9. Radiation therapy: linear energy transfer, penetration depth, Bragg-peak. Devices for radiation therapy. ALARA principle, dose limits. Flashcards
What is radiation therapy?
A medical intervention, where the pathological tissues are destroyed by ionizing radiation
What is the aim of radiation theory
to deliver a curative dose of irradiation to a defined tumor volume with as minimal damage as possible to surrounding healthy tissue.
What are the 6 well-known ionizing radiations used in radiotherapy?
- Alpha radiation
- Beta radiation
- Electron radiation
- Gamma radiation
- X-ray
- Proton radiation
Consequences of the absorption of ionizing radiation.
What are they? (3)
- Physical events
- Chemical reactions
- Biological consequences
Consequences of the absorption of ionizing radiation. - 1. Physical event
What does The amount of secondary ionization depend on?
on the material
→ it can be up to 10 times the amount of primary ionization.
Consequences of the absorption of ionizing radiation.
Describe the physical events
Using direct or indirect ionization
- Charged particles interact strongly and ionize directly
- Neutral particle;es interact less, ionize directly and penetrate farther
Consequences of the absorption of ionizing radiation.
Describe the chemical; events
Cell death, death of living system
Carcinogenesis, genetic transformation
Consequences of the absorption of ionizing radiation.
Describe the biological events
Direct or indirect effects occur
- Direct effect - Direct ionization of the macromolecules
- Indirect effect - Reactive ions (e.g. OH-) and/or radicals (e.g. *OH) are generated mainly from water molecules.
→ Reactive species induce damages in macromolecules and membrane structures.
3 approaches for radiotherapy
- Palliative radiotherapy
- Radical radiotherapy
- Adjuvant treatment
Purpose of Palliative radiotherapy. (+ examples)
to reduce pain and address acute symptoms
– e.g. bone metastasis, spinal cord compression etc.,
Purpose of Radical radiotherapy. (+ examples)
primary modality for cure
– e.g. head and neck tumours
Purpose of Adjuvant radiotherapy. (+ examples)
an additional cancer treatment given after the primary treatment such as surgery
→
What is Linear ion density?
the amount of n ion pairs produced on a path of length l
→ give the quantity n/l
→ characterize ionizing power
What is LET (Linear Energy Transfer)?
the energy transferred to the material surrounding the particle track, by means of secondary electrons
→ (nEion pair/l) → i.e, the product of the linear ion density and the energy necessary for creating an ion pair
Characteristics of α-radiation in radiotherapy
Internally deposited radioactivity
High ionizing power → high biological effect
How can α-radiation be used in radiotherapy
Internally deposited radioactivity
- α-radiation reach only the body surface → have to introduce the isotope itself into the target
- The isotope is bound to a carrier molecule (e.g, antibody) → reach the tumor cells ASAP → exerting damaging effects
Characteristics of β- radiation in radiotherapy
- Internally seeded radioactivity
- Particle energy is not optimal
- continuous energy spectrum
- typical energy: few MeV
Disadvantages of β- radiation in radiotherapy
- typical energy: few MeV → not penetrating deeper
- continuous spectrum
Why is continuous spectrum a disadvantag of β- radiation in radiotherapy
- One portion of electrons has energy that is below the average → readily absorbed
- Other part has above-average energy → can penetrate deeper
Characteristics of electron radiation in radiotherapy
- accelerated electron
- production: linear accelerator
- Efficient distance
→ can be used for treatment of superficial tumours
2 differences between β- radiation and electron radiation
β- radiation VS. electron radiation
- β- radiation - energy is determined by the isotope and its decay
- electron radiation - energy can be controlled
- β- radiation - continuous energy spectrum
- electron radiation - energy spectrum is dismissed → each accelerated electron has the same energy
How can β- radiation and electron radiation be used together?
Drawbacks of β- radiation can be useful accelerated electrons with some kind of particle accelerator
Why is electron radiation suitable for treatment of superficial tumors?
- The energy loss of accelerated electrons is hardly changed until a given penetration depth
What is dose-depth?
The distribution of absorbed dose in the direction of radiation
Describe gamma radiation in radiotherapy
- Having an indirect ionizing effect
→ Site of absorption is different from site of ionization which is also site of damage
- Penetration distance is energy dependent
Describe Electron PDD (percentage depth dose ) curves of gamma radiation
- PDD curves at voltages (see X-ray) and various photon energies
Describe gamma knife (radiotherapy)
- Focus multiple radiation sources onto a target from different directions simultaneously
→ The radiation isocenter is the point in space where radiation beams intersect
Describe characteristics of X ray used in radiotherapy
The X-rays are generated by a linear accelerator .
Few MeV photon energy.
Describe characteristics of X ray used in radiotherapy
The X-rays are generated by a linear accelerator .
Few MeV photon energy.
Describe characteristics of proton radiation used in radiotherapy
- A beam of protons accelerated by a particle accelerator
- Would be ideal, but very expensive!
Describe characteristics of proton radiation used in radiotherapy
It depends on velocity → protons ionize differently along different points of their path
What is relative depth-dose curve?
The curve of the dose distribution in the direction of spreading of the radiation
What is Bragg peak?
The number of generated ion-pairs gives such a peak at the end of the beam
→ this beam is where the proton slowed down and has concentrated ionization
Describe characteristics of NEUTRON radiation used in radiotherapy
- collision of high energy protons (66 MeV) into berillium target ( p(66) +Be) → Neutrons induce nuclear reactions.
- High LET
Sources of natural background OF RADIATION
- cosmic radiatiom
- radon
- potassium:
How can cosmic ray contribute to dose rate?
Cosmic ray contributions to dose rate as the function of the altitude
3 aims of radiation protection
- Prevention from deterministic effects (except in radiotherapy those that are intentionally produced)
- Keeping the occupational risk of the users of the sources at the level of occupational risk of other professionals.
- Keeping the public risk from ionising radiation sources at the level of public risk of other civilization related harms.
3 principles of radiation protection
- Optimization
- Justification
- Limitation
3 principles of radiation protection
What is optimization?
All exposures should be kept As Low As Reasonable Achievable (ALARA)
3 principles of radiation protection
What is ALARA principle?
All exposures should be kept As Low As Reasonable Achievable (ALARA)
X : cost of radiation protection
Y : cost of treatment
X+Y: total cost
→ Optimum is the minimum
3 devices that can be used for Detection of particles
• Cloud chamber
- Spark chamber
- Bubble chamber
What is a cloud chamber?
A particle detector consists of a sealed environment containing a supersaturated vapour of water or alcohol.
What is a spark chamber?
A particle detector useful for the investigation of subatomic particles in high-energy particle physics.
It consists of a series of thin metal plates parallel to each other, separated by small gaps, and enclosed in a container filled with neon or another inert gas.
What is a Bubble chamber
?
– particle detector
– superheated transparent liquid (H2, Ar, Xe)
– entire chamber is subject to a constant magnetic field
3 principles of radiation protection
What does Justification principle state?
no practice shall be adapted unless it produces a positive net benefit
3 principles of radiation protection
What does LMITATIOHNprinciple state?
the effective dose (E) to individuals shall not exceed the limits recommended by the ICRP (maximum permitted doses)
