proton beam therapy

Question 1

what machine accelerates protons during PBT

Answer 1

cyclotron

Question 2

what type two types of cells are radioresistant and why

Answer 2

red blood cells as they have no DNA and nerve cells as they rarely divide.

Question 3

PBT produces a pencil beam, what is a limitation of this?

Answer 3

the Bragg peak will very deep and if the tumour is close to the skin the tumour will receive no/low dose.

Question 4

what are the two methods to overcome the limitations with the pencil beam produced during PBT

Answer 4

1. using a scattering device to produce a broad beam of protons
2. using the narrow pencil beam to scan through the tumour volume similar to 3d printing

Question 5

describe the bragg peak of PBT in comparison to photon therapy

Answer 5

Proton therapy has a definitive zero dose distal to the Bragg peak whereas conventional photon therapy has a build-up effect to the Bragg peak which reduces exponentially after.

Question 6

why is proton therapy used in paediatrics

Answer 6

1. Low dose to surrounding tissues thus little side effects.
2. Reduces induction of secondary malignancy by 21%.

Question 7

name and describe the 5 differences between PBT and RT.

Answer 7

1. Tissue depth – protons stop in tissue at a depth which depends on tissue characteristics and proton energy, there is no dose distal to the Bragg peak.
   a. Photon beams do not have a corresponding depth beyond where there is no dose – and can be exploited clinically.
2. Effect on skin – photon beams have a skin-sparing effect whereas in PBT small amounts of dose are deposited on the skin surface. Although it is lower than the Bragg peak it means that the skin dose will be higher with protons than with photons.
3. Range uncertainty – for photons density on CT scans can accurately predict how the photons will interact with tissues whereas in PBT there is uncertainty as the CT HU is converted into relative stopping powers (RSP) for protons, but this is an approximation.

a. When working with protons the HU don’t hold as true so they have to be converted into RSP this is because the Bragg peak of protons is unclear which leads to range uncertainty.

4. Beam edge – lateral proton beam edge is typically sharper than a photon beam at superficial depths which can be useful when irradicating a tumour next to a radiosensitive organ.
5. Biological effect (depends on fractionation, dose & type of radiation) – thought that the biological effect of protons is greater by a factor of 1:1

Question 8

what are modulator nozzles used for in PBT?

Answer 8

enable a conformal shape so there is a spread of Bragg peak (SOBP), all the tumour can be irradiated.