Planning Terminology

Question 1

What does the word cover refer to?

Answer 1

This means how well the PTV is covered by the 95% isodose curve.

• Does it conform to the shape of the PTV
• Are there any areas not covered or covered too generously?

Question 2

What does dose variation refer to?

Answer 2

The distribution of dose across the PTV

ICRU makes recommendations about this, What is your highest dose and what is your lowest dose?

Question 3

What does Dose Homogeneity refer to?

Answer 3

This is how EVENLY spread the isodoses are across the PTV.

(It is not homogenous if you have more dose in one part of the PTV than the other)

Question 4

Peripheral dose defiintion

Answer 4

Refers to any dose or tissue outside the PTV

Question 5

Definition: Relative Electron Density

Answer 5

Compares the electron density of tissue to the density of water (The electron density of water is one what is the electron density of the tissue relative to water?)

Question 6

Relative electron density of soft tissue?

Answer 6

1.00

Question 7

Relative electron density of air

Answer 7

Air = 0

Question 8

Relative Electron Density of Lung

Answer 8

Lung = 0.3

Question 9

Relative electron density of bone (range):

Answer 9

Bone= 1.3-1.7

Question 10

When thinking about beam arrangement, what are the three things we consider about the PTV:

Answer 10

1) Size
2) Shape
3) Location
4) OAR

Question 11

What is an isodose curve?

Answer 11

They are lines passing through points of equal dose.

They are drawn at regular intervals of absorbed dose and are expressed as a percentage of dose at a reference point.

Question 12

The dose at any given 1 is _ 2_on central axis of the beam and gradually 3 toward the edges of the beam.

(Depth/Height, Greatest/Smallest, Decreases/Increases)

Answer 12

1) Depth
2) Greatest
3) decreases

Question 13

What is a build up region?

Answer 13

When radiation interacts with tissue a certain depth of is required before we see the maximum amount of radiation. (This is responsible for skin sparing effect)

Question 14

Energy fill in the blanks:
The 1 at dmax is defined as 100% and then the radiation dose 2 as the depth 3 the energy being absorbed within the tissue.

1) Dose/Energy
2) Decreases/Increases
3) Decreases/Increases

Answer 14

The 1 at dmax is defined as 100% and then the radiation dose 2 as the depth 3 the energy being absorbed within the tissue.

1) dose
2) decreases
3) increases

Question 15

The depth of d-max and the size of the build up region is energy dependent.
We refer to dmax for SXR/DXR (KV) as:

Answer 15

On the skin surface

Question 16

The depth of d-max and the size of the build up region is energy dependent.
We refer to dmax as ? cm for 6MV

Answer 16

1.5cm

Question 17

The depth of d-max and the size of the build up region is energy dependent.
We refer to dmax as ? cm for 10MV

Answer 17

2.5cm

Question 18

The depth of d-max and the size of the build up region is energy dependent.
We refer to dmax as ? cm for 18MV

Answer 18

3.5cm

Question 19

What is the definition of field size?

Answer 19

Field size is the lateral distance between the 50% isodose lines at a reference depth
(On the skin surface for fixed SSD, At the isocentre for isocentric techniques)

Question 20

Where is field size determined for fixed SSD beams?

Answer 20

For fixed SSD beams, the field size is determined where the 50% isodose curve cuts the skin surface at 100cm SSD

Question 21

Where is field size determined for isocentric beams?

Answer 21

Field size is determined where the 50% isodose curve cuts the isocentre at 100cm SAD.

Question 22

What is the definition of beam weighting?

Answer 22

The weight of an individual beam is its relative contribution of dose to the overall treatment dose

Question 23

According to ICRU50 what is the definition of Irradiated volume?

Answer 23

Tissue receiving a dose considered significant in relation to normal tissue

Question 24

ICRU 50 - Treated volume definition

Answer 24

Tissue irradiated to include PTV

Question 25

Planning tumour (target) volume according to ICRU 50

Answer 25

CTV plus margins for motion and set up

Question 26

What is CTV according to ICRU50?

Answer 26

Clinical tumour (target) volume
Is the Gross tumour volume (GTV) + margin for microscopic tumour

Question 27

What is GTV according to ICRU50?

Answer 27

Gross tumour volume is the palpable or visible macroscopic tumour