random questions Flashcards
Who is ultimately responsible for the dose deposition into the target?
Electrons give the dose.
The energy of the proton beam is transferred to secondary electrons through Coulomb interactions. Electrons are directly ionizing particles (since have charge and mass) and deposit the dose into the patient through hard collisions.
How do photons interact with matter? Give a general description of the quantities used to describe the phenomena and the interactions that take place.
ATTENUATION when we are in the presence of an absorbing/scattering material. The attenuation law is the Beer-Lambert law –> depends on the attenuation coefficient, associated to each of the possible interactions
Tissues have approximately Z=7:
1) Photoelectric effect: interaction of photon with inner shell electron.
E=1 - 50 keV
mu prop. to (Z/E)^3
2) Compton effect: interaction of photon with outer shell electron. Both get scattered.
E=50 keV - 20 MeV
mu prop. to (E’/E)
3) Pair production: interaction of photon with Coulomb field of nucleus. Pair of electron/positron is emitted.
E = 20-100 MeV
Rayleigh scattering: no energy deposition in the material
Photonuclear effect: produces neutrons but again no energy deposition
How do charged particles interact with matter?
SOFT COLLISIONS: large number, low energy transfer
HARD COLLISIONS: low number, high energy transfer
RADIATION COLLISIONS: elastic or inelastic. Inelastic cause the production of Bremsstrahlung x-rays [cross section prop. to (Z/M)^2]
What is the stopping power? How does it change for heavy and light charged particles?
The stopping power is defined as the energy loss per unit path length of the primary particle. Given the different types of collisions we can define different types of stopping powers.
For heavy charged particles, which interact mainly through collision, the radiative stopping power is 0.
For light charged particles at low temperature the collisional stopping power overcomes the radiative one; the inverse situation occurs at high temperatures resp. kinetik energies.
Light charged particles follow a more tortuous path into the target medium wrt. heavier particles. They are subject to more scattering and deposit large amount of dose in the entrance region; the dependence of the collisional stopping power on the inverse square of the velocity is the reason for the formation of the Bragg peak.
What is the difference between x-rays and gamma-rays?
x-rays are photon radiation produced in scattering interactions. Are less energetic than gamma rays and are therefore used for imaging purposes. The energy ranges between 124 eV and 1.24 MeV
gamma rays are photon radiation produced by the gamma decay of a radionuclide. They are more energetic than x-rays and can therefore be used in radiotherapy. The most common nuclide used is 60Co [E = 50 keV - 3 MeV].
What are typical energy ranges that we use for treatment and for imaging?
Thus what are the relevant interactions?
For imaging: 10-150 keV –> Photoelectric effect
For treatmen: 6-25 MeV –> Compton effect
Name devices that we can use to produce radiation of both photons, electrons and heavier particles.
1) X-RAY TUBE: used to produce photon beams (mainly bremsstrahlung x-rays and characteristic x-rays)
2) LINAC: can be used for both photon beams and electron beams
3) CYCLOTHRON / SYNCHROTRON: production of proton beams or heavy ion beams.
4) Cobalt-60 machine can still be used and there we have encapsulated 60-Co sources radiating the photons (gamma rays).
Define important quantities in dosimetry and describe how they are related to one another.
The most important equation in dosimetry relates the physical absorbed dose, the collisional (mass) stopping power of the target medium and the fluence of the beam.
- The physical absorbed dose is defined as the energy absorbed per unit target mass. Usually indicated in Gy.
- The fluence of the beam is defined as the number of particles crossing an infinitesimal area dA. dN/dA, usually given in 1/cm^2
- The mass stopping power is the density normalization of the stopping power of the target material. We only consider the collisional mass stopping power as this is the only way we can deposit energy into the target.
The fundamental dosimetry equation tells us that: D = S phi
List some dosimetry methods/instruments, their working principle and the quantity whose change they are sensible to.
1) CALORIMETRY: absolute method of dosimetry
- calorimeter: measures the temperature change induced by irradiation
- almost all the absorbed energy is converted into heat
- gives the primary dose-to-water standard for the institute it is produced in
2) IONIZATION CHAMBERS: calibration needed and done with help of calorimeters.
- gas contained in the chamber gets ionized by the incoming radiation and we observe ion pair production. We capture the produced ions with the help of an electric field and measure the number of produced ions per irradiation time.
- proportionality relation to absorbed dose is D = Q/M (W/e)
- typical voltage range between 200 and 400 V.
3) LUMINESCENCE DETECTORS: Scintillators vs. thermally and optically stimulated detectors
- Scintillating detectors respond to irradiation spontaneously, without being stimulated from outside and without storing the received energy. The electrons decay spontaneously from the excited states and release a photon.
- Stimulated detectors store the energy as excitation energy for the electrons of the valence band, which is then re-emitted as photo radiation once the detector is stimulated, either thermally or optically.
4) Gafchromic films:
- irradiation ignites chemical reactions in the foil which cause the color to change.
- the signal can be read by a regular scanner.
Describe the general structure of a radiation dosimeter.
Give the definitions for active and passive dosimeters and classify the detectors into those categories.
We have a detector reacting to the irradiation and producing a signal which is received and displayed by the reader.
Depending on whether the detector and the reader are directly in contact or not during the irradiation we can distinguish between active and passive detectors.
Among those we have seen we have:
A) scintillators, calorimeters, IC
P) luminescence detectors, gafchromic films
Describe what the TRS 398 protocol is and how the workflow for the calibration of ionization chambers works.
TRS 398 protocol is a collection of data and protocols that provides the guidelines to follow when performing dosimetry with ionisation chambers.
For the calibration process, needed to assess the correct dose to water, 3 steps are required.
1) reference dosimetry done by the national metrology institute: They do the measurements for the dose to water with a beam of reference quality, whose dose-to-water value has been determined with calorimetry.
- this provides the correction factor N_(D, W, Q0)
2) Reference dosimetry with hospital radiation beam of quality Q
- with help of tabulated correction factors we can relate the measurement for the dose-to-water at reference quality and user’s quality.
- provided correction factor: k_(Q, Q0)
3) Cross calibration of all the ionization chambers of the hospital with respect to the calibrated reference IC.
All the correction factors are needed to relate the value displayed by the reader with the dose to water for the chamber.
The displayed value needs to be corrected as well with some correction factors: the two main contributions come from the temperature-pressure correction factors and the ion recombination correction factors.
How do we relate the physical absorbed dose of two different media?
Theoretical model: Bragg cavity theory
- cavity doesn’t impact the fluence of charged particle into the external medium
- dose absorbed in the cavity is entirely deposited by the charged particles crossing it
- charged particle equilibrium is satisfied
–> DOSE RATIO EQUALS THE MASS STOPPING POWERS RATIO
