Introduction to Treatment Planning Principles

Question 1

AIM OF TREATMENT PLANNING

Answer 1

Irradiate Target volume with the dose prescribed

Reduce the dose to normal tissue as much as possible

Create the optimal dose distribution

Question 2

2D and 3D Conformal Treatment Planning (CTP)

Answer 2

• performed using forward planning
• relies on planners experience
• Planner manually chooses all aspects of the plan:
  no. of beams , beam geometry and wedges

Question 3

Fixed FSD technique

Answer 3

Same FSD for all beams (100cm)

Question 4

Isocentric vs. Fixed FSD – Single Field

Answer 4

fixed: Field size is defined at the surface of the phantom/patient
isocentric: defined at isocentre

Question 5

Isocentric vs. Fixed FSD - Parallel Opposed

Answer 5

• Higher monitor units for fixed FSD plan (distance factor)
• Need to move couch between fields to reset FSD (chance of error in this)
• Field sizes increases for isocentric technique to cover the same volume

Question 6

Penumbra

Answer 6

distance between the 20% and the 80% (10% and 90%) isodose lines

region near the edge of the field where the dose falls off rapidly

Question 7

Width of penumbra depends on

Answer 7

• Size of ‘source’
• SSD/FSD (lower SSD, higher penumbra)
• Energy (↑penumbra with ↑energy : increased field size)

Question 8

Dose on the field edge is

Answer 8

50%

Question 9

Wedges
purpose
how is it done
what is the wedge angle

Answer 9

shape the isodose distribution.

reducing the radiation intensity progressively along a beam.

angle through which the isodose curve is tilted in relation to the normal position at the central axis of the beam at that specific depth

Question 10

Why shape isodoses with wedges?

Answer 10

to create a uniform dose distribution when beams are arranged at angles at one another

compensate for surface obliquity

to shape the dose to match the target volume better

Question 11

Beam Weighting
function
start with

Answer 11

improves dose distribution and reduces exit dose to OAR eg. parotid and lung

conventional weighting (all beams equal) and modified based on patient and situation

Question 12

bolus
what is it
purpose
counteracts with

Answer 12

• tissue -equivalent material placed on patients skin
• increase dose on surface
• skin sparing effect of of mega-voltage x-rays ,while retaining penetration

Question 13

options of Isocentre Placement

Answer 13

1. at CT reference point
2. at centre of planning target volume (PTV)
3. elsewhere within PTV

Question 13

if isocentric placement is at CT Ref point

Answer 13

• will not need moves /verification

- not always suitable for ipsilateral target

Question 14

if isocentric placement is at Centre of PTV

Answer 14

require daily moves in all directions and verif

Question 15

Normalisation
what is it
plans are usually normalised at

Answer 15

normalisation point is the point where the dose is forced to 100% and the dose everywhere is changed to the same ratio

geometric centre of target volume (usually isocentre)

Question 16

when do need to normalise

Answer 16

if isocentre is:

• at post edge of beam
• in inhomogenous tissue
• near a field edge or shielding