Intro to Treatment Planning

Question 1

What makes a good plan?

Answer 1

• Uniform dose to PTV (ICRU -> 95-107%)
• 95% Isodose conforms closely to shape of PTV
• Critical doses to OAR not exceeded
• Integral dose kept to a minimum

Question 2

What can alter the dose distribution fo a plan?

Answer 2

1. Treatchment techniwue
2. Beam Arrangment
3. Beam Energy
4. Field Size
5. Beam Weights
6. MLCs, blocks, wedges

Question 3

Treatment Technique:

Fixed FSD vs ISOCENTRIC:

explaination

Answer 3

Fixed FSD

• Patient is moved so that the skin is at a constant distance (100cm) for each beam orientation

Isocentric

• Patient is positioned so that fixed point is at the machine isocentre & machine is moved around patient. FSD vaires. No re-positioning between beams

Question 4

Treatment Technique:

Fixed FSD: 10x10 field 1Gy prescribed to dmax SSD = 100

ISOCENTRIC: 10x10 field 1Gy prescribed to isocentre SSD = 90

Are the dose distributions the same?

Answer 4

No.

• Fixed FSD has 10x10 at the surface, isocentre as 10x10 at the isocentre (midline) To irradiate the same area of the patient, the isocentric plan would need to be increased to 11x11
• PDDs fall off due to attenuation and ISL: Fall off due to attenuation is similar but the ISL will impact the isocentric set-up more because the patient is closer to the source

Question 5

Treatment Technique:

Fixed FSD: 10x10 field 1Gy prescribed to midline SSD = 100

ISOCENTRIC: 11x11 field 1Gy prescribed to midline SSD = 90

Which plan requires more MUs?

Answer 5

Fixed SSD

• Patient is closer to source of radiation in isocentric plan, less ISL, hence more radiation reaching the midline, requires fewer MUs
• NB// as FSD increases, PDD increases but absolute dose goes down.
• NB// collimator setting was increased to irradiate same area of patient at midline

Question 6

Treatment Technique:

Influence of SSD on PDD

SSD = 85cm, 100cm, 130cm

NB// field sizes adjusted to make 10x10 at surface and isodoses normalised to 100% at dma

Answer 6

• Smaller SSD means
  
  • Reduced ISL attenuation
  • Increased dose as depth, isodoses linealry moving downwards

NB// go over this in the ppt.

Question 7

Beam Arrangement:

What do you need to think about?

Answer 7

• Where is PTV
• What shape is PTV?
• What are you entering through?
• What are you exiting through?
• Any other limitations e.g. collisions, couch etc.

Question 8

Beam Energy:

Why does dmax get deeper with increasing energy?

Answer 8

Beam is more attenuating

Question 9

Beam Energy:

Parallel opposed fields,

Larger patient separation, do you want 6 or 10MV?

Answer 9

10MV

• Parallel opposed, exit dose from one beam and entrance dose from other are summed
• Amount of dose is proportional to patient separation and beam energy
• 10MV would be attenuated less, and would have a higher exit dose, and greater dose at depth. This means you would need fewer MUs to provide the dose at midline.
• Also, 10MV means you would have more uniform depth dose profile and fewwer hotspots at skin

Question 10

Penumbra

Feild Size:

How does field size affect isodose charts?

Answer 10

Small field sizes - a large part of field is made up of penumbra region. Isodose charts dramatically change shape, depending on source size, collimation etc.

Question 11

Field Size:

Comments

Answer 11

As field size increases:

• Beam becomes more penetrating due to increased scatter
• Large field size (>10x10) flattening filter isn’t doing it’s job properly and it looks a bit horny at shallow depths

Dose profile for smallest field size is very peaked

Question 12

Field Size

• Why does PDD increases with field size
• Will PDD keep increasing with increasing field-size?
• How will this change with energy, why?

Answer 12

• Increase in scatter at depth, hence PDD increases with field size
• Effect plateauxs out as field size becoems so large that additional scatter at the edges can’t travel far enough to contribute to central axis
• PDD has less dependence on field size for higher energies because the probability of scatter decreases with increasing energy (in MV photon range) & higher energy photons are scattered more in the forward direction

Question 13

Wedges

Why does the angle of the wedged isodose lines change with depth?

Wedged fields can be created using:

a) physical
b) motorised/universal
c) dynamic/virtual wedges

How do these different methods work?

Answer 13

Physical - wedges of different angles that you would go put in the beam

Universal - 60deg wedge that can be used in combination with open fields to create different wedge angles

Dynamic/virtual - move the jaws to create any wedge angle you like

Question 14

PDDs

How would you expect the PDD to change for a virtual/dynamic wedge?

Answer 14

PDD for physical wedge should be greater at depth because the beam has been hardened

Question 15

Rule of thumb for wedge angles:

Answer 15

theta = (180-phi)/2

phi = hinge angle

Question 16

IMCRT>CRT>Conventional

Answer 16

Conventional = hard to get dose volume to conform to weird shapes of PTV

CRT = Collimator adjustment allows you to get the weird shape of the PTV but difficult to get complex dose distributions

IMCRT = Intensity modulated is the best

Question 17

THERE ARE VERY GOOD EXAM QUESTIONS AT THE END OF THIS LECTURE DO NOT MISS THEM