Proton Therapy Flashcards
What are the advantages and disadvantages (in the physics) of proton therapy with respect to conventional radiotherapy?
A) Protons don’t get absorbed, they deposit few energy through Coulomb interactions, they have the skin-sparing effect
D) they are much more subject to scattering, they interact with neighboring nuclei producing secondary electrons that can give dose
What is a SOBP and how is it obtained? (in PS and AS)
The spread-out Bragg peak is the energy deposition pattern we try to create when performing proton therapy. It consists in the superposition of multiple Bragg peaks with different energies and ranges which produces an homogeneous dose distribution in the whole area of interest where the tumor is.
What are the differences and analogies of the LINAC setup and the PS?
A) When we use a LINAC for electrons we have an incoming pencil beam which gets scattered and broadened laterally. The lateral shape is modeled by collimators.
D) In the LINAC we have the MLC for dose conformity, whereas in the PS we have collimator and compensator. The protons are produced in a generator, the electrons by the electron gun. The PS is patient-specific, the LINAC is used for multiple patients.
Why is proton therapy better than conventional RT (regarding clinical outcomes)?
1) Dose deposition pattern: The mid-to-low dose levels to healthy tissues in comparison to photons are drastically reduced. –> reduced toxicity
2) Improved post-treatment quality of life for pediatric tumors
3) Lower incidence in secondary induced tumors
4) Improved tolerance to radio-chemo therapy
What are the physical and dosimetric properties of heavy ions?
1) Comparable dose plateau to that of protons
2) higher mass leads to: sharper Bragg peak. more energy required for the same range, less scattering
3) fragmentation tail
What is the RBE? What quantities does it depend on?
Why is this an advantage/disadvantage?
RBE: relative biological effectiveness. Is the effectiveness of the radiation in damaging tissues compared to that of a reference radiation (Cobalt60). It is end-point, dose and tissue dependent.
What are the differences (quantitative) between the RBE for protons and heavy ions?
- The RBE for protons is very different from that of heavy ions and that is due to the difference in the ionization pattern. Heavy ions are more densely ionizing.
- The RBE for protons is 1.1 for all tissues and doses, whereas for heavy ion it can vary up to 4. –> large differences radio-biologically
- From in-vitro experiments we find that the RBE increases as dose/energy decreases (sharper trend on the cell survival curve).
Why can the difference in RBE and LET between protons and photons be problematic? (in clinical cases)
For clinical cases the RBE advantage of heavy ions may not be as high as expected –> look at the RBE distribution. They may be higher in healthy tissues than in tumors…
What is the LET? What quantities does it depend on?
How does the RBE depend on the LET?
LET = linear energy transfer. The mean energy transferred to the target material by a particle traveling the distance of 1 micrometer.
It depends on particle energy, charge and mass.
The RBE increases with the LET up to a maximum, then decreases.
