RT Treatment Planning

Question 1

What is meant by a good RT plan?

Answer 1

• uniform dose to PTV (95 - 107%)
• close 95% isodose conformity to PTV
• dose to OARs below the limits
• low integral dose
• consideration of practical set-up and deliverable

Question 2

What patient factors may affect the plan?

Answer 2

• intent i.e. is it radical or pallative
• other concurrent treatments
• risk factors
• age
• patient choice

Question 3

What parameters can be changes to affect the dose distribution?

Answer 3

• beam arrangement
• wedges
• energy
• MLCs
• FS
• weightings

Question 4

What affect dose increasing the SSD have on the PDD and why?

Answer 4

Increase the SSD, makes the dose tail off more gradually as the ISL has less of a noticable effect at greater distances.

Question 5

What should be considered when deciding the optimal beam arrangement?

Answer 5

• location of the PTV
• shape of the PTV
• location of OARs
• entrance and exit dose
• practical issues with delivery

Question 6

How does increasing the beam energy affect the PDD?

Answer 6

Increase in beam energy results in:

• more gradual tail off due to harder beam and therefore more penetrating
• increase in D_max due to longer mean free path of secondary electrons so a longer build-up
• decrease in surface dose due a less backscatter

Question 7

What affect dose the beam energy have on isodose plots?

Answer 7

Ideally at 10 cm the isodose should be flat at the correct energy. At higher energies the profile becomes more rounded due to increased penetration of the beam. At lower energies, the beam cannot penetrate the central portion of the FF effectively resulting in a horny profile.

Question 8

How does changing the FS affect the PDD?

Answer 8

If we increase the FS then the PDD has a more gradual tail off, mainly due to an increase in phantom scatter. There is a slight contribution from the increase in FF scatter and electron contamination but this will not have much effect at greater depths.

Question 9

Why are wedges used? What effect do they have on the PDD and why does the angle appear to change on an isodose plot?

Answer 9

• used to account for surface shape and to even out dose gradients
• adding in a wedge hardens the beam and therefore results in a more gradual tail-off
• the angle of the lines changes due to the hardness of the beam changing with depth