Biological Effects of Radiology Flashcards
interactions of xrays with matter
incident xray => beam attenuated (reduced in intensity) c through interactions with tissue => remnant beam strikes receptor to create image
attenuation
1) individual photons in the beam can be absorbed by atoms in the tissues
- cease to exist after absorption interactions
2) scattered out the beam with tissue atoms
- theyll move in a different direction
3) absorption
- more likely in bone
4)scattering
- more likely in soft tissue
percent of interactions
1) 9% no interaction
2) 7% coherent scattering
3) 27% photoelectric absorption
4) 57% compton scattering
photoelectric absorption
1) incident photon interacts with electron in inner orbital
2) atom is ionized and inner electron is ejected
- photoelectron
3) travels short distance before it gives up energy through secondary ionization
characteristic radiation
1) released when the electron deficiency is filled
2) low energy absorbed by the patient, not contribute to image degradation
radiographic contrast
1) differential absorption in various tissues and objects (restorations)
2) probability of photoelectric absorption in bone is 6.5x greater than in equal thickness of soft tissue
3) different degrees of radiopacities on the image is due to differential photoelectric absorption of photons in those structures
compton scatter
1) incident photon collides with outer orbital electron
2) the outer orbital electron receives some kinetic energy from the photon and recoils
3) ionization
4) path of incident photon is deflected and scattered in a new direction
- additional ionizations at tissue sites outsize the circumference of the incident beam
5) we dont like it
- more radiation exposure
- degrades image contrast
6) independent of atomic #
coherent scatter
1) low energy incident photon interacts with a whole atom
2) momentarily excited and the incident photon ceases to exist
3) excited atom returns to the ground state and generates another photon with the same energy as incident photon
- but at a different angle
4) insignificant bio effects and image degradation
biological effects of ionization radiation
1) direct actions
2) indirect actions
3) both yield unstable free radicals
direct actions
1) photon directly interacts with and ionized a biological macromolecule
2) secondary electrons may also interact
indirect actions
1) photons and secondary electrons interact with water
2) 2/3 of the biological damage due to hydroxyl radical
double strand breaks
1) detrimental event for cell killing, tumors, and inheritable effects
2) repair mechanisms is very ERROR prone
- nonhomologous end joining and homologous recombination
3) most important
damage clusters
1) though to be critial lesions, responsible or cell killing ,carcinogenesis, and heritible effects
2) can be causes by a single xray photon
DNA aberrations
1) occur due to failure of chromosome to rejoin
2) chromosome aberration
- DNA strand break occurs prior to chromosomal duplication
- the break is replicated
3) chromatid aberration
- DNA strand break occurs after
- only in one sister chromatic
–
basically, cause cell death OR tumor induction and heritable effects
stochastic effects
1) no dose threshold
2) diagnostic radiation place at risk
- but either you have it or not
3) sublethal DNA changes
- ex. survives with mutation
4) type of cell damaged has effect on the manifestation
- germ cell = heritable
- cell growth cell = carcinogenesis
deterministic effects
1) manifest only when radiation dose exceeds a certain threshold
2) once past threshold, pretty much everyone will have the effect
stochastic effects are though to occur without a dose threshold
1) a single xray photon has the potential
2) smallest dose COULD cause heritable effect or cancer
4) but the probability increases with dose
radiation induced cancer
1) clinically and histologically indistinguishable from other cancers
2) certain tissues are more sensitive to it
3) long latent period between radiation exposure and cancer
- years to decades
4) higher in children than adults
- 3x
main radiation induced cancers
1) leukemia
- exposed bone marrow to radiation
- highest risk in children up to 7
2) thyroid chancer
- highest risk in children
- females 2-3x more susceptible
3) salivary gland tumors
- incidence of both benign and malignant tumors is increased in patients treated with H&N radiation and atomic bomb survivors
breast cancer
1) female breast tissue is highly sensitive to radiation induced cancer
2) linear relationship between risk and dose
3) higher risk before 20
brain and nervous system cancer
1) in utero and to therapeutic doses in childhood and adulthood show higher incidence of malignant and benign tumors
2) association between intracranial meningiomas and previous radiography
risk and age
1) risk for younger patients
heritable effects of radiation
1) result from DNA damage in germ cells
2) information from atomic bomb survivors
- no significant adverse outcomes in the offspring of the survivors
3) it is possible that damage occurs at low frequence
deterministic effects
1) need threshold dose to manifest
- higher the dose, more severe the effect
2) the dose from diagnostic radiation is less than threshold dose
general rule of organ sensitivity
1) rapidly dividing cells are more radiosensitive
2) post mitotic specialized cells are more radioresistant
3) relevant exceptions (due to death by apoptosis instead of mitotic deth
- lymphocytes are more sensitive
- serous acini of salivary glands are as well
deterministic radiation effects on embryos and fetuses
1)embryos and fetuses are mroe sensitive
2) preimplantation (0-9 days)
- 100 mGy (14,000x higher than the fetal dose)
- embryonic death
3) 8-25 weeks
- 0.3 Gy (42,000x higher than fetal dose)
- microcephaly and mental retardation
radiotherapy involving oral cavity
1) H&N is administered by fractions
-greater tumor destruction
- increases cellular repair
- increase mean oxygen tension in the tumor
- helps generate free radicals to kill cancer cells
2) 2Gy is delivered daily for a weekly exposure of 10 Gy until a total of 60-70 Gy
oral complications for H&N radiation therapy
1) any changes that last 2 years, pose lifelong risks
mucositis`
1) discomfort is most severe at the end of therapy
2) typically heals after 2 months
3) lasting effects: fibrotic underlying CT, which can cause ulcerations
4) ulcerations may represent tumor recurrence
- biopsy
taste buds
1) loss of taste 2-3 weeks of radiotherapy
2) decreases by factor of 1000-10000 during the treatment
3) typically bitter and acid
- posterior 2/3
4) salt and sweet
- anterior 1/3
5) reversible in about 2-4 months
salivary glands
1) 50% reduction in salivary flow in the first week of radiation therapy
2) parotid > radiosensitive than submandibular
3) xerostomia results from a loss of saliva production
radiation caries
1) radiation induced xerostomia
2) widespread lesions at buccal, occlusal, incisal, and palatal
- more common
3) more likely to have PA inflammation and disease
tooth development
1) effect depends on stage of tooth development
2) early development may destroy tooth bud
3) or later, arrested development
4) eruptive mechanism is relatively radiation resistant
5) failure to form tooth, root, or dwarfed teeth
osteoradionecrosis
1) damage to vasculature of periosteum and cortical bone
2) this is devitalized area of irradiated bone
- 5-7% incidence for conventional radiotherapy, IMRT, and brachytherapy
3) unlikely when dose <60 Gy
4) can develop anytime after the therapy
- often 6-12 mo
5) mandible is more susceptible due to lower vascularity
osteoradionecrosis features
1) widening of PDL
2) lytic changes (radiolucent areas of bone destruction)
3) sequestrations (necrotic bone)
4) fractures
to minimize risk of ORN
1) prior
- restore carious teeth
- implement oral hygiene
- consider extracting teeth with severe caries and periodontal bone loss
2) post
- monitor for disease
- minimize surgical trauma
- adjust dentures to avoid dental sores
musculature
1) the muscles of mastication may experience contracture and trismus due to inflammation and fibrosis
2) masseter or pterygoid are commonly affected
3) progressive restricted mouth opening 2 mo after completion of radiation therapy
