Contouring and Localization

ONCOL 355 - Planning and Dosimetry. University of Alberta

External Countour

Outline of the patient’s skin surface usually done through the transverse plane

Internal Contour

Outline of the body's internal structures, depicting organ shapes and relationships, through the transverse plane

three reasons contours are made

1. Patient shape and size

   • depths from surface to isocenter

2. Location of Organs

   • amount of organs in the radiation path to calculate dose to these organs

     • we need to have a visual of what the radiaiton beam is travelling through

3. Set-up data for the radiation therapists

   • Contours needed to assist radiation therapists in setting up the patient

   • Depths important for the planner, helps with MU calculations

why do we need critical organs countoured?

they are radiosensitive and dose limiting

• this limits the amount of dose we can give to the tumor

Serial Structures

Structures that when damage to the organ occurs in one spot, the organ completely loses function

• example: spinal cord

• have max dose that can be applied (4500-5000 cGy)

Parallel Structures

structures that hen damage to the organ occurs in one spot, funcitonal impairment in that spot occurs, but the rest of the organ can keep working

• example: lung and kidney

  • :we need to know how much of that structure gets dose, is a volume relationship

  • how much of the lung is getting 3000 cGy

4 old methods of external countouring

1. Simulator

2. Bendable Wire

3. Plaster of Paris Bandages

4. Immobilization Shell

Simulator

Simulator looks like a treatment machine

• Rotates around patient's isocenter 10 degrees and read the ODI's of patient surface, then can extract an external countour

Bendable wire

Put wire on patient and get crude contour, apply to digitizer

Plaster of Paris Bandages

Bandages used to create external contours by wrapping them around the patient's body, hardening to form a rigid shape for treatment planning and localization.

Immobilization Shell

a shell which reflects a patient’s shape is used and a contour is traced from inside the shell.

• a jig with a pointer attached to a pencil traces the shape and applies it to paper

how are internal contours created alongside the 4 outdated methods of external contouring?

orthogonal x-rays images

• A/P and lateral images to gain A/P, Sup/Inf and lateral information

disadvantage of 2D countouring

only reflect’s patient’s countour at one point, there is a change in organ shape within each slice that can’t be see without a CT

what machine is now used for both internal and external countour

CT images

• multiple planes: 2D transverse, saggital, and coronal images are used to give a 3D view of the body/tumor

• we now get multiple slices in each plane

3D CT countours give information about …

the size and shape of the patient, internal structures, and the tumor

GTV

Gross Tumor Volume

• visable extent of the tumor, easy to put contour line around

CTV

Clinical Target Volume

• accounts for microscopic cells of the tumor, puts extra room around GTV

ITV

an additional margin (internal margin) is palces to account for motion of the patient

• can only do this if we have 4D CT information

Formula for ITV

ITV = CTV + IM

SM

Set-up margin

• additional contour placed as a geometrical concept as patient set-up on bed will always have discrepencies

• placed on top of the CTV or ITV

PTV

Planning Target Volume

is the volume that includes the ITV plus the setup margin (SM) to ensure that the prescribed dose is delivered to the CTV despite variations in patient positioning and internal motion.

PTV formula

PTV = CTV (or ITV) + SM

TV

Treated Volume

•  high dose region that is treated when all the beams are on, what is prescribed by the RO

IRV

 total treated volume account for all areas where beams enter or exit

ITV accounts for _____, PTV accounts for _____

motion, set-up discrepancies

describe the orders of the volumes as they are created

GTV —> CTV —> ITV —> PTV

OAR

Organ at risk

• normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose

PRV

Planning risk volume

• additional margin around the OAR may be recommended to account for organ motion and set-up uncertainties

• helps ensure sparing of OAR

PRV formula

OAR + Internal/set-up margin = PRV