Fr - Some main points of the imaging weeks

Question 1

What is the therapeutic window? Explain

Answer 1

The difference in dose between tumor control and tissue damage. (The difference between the Tumor control line and the complication line.) Larger window means for the same tumor control probability, the tissue control probability is much lower.

Question 2

What would happen in hypoxia with the therapeutic window? Hypoxic tumors

Answer 2

The hypoxic tumor cells are more resistant, therefor the therapeutic window becomes smaller. The line for the normal tissues stays the same, but the blue line (the tumor line) shifts to the right.

The tumor dose must be higher and for that reason the therapeutic window must be smaller.

Question 3

Be sure to know: GTV, CTV, PTV
Which one of these is the largest? Why? (2 reasons why)

Answer 3

PTV (Because of the margins around the tumor.
Because there may be micro metastases around the GTV, to include this we need a margin around it. Another reason is that the patient might move a bit.

Question 4

Brain tumors: why is MRI used for imaging the healthy parts of the brain?

Answer 4

To get functional information about the cortical brain areas and about the nerve tracts.

Question 5

Brain tumors: why is MRI used for imaging the healthy parts of the brain?
To get functional information about the cortical brain areas and about the nerve tracts.
Why do we want to know that?

Answer 5

So they know which areas to avoid with radiotherapy/surgeries. Because saving the important brain areas is crucial for quality of life. In surgery and radiotherapy. e.g.: Motor function of hands, vision.

Question 6

Describe the key tissue property that allows fiber tracking of white matter fiber

Answer 6

The scientific name for that difference is fractional anisotropy. This allows the fibre tracking.
FA does not work in tumors, because it does not have ordered fibre structure such as in the nerve tracts.

Question 7

Why is the tumor high signal on DWI?

Answer 7

Because there is a lot of diffusion restriction, and diffusion kills the signal. Tumors are highly dense.

Question 8

Why is the tumor signal low on ADC?

Answer 8

Because bright indicates the amount of diffusivity. High diffusivity, high value.

Question 9

Why is the MRI signal intensity in tumors higher on ce-MRI, than on MRI without contrast agent? (2)

Answer 9

So there are fewer support cells and the new vessels tend to be very leaky, meaning it leaks in the interstitial space. Therefore it stats in the tissue for some time.
Tumors are highly vascularised with leaky tissues, the contrast agents slips through these leaks.

Question 10

Ce-MRI is used to measure if a tumor responds to therapy. What change do you expect in the ce-MRI signal intensity, if the tumor responds favourably? Motivate your answer.

Answer 10

The contrast enhancement is much lower.

Question 11

Is it because of coincidence detection that we know the line of the PET signal

Answer 11

Yes

Question 12

Why does PET have better spatial resolution than other nuclear techniques?

Answer 12

Because of coincidence detection

Question 13

Why is a pet scanner always combined with a CT? (2)

Answer 13

For structure/anatomy localisation of the PET image
For correction of attenuation of the radiation within the patient’s body.

Question 14

Why is 89-zirconium used for monoclonal antibodies?

Answer 14

Because antibodies need around 2 days to be taken up by their target

Question 15

What is the specific radiotracer for restaging prostate cancer?

Answer 15

Prostate Specific Membrane Antigen

Question 16

Why do you need modelling of the amount of tracer in blood, in the interstitial space and bound to the target-specific receptor?

Answer 16

The total PET signal is the sum of
- The blood fraction
- The fraction in the interstitial space
- And the fraction of target-mediated binding.
Thus after measuring the total PET signal, you need modelling to calculate the true target-mediated binding. This is called tissue compartment modelling.