5.D Verification Imaging

Question 1

Radiographic Film:
Dose reaching the image receptor depends on

Answer 1

• Energy
• Patient separation
• Inverse square law
• Everything that affects the scattered component
  ‒ Field size, thickness, air gap

Question 2

Quality of film is dependent on

Answer 2

• Latitude
• sensitivity
• speed
• characteristic curve

Question 3

EPIDS
* Advantage

Answer 3

-the spatial resolution is not limited by the EPID design
-contrast resolution is high but image contrast is limited by MV radiation
-speed (real time)
-remote access
-low profile when not in use

Question 4

EPIDS
* disadvantage

Answer 4

• radiation damage limits lifetime
• size maybe too small for very large fields.
• increases the backscatter component

Question 5

EPIDS = How does it work

Answer 5

-ray traced doses are computed into portal imager

-portal dose images are predicted at planning then compared at measured values

-dont match=difficult to relate this back to dose distribution in the patient

Question 6

What is Transit dosimetry

Answer 6

-darkness of image ===== the dosage at that point

Question 7

Transit dosimetry:forward projection

Answer 7

portal dose calculated BY TPS at the Epid plane are predicted at planning then compared at measured values from EPID

-IF DONT MATCH=difficult to relate this back to the dose distribution in px

-easy to do

Question 8

Backprojection

Answer 8

-measured EPID dose is backprojected to any plan of the patient
-more complicated algorithm

Question 9

Gamma evaluation (1)
Dose difference

Answer 9

• Just subtract the doses for each (x,y) coordinate.
• Very good for homogenous (dose) regions
• Poor where regions have high dose gradients due to misalignment

Question 10

Gamma evaluation (1)
Distance to agreement (DTA)

Answer 10

Distance between a point on the dose distribution and the nearest point on the calculated distribution with the same dose.

Question 11

Ghosting

Answer 11

charge trapping in the EPID
-problem for low MU fields

Question 12

Disadvantages

Answer 12

-Non water/tissue equivalence of amorphous silicon
-Oversensitive to low energy photons

Question 13

Disadvantages:
Pixels at the edge of the field will have a different low energy profile to those in the middle because of

Answer 13

-Differential beam hardening because of the shape of
the flattening filter
‒ Distance from the patient
‒ Path length in the patient

Question 14

Disadvantages of Radiographic film

Answer 14

*Changes in processing conditions
* Interfilm emulsion differences
* Artefact caused by air pockets adjacent to the film

Question 15

Radiographic film is Still useful for checking

Answer 15

• Radiation fields
• Light field coincidence
• Flatness and symmetry
• qualitative radiation distributions