Theraspheres

Just got back from Chicago... (fun town), on a training seminar in regards to TheraSpheres.  For those in the United States and Europe, this might be routine practise in treating liver cancers (hepatocellular carcinoma HCC), but not so much here in Canada.  There are only a few places in Canada that are performing these procedures (BC, Alberta) but not sure what type of volumes they have in regards to this type of treatment.  The Nordion sales reps have stated that they have some type of involvement within these regions.

At any rate what is a TheraSphere?  They are glass beads with Yittrium-90 attached to the surface.  The beads themselves are biocompatible and insoluble and have a mean size between 20 - 30um.  These beads are injected through a femoral line while undergoing a hepatic angiogram in Interventional Radiology (IR) and the beads are deposited close to the site of the liver lesion that you want to treat.

What's tricky about this is, figuring out the blood supply to the region of the liver in and around the tumour site(s).  For the most part the blood flow to the liver is fairly predictable, but with respect to some patients it's a bit more complicated because of a variety of arterial variants, parasitization of flow, accessory arteries, retrograde blood flow etc. that if not found with a thorough investigation, the deposition of the TheraSpheres will not go entirely to the tumour, but elsewhere within the body.  This is not a good thing, since adverse reactions may potentially occur.  The case studies illustrated these quite well at the training seminar.  However, for the most part Yittrium-90 radioembolization of the HCC tumours has several advantages in that it has a lower toxicity profile in comparison to transarterial chemoembolization.

So what does TheraSpheres have to do with Nuclear Medicine?  Well for the most part we assay and deliver the Yittrium-90 to the angiography suite, and from there the IR techs and the IR doctors take care of the rest with respect to the injection and the clean up.  This will vary from site to site, depending upon the level of comfort and training in dealing with radioactive materials.

However even before this occurs, Nuclear Medicine is important in determining extra hepatic shunting to the lungs or gastrointestinal tract as part of the selection process in figuring out who are good candidates for this treatment.

150MBq of Tc-99m MAA is injected with a microcatheter into the hepatic artery after coil embolization of all visible non hepatic arterial flow.  Basically, whatever blood flow that doesn't deal with the liver, they get clamped down with coils, in order to figure extrahepatic flow.  The image below is an example of this type of imaging.

Fig. 1  ROI's drawn over the lungs and liver to determine the counts for geometric calculations to determine the Lung Shunt Function (LSF)

Anterior and posterior images are taken to determine regions of interest (ROI) to find out the Lung Shunt Fraction (LSF).  The geometric means are calculated from the lungs and liver using the numbers from the ROI's  and are used in this equation:

LSF = Lungs / (Liver + Lungs) *100

The reason why this is important is because it helps in determining the dosimetry calculations for pre and post treatment.  This is important since you need to know how much radiation to give given a specific volume of liver that you plan to treat without affecting other parts of the body (ie. lungs).

There is the question of planar versus SPECT/CT imaging with MAA.  At the training seminar, planar imaging was described, but others have suggested performing SPECT/CT.  Since at our site we have limited experience with this protocol, we would need to speak to the IR doctors and the Nuc Med doctors to figure out what they want.

The current imaging protocol for Tc-99m MAA:

Static Imaging Protocol (Northwestern Hospital, Chicago):

Dosage:

- 37 -185 MBq Tc-99m MAA, injected in IR and the patient delivered to Nuclear Medicine

Equipment:

- Any large FOV dual detector gamma camera, with LEAP or LEHR collimation

Imaging:

Option 1:  The patient is positioned supine under the gamma camera and 4 images are acquired.  Anterior and posterior images of the abdomen and of the thorax are acquired separately

Option 2:  The patient is positioned supine under the gamma camera and a whole body scan is acquired

Camera Parameters:

- Acquisition matrix = 256 X 256

- Zoom = 1.45 or less to ensure all activity visible in FOV; total counts >1M

- Counting time - 5mins per 74 MBq administration of Tc-99m

SPECT/CT Imaging Protocol (University Hospital Essen, Essen)

Dosage:

150 MBq Tc-99m MAA

Equipment:

- Dual headed gamma camera, with SPECT/CT capability

Imaging:

- 30 minutes post injection of Tc-99m MAA, anterior and posterior planar images of the whole body 

- SPECT/CT afterwards

Camera Parameters:

- SPECT - 128 X 128 matrix

- 128 frames (25 secs/frame)

- CT - 130 keV, 17 mAs, 5mm slices

There is also talk about PET/CT imaging as well.  Since Yittrium-90 is a beta emitter, having the patient come back the following day after treatment allows imaging of the distribution and deposition of the microspheres within the body.  

Anyway, there is a lot to know about this procedure and I am only scratching the surface. At our facility we have had some experience with this many years ago, but now there is a real push with some of the doctors at the hospital to revisit this type of treatment again.

Stay tuned........

Update:  Check out Theraspheres Part Deux, in this blog site.  We've performed out first LSF and treatment.