THYROID CANCER ABLATION

After thyroid surgery, use 80mCi if patient has been pre-treated with thyroxine withdrawl, or 100mCi if they have been pre-treated with Thyrogen.

After the ablation dose, they will have a follow-up scan. If this shows remnant disease that you don't think is malignant, give 70mCi. However, if it shows remnant disease that you think is malignant, or there are metastases seen, then give 150mCi.

URINOMA

This shows up as a photopaenic defect.
Delayed images - at 24 hours - will show the previously photopaenic area as full of tracer, because the non-radioactive urine has been mixed with radioactive urine.

It is due to rupture (the renal equivalent of a AAA) of the pelvicalyceal system, usually during a period of strain such as when there is acute obstruction of the ureter, or when there is massive VUR, or from a retrograde pyelogram.

Thus, you get an encapsulated retroperitoneal urine collection sitting in the perinephric space.
Once it gets big enough, it will obstruct drainage from the kidney.

CSF STUDIES

Tc-DTPA or In-111 is used. It has to be injected into the CSF via a lumbar puncture. You can then observe whether:

1. Any appears in the nose = fractured cribriform plate causing CSF rhinorrhea
...an example of a negative study because you see the CSF bathing the brain and spinal cord, but nothing is seen in the nose.

BONE DENSITOMETRY

Osteoporosis is defined as T<-2.5. Osteopaenia is defined as T -1 to -2.5.

i) T score shows your bone density relative to the mean bone density for Thirty year olds of the same sex and ethnicity. A positive number means higher than the mean, and a negative number, lower than the mean.

ii) Z score is the same thing, only for your age compatriots. This is what we use when we evaluate kids for their development. It was these scores that were used when the bone density machines were first invented in the 70’s.

The data for T and Z scores comes from the NHANES study for the hip. However, there is no unifying data for the spine, and so each machine manufacturer uses their own reference range.

The pictures that you use are:

So, what you measure is:

1. The femoral neck, greater trochanter and intertrochanteric region. You then sum these to give you the "total hip" number.

2. The first four lumbar vertebrae.

The reasons why the scan is typically done wrong are:

1. Hip rotation. If the lesser trochanter does not have the same shape then there has been rotation.

2. New spinal pathology that increases the attenuation of the X-ray beam - Paget's, wedge fracture,spinal rods, ...as well as aortic calcification.

So, the first thing to do is to check the images to rule these things out. Then, you check the graph:

You now have an immediate idea of whether their bone density is out of the reference range for them or not.

You confirm this by looking at the next table, which will show the actual bone mineral density, the T score and the Z score.

When you are doing this, there are a few things to be checking off in your mind, to make sure that no technical errors are fudging the results:

1. Are the neck, trochanteric, and intertrochanteric value within a 1/2 standard deviation from the total hip?

2. Are the four vertebrae close to each other in bone density? If not, then this is either a technical problem, or one of those vertebrae has undergone something pathological.

3. On the Lunar company's machines, the Z scores are adjusted for weight. This mucks things up in anorexic and obese people, and is why the Z score can be higher than the T score! (e.g. an anorexic 50yo is compared to other anorexic 50yo's and therefore gets a better Z score than if she were compared to all 50yo's).

The machine calculates the BMD by measuring the BMC (bone mineral concentration) in an Area that the technoloist has drawn lines around. The formula is BMD=BMC/A.

There are some important things to know that stem from this:

- when the spine BMD is reported, the typical value that is mentioned is the L1-L4 BMD. This is not the average of the four BMD's. Rather, this value comes from all four of the BMC's being summed and then being divided by the sum of the four A's. Thus, if one vertebra is no good, a summed lumbar BMD score can still be calculated by using the others.

One other value that is mentioned in reports is the BMAD (Bone Mineral Apparent Density) - this is a volumetirc measurement and is used for short people or kids.

How to report the DEXA:

1. The bone mineral density was done using a Hologic/Lunar/Norland machine.

2. The lumbar spine was X g/cm2 which is X standard deviations below (above) the mean for age-matched persons.
3. The total hip BMD was X g/cm2 which isX standard deviations below (above) expected for her age.
4. The T-score was X at the spine and X at the total hip. This places her in the WHO category of (normal, osteopenia, osteoporosis, established osteoporosis) [NB: need to know about fractures before you can tell the correct category.]...or, for males: The World Health Organization has not specified cut-off ranges for males.

5. Comparison to the previous study...

NB: There are three important things to know about comparisons to the previous studies:-

• Prior to 1997, there was no NHANES data, and so the reference range was quite different and therefore the T and Z scores were created from different reference ranges.
  
• There is a deviation of 2% between results taken at different times on the same machine. A significant difference is therefore only said to be present when there is a 6% difference.
• If the previous bone density was done on a machine from a different manufacturer, then to make a comparison to current bone density, both densities must be converted to standardized units.
  

6. The 10 year probability of fracture is ...

BONE SCAN LIBRARY

Here are the commonest bone scan pathologies you will come accross:

SHIN SPLINTS:
This occurs because the stress on the bone leads to more active osteoclasts clearing the bone so that more can be trowelled on. This trowelling is the periosteal reaction that you see on the scan.
Classic locations are the posteromedial tibia middle-distal 1/3 border. Appearance can be either fusiform or linear.



PAGETS DISEASE:
This will,4 out of 5 times, involve more than one bone. It starts at the ends of long bones and the leading edge of the hot patch has a flame-like appearance.
The bone is bent and thickened.

The differential is damage bone from chronic osteomyelitis, fibrous dysplasia, and osteosarcoma.

OSTEOMYELITIS:
All 3 phases on the bone scan will be hot, unlike in cellulitis, when only the first two will be hot.

GFR STUDY