the liver are obtained using regions of interest
• The shunt fraction is determined:
   ,
  where GM_Lung and GM_Liver are the geometric means of the counts from the lung and liver, respectively. Geometric means are calculated from the anterior and posterior counts (counts_ant and counts_post, respectively) as:
  
• The shunt fraction is used along with information from SPECT images and CT to ensure that the radiation dose to the normal liver and lung parenchyma is safe.
• The final dose for ⁹⁰Yttrium can be determined using the empiric method or the partition method.
• Empiric method
  • A standard amount of activity is prescribed based on the percentage of the liver that is replaced with tumor (based on CT).
  • A variant of the empiric method also takes into account the patient's body surface area.
    

    Lung shunt fraction	Reduce implanted activity by
    0% – 10%	No reduction required
    11% – 15%	Reduce by 20%
    16% – 20%	Reduce by 40%
    > 20%	Do not treat

• Partition method
  • The partition method method tries to implant the highest possible activity to the tumor while limiting radiation dose to sensitive tissues such as the lung (< 25 Gray) and the normal liver parenchyma (< 80 Gray in normal livers and < 70 Gray in cirrhotic livers).
  • Practically speaking, this method can only be used in cases of a discrete tumor mass that allows the tumor to be drawn as a region-of-interest on SPECT images. This is usually only possible for patients with primary hepatocellular carcinoma, where there is often a large single tumor mass.
  • CT is used to determine the volume of tumor and normal liver.
  • SPECT images are used to measure the proportion of ^99mTc-MAA activity that lodges in the tumor, in the normal liver, and in the lung.
  • The shunt fraction is then used along with tumor volume and liver volume from the SPECT images, to calculate an optimal dose for the patient.

References

• SIR Spheres. User Manual.