2. Tx Volume And Uncertainties

Question 1

2 ways to create treatment plans

Answer 1

Field based

Volume based

Question 2

Isocenter defined by a 5 year old

Answer 2

Like a wagon wheel where all fields intersect, couch and gantry rotate around isocenter

Question 3

Anatomic uncertainties

Answer 3

Refers to the clinically suspected but undetected disease involvement (we can’t see it)

• regional lymph nodes
• Microscopic extensions of the disease

Question 4

Geometric uncertainties

Answer 4

Refers to small spatial uncertainties associated with targeting

Planned isocenter in patient vs isocenter of machine

Question 5

How can geometric uncertainties be classified?

Answer 5

By their source:
Variations in patient anatomy or set up

By when they occur:
Interfractional- changes that occur between fractions (bladder filling, immobilization devices)

Intrafractional- changes that occur during treatment(within the same fraction) so breathing, patient movement

Question 6

GTV

Answer 6

Gross tumour volume

Gross demonstrable extent and location of malignant growth

Consist of the primary tumour

Question 7

CTV

Answer 7

Clinical target volume

Tissue volume that contains the GTV plus any microscopic extensions.

(If the tumour has been removed surgically there may be a CTV in absence of the GTV)

GTV+margin=CTV

Accounts for anatomic uncertainties

Question 8

PTV

Answer 8

Planning Target Volume

Geometric volume rather than clinical volume such as CTV

PTV=CTV+margin for geometric uncertainties

To select appropriate beam size and arrangements to ensure the prescribed dose is actually delivered to the CTV

Question 9

What is the differences between the CTV and PTV

Answer 9

CTV- is a clinical volume

PTV- is a geometric volume

Question 10

IM

Answer 10

Internal Margin

Accounts for expected physiologic movements and variations in size, shape, and positions in f the CTV

Ex- breathing, variation in bladder/rectum, swallowing, heart beat, BM

Question 11

SM

Answer 11

Set-up Margin

Accounts for uncertainties in patient setup and alignment of the beams through all sessions

Wiggle room

Question 12

CTV+IM=

ITV+SM=

Answer 12

1. ITV

2. PTV

Question 13

ITV

Answer 13

Internal Target Volume

Consist of an internal margin added to CTV to compensate for internal movements

Question 14

Treated volume

Answer 14

Volume enclosed by the prescription Isidore

Is the tissue volume is planned to receive at least a dose selected by the RO team to achieve treatment

Question 15

Irradiated volume

Answer 15

The volume which receives a dose that is considered significant in relation to normal tissue tolerance

Greater than 50% of the specified target dose

Question 16

OAR

Answer 16

Organs at risk

An organ if given excess dose would compromise the success of RT

Tissue or structure that at risk to radiation sensitivity may influence treatment

Question 17

PRV

Answer 17

Planning organs at risk volume

Refers to margins around organs at risk

Question 18

Plan evaluation

Answer 18

Dose to the PTV should be as homogeneous as possible

-some Heterogeneity is accepted due to technical reasons

-5% and +7% of the prescribed dose is acceptable
95%-107%

Question 19

Dose volume histogram

Answer 19

Graphically shows volume of structure vs dose
Used to evaluate plan
Compare plans
Doesn’t indicate where within the volume the dose is received