Radio Principle 9a & b Flashcards
BIOLOGICAL EFFECTS OF RADIATION
What is Ionisation?
- Ionisation is teh ability to creat an ion from an atom
- An X-ray photon can remove an electron from an atom
- damaging effects were noted almost immediately after the discovert of x-rays
How does radiation affect the himan body and DNA?
-It creates irreversible chemical change responsible for biologic effects from radiation appears to be damage to DNA molecules.
- Most induced DNA damage is repaired within a few mintes to hours by enzymativ processes within the body.
- However efficacy and accuracy of DNA repair following radiation exposure may determine whether the biological effects of radiation result in permanent damage.
Radiation Effects:
There are 2 primary effects to consider
- Stochastic
- Detterministic (non-stochastic)
We must also consider whether the effects are somatic, carcinogenic, terayogenic, or genetic
Stochastic effects of radiation
-effects increase as dose increases
-expressed throughout the dose range
-linear, non-threshold model for dose/response
(there is no dose level under which the risk of developing a particular effect is zero
-see BEIR V11 for more information
Most common manifestations: cancer or genetic damage.
Binary effect: all or none
eg pt either gets leukemia or doent
Effects doesnt get worse with dose
see slide 8 of 9a lecture
Deterministic (non-stochastic)
-likelihood of occurrence is fairly certain after a a specific dose level
-severity increase as dose increases
-threshold model
(know r suspected dose level under which the risk of developing a particular effect is eliminated)
Non-random effect
- Certain dose levels produce consistent changes across the population
- Doses below a threshold do not produce these types of changes
e. g. 1Sv then the higher the dose, the worse the effects
(see slide 10)
What are the long-term Somatic (carcinogenic) Effects
- Result of chromosomal mutations occuring within any cells
- excluding germinal cells in the gonads
The changes are not sever enough to affect survival or division of the cell, but may be initiating factor for cancer development.
Long term somatic (carcinogenic) effects
It should be noted:
Humans are significantly more resistant to developing radiation-induced cancer than lab rats (on which most studies have been done)
It appears that there are mechanisms in place in humans that suppress cancer development from these mutated cells
The latency period for many forms of cancer development in humans is several years
The development of a dose/response relationship for radiation induced cancer is problematic
Carcinogenic effects are currently classified under the linear, non-threshold model.
Cancers developing in association with high dose radiation exposure include:
- leukemia
- breast, bone, lung, and thyroid cancer
Low dose/risk data is less conclusive, but suggests an increase, with children appearing almost twice as susceptible as adults
Common sense: precautionary principle
For example: of 1.6 mill children in the US who will receive a CT this year , 1500 will eventually die from radiation-induced cancer.
What are the acute somatic, and Teratogenic Developmental effects
These effects are primarily due to inhibition of mitosis, which leads to cell death.
These changes will either directly affect the person(s) being exposed or the developing foetus.
Acute somatic effects include:
skin erythema, hair loss, cataract formation, temporary sterility, and radiation syndromes
What are the acute radiation Syndromes?
what are there phases?
Acute radiation syndrome:
Organ systems affected include
- Hematopoietic
- Gastrointestinal
- Cerebrovascular
4 phases -Prodromal (48 hours to 6 days) -Latent (several days to a month) -Manifest illness (weeks) Recovery or death
Haematopoietic Syndrome
Haematopoietic Syndrome
Exposures exceeding 1 Gy
Crisis occurs weeks after exposure with bone marrow hypoplasia or aplasia
There is 50% decline in lymphocyte count within 24 hours followed by other cytopaenias, depending on dose
Gastrointestinal Syndrome
Gastrointestinal Syndrome
- The GI syndrome has mortality exceeding that of the hematopoietic syndrome at exposures above 12 Gy
- Acute vomiting and diarrhea with cramps are immediate, followed by a latent phase of 5 to 7 days and then manifest illness
- Malnutrition, bowel obstruction, anemia, cardiovascular collapse, sepsis, and renal failure are potential complications
Cerebrovascular Syndrome
Cerebrovascular Syndrome
The cerebrovascular syndrome occurs with a supralethal exposure (more than 20Gy)
Characterised by confusion, ataxia, papilloedema, and absent corneal and deep tendon reflexes.
It mimics septic shock and is associated with death within 2 days.
Acute Somatic, and Teratogenic/Developmental Effects
Teratogenic effects will not occur during the first two weeks of pregnancy, as no organogenesis is occurring.
Damage to the embryo at this stage is never identified, as the embryo will either be unaffected, or die and be passed during the next menstrual cycle – the mother rarely knowing she was pregnant.
During major organogenesis (3rd – 7th week), congenital abnormalities can occur following radiation exposure, but rarely below exposure levels of 100 mGy, and typically only seen above 250-500 mGy
NB: medical diagnostic exposures generally provide exposure levels to the fetus under 10mGy for lumbar spine, and substantially less for other body areas.
From the 8th week until birth, growth and brain development are primarily occurring.
These can be affected by radiation doses in the range similar to those mentioned above.
Changes that can occur include:
microcephaly and impaired cognition (from neuroblast cell depletion);
growth retardation (from cartilage damage);
childhood cancer development (particularly leukaemia).
Somatic effects follow the threshold (deterministic) model
Teratogenic effects are linear, non-threshold (stochastic).
Genetic effects
Result of chromosomal mutations occurring within the germinal cells of the gonads
Have the potential to affect the progeny of the person exposed.
Most chromosomal aberrations are lethal to the cell
Less severe damage also possible
-translocations
-small deletions
-may be transmitted to the daughter cells, and thus to future generations.
Almost all harmful (recessive) mutations are expressed in the heterozygous state and would therefore manifest in the first generation. As such, long term population risks are quite low.
Cell sensitivity
Law of Bergonie and Tibondeau
Law of Bergonie and Tibondeau
- 100 years old
- Still the best “rule of thumb” for predicting the relative sensitivity of cell types or tissues to radiation.
The more mature a cell, the more resistant to radiation it is
The younger the tissue and organs, the more radiosensitive they are
-Particularly unspecialised types (ie, cells that will later become specialised, but which currently are not – eg, stem cells, fertilized ovum, immature blood cells, lymphocytes, erythrocytes
Tissues with higher metabolic activity are more radiosensitive
Rapidly proliferating cells are more radiosensitive
This principle applied in radiation treatment for cancer.
Based on this information
Children more sensitive than adults
Cells with continuous high turnover (e.g. epithelial tissue, red marrow) more sensitive than those with little turnover or activity (e.g. cortical bone, neurological tissue).
Lethal Dose
The LD50/60 (mean lethal dose required to kill 50% of humans at 60 days) for whole-body irradiation:
3.25 to 4 Gy in persons without supportive care
6 to 7 Gy when antibiotics and transfusions are provided.
Background Radiation:
The most common source of background radiation is radon
- Product of uranium radioactive decay.
- Accounts for ~70% of all background radiation
- Location dependent
- Some rocks have higher uranium content (e.g. granite).
Other ground sources, including thorium, isotopes of potassium, and cosmic rays account for the remainder of exposure
COMPUTER TOMOGRAPHY
ELECTRON BEAM TOMOGRAPHY.
Uses pencil-thin beams of x-rays
Ionising radiation!
Tube and detectors rotate around patient
Detectors record amounts of radiation transmitted through patient
Axial images acquired directly
Software can reformat to indirectly visualise in any plane, even 3-D
mehh
What are the advantages of a CT
- Excellent depiction of osseous structures
- Rapid
- Demonstrates hyperacute intracranial haemorrhage better than MR (?)
Limitations of CT
Relatively high dose of ionising radiation:
- contraindications are the same as for plain films including:
pregnancy
sensitivity to contrast
- plus:
beam hardening artefacts: Beam hardening artefacts appear as streaks and shadows adjacent to areas of high density such as the petrous bones, shoulders, and hips. The artefact occurs because the high density anatomy absorbs the lower energy photons while the higher energy photons pass through to the detectors which results in the beam becoming ‘harder’.
dense bone
metal
CT Procedure Overview
The computer software takes this information and creates the grey scale which forms the anatomical detail
For each rotation of the tube around the patient, an image “slice” is created
This slice is balanced against adjacent slices (as there is usually slice overlap to avoid gaps in the image) to create the final image
Each image slice is numbered, and can be compared to a scout view to localise the region of the slice
Images are generally acquired in the axial plane, and must be reformatted by the computer to attain a different plane – which results in loss of image resolution (newer generation scanners, e.g. spiral, multi-slice detectors, are not limited by this problem)
The image scanner can be tilted to image through specific planes (eg, discs)
Contrast and density of the tissues (windows) can be altered to emphasise bone or soft tissue
Terminology: CT
Hypodense
Isodense
Hyperdemse
Windowing (selected contrast density)
Soft tissue window
Bone window
Tissue density
Hypodense = black to dark grey
Isodense = same as some other reference tissue
Hyperdense = white to light grey
Windowing (selected contrast and density)
Soft tissue window = see soft tissue resolution well, bone resolution is poor
Low window width and level factors on image
Bone window = see bone resolution well (ie, corticomedullary distinction), soft tissues resolution if poor
High window width and level factors on image
What is electron beam tomography?
AKA EBT, EBCT
Magnetically controlled beam of e- hits targets around the patient
X-rays shoot through patient from the targets
Extremely rapid image acquisition
Low dose (Dose equivalent to abdominal plain film 1/10th to 1/38th that of standard multi-detector CT)
Much marketing in U.S. and U.K. in mid 2000s (heart scan, virtual colonoscopy, yuppie scan (whole body)) - now stopped.
