Angela's Radiation therapy Flashcards
What is an alpha particle?
Two protons and two neutrons (+ charge) emitted from the nucleus during a form of radioactive decay, called alpha-decay. An alpha-particle is identical to the nucleus of a normal helium atom.
“An alpha man is full of helium”
What is a beta particle?
Single electron (- charge) - high energy, high speed electrons (β-) that are ejected from the nucleus, [Can also form a positron (β+).
What is the penetrance of a beta particle?
None - it is stopped by sheets of aluminum or a few mm of tissue.
What is a gamma particle?
Gamma particle is a photon (a unit of electromagnetic energy), that is emitted by the nucleus of some radionuclides following radioactive decay. Gamma photons are the most energetic photons in the electromagnetic spectrum.
High enerG - G - gamma
What is an X ray?
A penetrating form of high-energy electromagnetic radiation. An X-ray is a packet of electromagnetic energy (photon). X-rays are produced by electrons external to the nucleus.
Where do x-rays originate?
OUTSIDE the nucleus - from the electron cloud of an atom. This is generally caused by energy changes in an electron, which moves from a higher energy level to a lower one, causing the excess energy to be released.
How are X-rays produced?
X-rays are commonly produced in X-ray tubes by accelerating electrons through a potential difference (a voltage drop) and directing them onto a target material (i.e. tungsten).
What are properties of alpha particles?
Alpha particles are highly ionising because of their double positive charge, large mass, and because they are relatively slow. They can cause multiple ionisations within a very small distance. This gives them the potential to do much more biological damage for the same amount of deposited energy. They are unable to penetrate very far through matter and are brought to rest by a few centimetres of air or less than a tenth of a millimetre of biological tissue.
What is a proton?
Large, positively charged elementary particle. Created by by ionization of a hydrogen atom - stripping an electron from a hydrogen atom via acceleration at 300MeV energies.
Sport Pros stay HYDRidated. Protons are from hydrogens
What is a neutron?
Large, neutral elementary particle.
How are neutrons produced?
Neutrons are produced when alpha particles are accelerated at and hit any of several light isotopes including isotopes of beryllium, carbon, or oxygen. Also can be produced by fission.
What is a pi meson (pion)?
Any of three subatomic particles: π0, π+, and π−
Each pion consists of a quark and an antiquark. They are unstable, decaying after a short lifetime. Neutral pions generally decay into gamma rays.
What is the clinical application of the photoelectric effect?
Diagnostic radiology
Is the photoelectric effect dependent on atomic number?
Yes, Z^3
(Z is the atomic number)
What radiologic phenomenon starts to occur at 1.02 meV?
Pair production (becomes dominant at 30 MeV)
Pair production is the production of a particle-antiparticle pair from the decay of a neutral particle or from a pulse of electromagnetic energy traveling through matter. The most commonly observed pair-production process is the materialization of an electron and a positron from a high-energy photon.
At energies greater than 30 MeV, pair production is the dominant mechanism of radiation interaction with matter. As photon energy increases, the dominant interaction mechanism shifts from photoelectric effect to Compton scattering to pair production.
Is pair production dependent on atomic number?
Yes
PP is related to the atomic number (Z) of attenuator, incident photon energy (E) and physical density (p) by Z E (- 1.022) p.
What is the Compton Effect?
The Compton effect (also called Compton scattering) is the result of a high-energy photon (X-ray or gamma ray) colliding with a target, which releases loosely bound electrons from the outer shell of the atom or molecule. The scattered radiation experiences a wavelength shift
“The Compton family is Phony” To remember PHOtones.
NOT related to Z (atomic number) but density of tissue (penetrates 0.5-3cm)
What is the clinical application of the Compton Effect?
Radiotherapy
What is the particle in the Compton Effect that interacts with DNA directly and H2O to form hydroxy free radicals?
Displaced electron
At what energy does the Compton Effect predominate in tissue?
~25 keV to 25 MeV
Most radiation treatments are performed at energy levels of about 6-20 MeV
keV is 1000 times smaller than a MeV
In the Compton effect, what does the incident photon interact with?
Loosely bound outer orbital electrons
In the Compton effect, what is % of damage to DNA is caused directly from the interaction of the photon with DNA
direct DNA interaction - responsible for 25% of damage due to radiation. The other 75% is caused by indirect action (the photon creating incidental free radicals when go on to cause DNA damage
In the Compton effect, what is indirect action? (ie how much DNA damage is not caused by the direct interaction of the electron with DNA)
Formation of free hydroxy radicals - responsible for 75% of damage due to radiation
Most DNA damage is induced indirectly by free radicals
In Compton Effect, is absorption dependent on tissue density or atomic number?
Not dependent on atomic number; is dependent on tissue density because Tissue density is correlated with electron density for biological tissues
Compton scatter is dependent on the number of available electrons; the electron density of the material; and on the physical density but not on the atomic number of the material.
The probability of a Compton interaction is directly proportional to the electron density of the absorber. The density of electrons in bone (5.55 × 1023/cm3) is greater than in soft tissue (3.34 × 1023/cm3); therefore the probability of Compton scattering is correspondingly greater in bone than in tissue.
At what energy range does the photoelectric effect predominates in tissue?
10-25 keV
keV is 1000 times smaller than a MeV
What are the most common sources of radiation for brachytherapy in GYN cancers?
Cesium-137 (half-life 30 years)
Iridium-192 (half-life 74 days)
What is Linear Energy Transfer (LET)?
The average amount of energy that is lost per unit path-length as a charged particle travels through a given material
What has high LET?
Densely ionizing radiation like neutrons, alpha particles, pi-mesons
What has low LET?
Sparsely ionizing radiation like gamma rays and X-rays, beta particles/high-energy electrons, photons, Protons
What are the benefits of high LET energy?
More effective in hypoxic tissue (necrotic tumor)
What has the highest LET?
Neutrons (more than protons, pi mesons, X-rays, photons). Carbons are also high LET sources.
Alpha > neutron if this is an answer
What has the lowest LET?
X-rays (250 keV)
What is the most potent radiation modulator?
Oxygen tension
What does the shoulder of cell survival curve represent?
The width of shoulder of the survival curve indicates the degree of sublethal damage, which is more readily repaired in the S phase. Normal tissue should have a greater ability to recover from sublethal damage then tumor.
What does increased oxygen do to the shoulder of the cell survival curve?
Shifts it to the left
Half lives of:
Iridium
Radium
Cesium
Cobalt
Iridium - 74 days
Radium - 1600 years
Cesium - 30 years
Cobalt - 5.26 years
Decay products of:
Iodine-125
Phosphate-32
Iodine-125 –> gamma ray
Phosphate-32 –> beta particle
“IG” -instagram
“PB”-peanut butter
What is the 100% isodose depth of Cobalt-60 (Co-60)?
5mm
P32 - 8 mm
Cesium - 2 mm
What dose of radiation sterilizes microscopic disease?
45-50 Gy
What is a benefit of hyperfractionation?
Higher total dose is delivered, with no increase in late side effects
What is a drawback to hyper fractionation?
Increased acute toxicity, more demanding schedule for patient
What is a common dose per fraction in hyper fractionated treatment?
1.15 Gy/fraction twice daily
What are the sources commonly used in LDR?
Cesium-137 and Iridium-192
What is the dose rate of LDR?
40-60 cGy/hour, usually done over 1-2 insertions.
Always less than 200cGy (or 2 gy) / hr
What is the benefit of LDR?
Late-responding tissue sparing
What are the sources commonly used for HDR?
Cobalt-60 and Iridium-192
What is the dose rate for HDR?
> 12 Gy/hr or 20 cGy (or 0.2 Gy) / min
What fraction of a total LDR dose is commonly administered during HDR?
HDR:LDR is 0.5-0.6:1
Chi textbook: the equivalent radiation dose (ERD) is a mathematical model that can be used to determine an appropriate dose for HDR based upon LDR techniques. These calculations show that one must give approximately 60 to 70% of the LDR dose with HDR (this conversion strictly for Brachytherapy)
—> HDR : LDR = 0.6-0.7 : 1.0
What preparation may improve late side effects with HDR?
Rectal retraction and packing
What is the dose to point A in a patient with stage II cervical cancer?
8500 cGy up to 9000cGy with bulky/barrel cervix
What is the brachy dose for endometrial cancer?
60 Gy to mucosa
30 Gy to 0.5cm deep to mucosa
What is associated with more complications in treatment of IB2 cervix cancer?
A) Radical hysterectomy followed by adjuvant RT
B) Neoadjuvant chemo then rad hyst
C) Radical chemoradiatoin
D) Radiation followed by extrafascial hyst
Radical hysterectomy followed by adjuvant RT
Which has the lowest tolerance for radiation?
A) Bladder
B) Rectum
C) Peripheral nerves
D) Small intestine
E) Ureter
Small intestine
What does TD5, TD50, TD100 mean?
TD5 = tolerance dose with 5% risk of injury
TD50 = tolerance dose with 50% risk of injury
TD100 = tolerance dose with 100% risk of injury
*[Sometimes written as TD50/5 meaning 50% risk at 5 years
*Specific to individual organs. Different values for single dose vs fractionated dose.
TD: maximum radiation dose or intensity of fractionated radiotherapy that is associated with an acceptable low complication probability
What is the radiation doses required to sterilize an ovary?
TD5-50 single dose: 2-6 Gy
TD5-50 fractionated dose: 6-10 Gy
Most adult women have ovarian failure after 20 Gy
(JS) Chi states: single dose of 4-8 Gy or fractionated doses of 12-20 Gy (depending on age) are known to produce permanent castration and sterility
What are the clinical properties of high-LET radiation?
Smaller shoulder to cell survival curve
What percentage of the dose at 2cm from the source is present at 1cm from the source?
400% (inverse square law)
What is the most common time frame for bowel radiation injuries to manifest?
6-18 months
What dose of radiation is needed to sterilize subclinical nodes?
4500 cGy
What is the most important factor in HDR dose?
A) Size of source
B) Age of source
C) Shape of source
D) Location of source
Age
What radiation source is most likely to be used for implanted seeds?
Iridium
Normal tissue tolerance for radiation (TD5 and TD50) for the following organs:
Bone marrow:
Kidney:
Liver:
Small intestine:
Spinal cord:
Large intestine:
Bladder:
Rectum:
Vagina:
Skin:
Stomach:
Bone marrow: Aplasia 250/450, Pancytopenia 3000/4000
Kidney: 1500/2000
Liver: 3000/4000
Small intestine: 4000/xx
Spinal cord: 4500/5500
Large intestine: 4500/xx
Bladder: 6000/8000
Rectum: 6000/8000
Vagina: 8000-8500/xx
Skin: 5000/xx
Stomach: 5000/xx
What is a benefit of HDR over LDR brachytherapy?
Shorter overall duration of acute GU toxicity for HDR vs LDR (but similar rates of late toxicity)
What is the outcome of radiation to embryo within the first two weeks of pregnancy?
Embryonic death
What is the outcome of radiation to fetus within 2-16 weeks GA of pregnancy?
Microcephaly, growth retardation
What is the outcome of radiation to fetus within 16-20 weeks GA of pregnancy?
Less severe microcephaly, growth retardation
What is the outcome of radiation to fetus after 30+ weeks GA of pregnancy?
Minimal effects
What is the outcome of radiation to fetus after 30+ weeks GA of pregnancy?
Minimal effects
What percent of DNA damage caused by radiation is due to indirect and direct interactions?
75% indirect via free radicals
25% direct via photon damage to DNA
What mechanism of radiation induced DNA damage ultimately leads to cell death?
Double-stranded DNA breaks lead to errors in telophase due to incorrect annealment and segregation, which leads to mitotic cell death
What is the biologically effective dose (BED)?
An attempt to quantify the biological effect (log cell kill) of radiation dose delivery. This is tissue-specific - cell-survival curves are modeled on a homogeneous tissue type.
What are the 4 Rs of radiobiology and what is the significance?
- Repair of DNA damage
- Redistribution of cells in the cell cycle
- Repopulation
- Reoxygenation of hypoxic tumor areas
4 R’s are the biological principles governing how/why fractionating radiation improves the therapeutic ratio.
success or failure of radiation treatment is determined by the 4 R’s of radiobiology
4 R’s of radiobiology: redistribution
Redistribution - cells are distributed in different phases of the cell cycle. Certain phases are radio resistant due to the specific mechanism of DNA repair at that stage. Cells tend to synchronize after multiple fractions of radiation (redistribute to different phases of the cell cycle), so eventually they will be at a radiosensitive time point.
S phase: most radio resistant
M phase: most radio sensitive
4 R’s of radiobiology: repair
Repair - different cells repair DNA at different rates. Normal cells with intact DNA are better able to repair radiation damage than cancer cells (“therapeutic ratio”).
4 R’s of radiobiology: repopulation
Repopulation of tumor cells occurs between fractions. Avoid treatment breaks!
4 R’s of radiobiology: reoxygenation
Reoxygenation - as tumor cells die, there is less competition for nutrients. Surviving cells become more oxygenated and thus there is more substrate for free radical formation.
What dose of RT is given for positive margins (i.e. microscopic high volume disease)?
55-65 Gy
What dose of RT is given for gross residual disease?
70+ Gy (biologically effective dose will account for differences in dose per fraction)
What is the typical radiation dose given per fraction?
EBRT - 1.8-2 Gy per day
Brachytherapy - 4-7 Gy in 3 to 6 Fx
?Jessa-thoughts on brachy dose
What is the inverse square law?
Radiation dose to tissue is inversely proportional to the square of the distance from the source of radiation.
What is meant by prescription depth?
Depth of the target dose (i.e. if target is 3cm deep to surface, you can prescribe XX Gy to 3cm; tissues shallower or deeper to that point will get a certain percentage of that dose)
