Heterogeneity Corrections

Question 1

In Monaco what is the difference between an interest point and and a marker.

Answer 1

Interest point- a point where you can report/monitor dose

Marker- a physical point - e.g. BB.

Question 2

What are some heterogeneties in the human body?

Answer 2

Bone
Teeth
Air cavities
Small intercalated spaces within the bone

Question 3

What are the 2 types of inhomogeneites

Answer 3

natural occurring internal inhomogeneites

Man made materials

Question 4

What is a CT-ED file?

Answer 4

CT number to electron density file.

Question 5

How do physicists come up with the CT-ED file?

Answer 5

a TISSUE CHARACTERISATION phantom (containing different densities) is scanned to establish the relationship between the electron density of various tissues and their corresponding HU/CT number.

This is then entered into the TPS.

The phantom does not have the density for metal - that’s why we have to account for it ourselves.

Question 6

What is the electron and physical density of titanium?

Answer 6

electron density: 3.8

physical density: 4.6

Question 7

What is the electron and physical density of steel?

Answer 7

Electron density: 6.8

Physical density: 8.1

Question 8

What densities do Monaco and Pinnacle use?

Answer 8

Pinnacle - Physical density

Monaco - Electron density

Question 9

Should we include or exclude BB markers in the contour

Answer 9

Exclude BB markers from contours as they can effect distribution.

Question 10

Do we need to contour/override the density of fidcucial markers

Answer 10

Yes.

Question 11

What are the effects of streaking artifacts treatment plans?

Answer 11

Effects contouring - difficult to delineate structures

Effects on dosimetry - because of the metal there will be more dose delivered to the patient (there will be very high dose close to the artifact).

Question 12

Why is it important to look at breast tissue expanders.

Answer 12

Some breast tissue expanders have a magnetic disk which is metallic. This has been proven to affect dose distribution (reduces dose to the target volume)

The metallic streaking from the disk makes it hard to contour OAR as well

Question 13

What must we consider for a hip prosthesis?

Answer 13

There is a femoral and acetabular component that have different densities. Therefore you must contour them as separate structures.

The density of the the acetabular component is higher than the femoral component.

Question 14

Is it better to use a higher or lower energy beam when treating near a metal heterogeneity.

Answer 14

Better to use lower energy beam as the interactions with the metal will be more with a higher energy beam.

For energies greater than 10MV there is increased risk of gamma ray production in the metal.

Question 15

How can we minimise metal artifacts in CT?

Answer 15

Check if the scanner has a metal artifact reduction algorithm or metal deletion technique (MDT) which will improve image quality.

This is applied after the patient has been scanned.

Question 16

What are the beam arrangement options for accounting for a prosthesis.

Answer 16

Option1: avoid the beams that pass through the device. However this can result in increasing dose to OAR. Also need to consider practical issues like gantry-couch collision, portal verification, etc.

Option2: (Dose compensation method) Contour prosthesis, make it a critical structure and assign a dose constraints - done using IMRT and VMAT. Can also use avoidance sectors (VMAT)

Option 2 is better as it protects OAR and avoids prosthesis.