lecture 8 meyer
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| 2 general uses of radiation in ca tx | as (neo)adjunct to surg; can be also used with definitive intent i.e. as the primary radical therapy for a certain type of tumor
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| types of radiation used for tx | ionizing radiation breaks down into electromagnetic (XR and gamma rays) and particulate (electrons, protons, neutrons, heavily charged ions) types
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| unit of measurement for radiation exposure | two are important: rad is 100 ergs/g absorbed dose of radiation // gray (Gy) is 1 joule/kg energy absorbed
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| mechanism of radiation effects on tissue | indirect: dominant for sparsely ionizing like XRs and gamma rays - free radical generation that leads to apoptosis or classical necrosis // direct: densely ionizing radiation like neutrons and charged particles, results in direct damage to DNA/proteins
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| 4 modifiers of radiation exposure | cell cycle, type of radiation, fractionation and oxygenation
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| how being mindful of cell cycle during radiation exposure can enhance the effect of radiation | the clinician will try to catch cells when they are in M or G2 phase b/c they're most sensitive to radiation; least sensitive in S phase
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| early and late side effects of radiation tx |
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| cells are much more sensitive to radiation damage when they are in oxygen-rich environment | less sensitive in a hypoxic state
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| how being mindful of the type of radiation during exposure can enhance its effect | for densely ionizing radiation, survival is an exponential function of dose (the higher the dose, the greater log kill); for sparsely ionizing radiation there is an initial linear slope followed by a shoulder then curve becomes straight at higher doses
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| how being mindful of oxygenation during radiation exposure can enhance its effects | radiation response can be enhanced by drugs that increase the oxygen content in the hypoxic area of the tumor
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| how being mindful of fractionation during radiation exposure can enhance its effect | providing radiation in a single dose or fraction gives a greater cell kill than giving the same dose of radiation in multiple, smaller doses due to the repair of sublethal damage btwn fractions
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| 2 methods of delivering radiation therapy | teletherapy and brachytherapy
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| teletherapy or external beam radiation | most freq used method: photons or electrons directed at the tumor from outside the body; may be from radioactive source (e.g. cobalt-60) or generated from electron acceleration (linear accelerator) and directed at several angles to pinpoint tumor
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| brachytherapy | intracavity or interstitial radioactive source & emits radiation to surrounding tissues; dropoff of radiation - inverse square effect
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| 3 types of information a radiation oncologist needs to make a confident clinical tx decision | histopathology (rapidly dividing undifferentiated cells are most radiosensitive), stage, goal of tx (palliative vs cure)
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| acute effects of radiation tx | within days-weeks in tissues with rapid turnover, result from death of large #s of cells, can be reversible if rapidly repaired by proliferation of stem cells
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| chronic effects of radiation tx | occurs after months-yrs primarly from slowly growing tissues, combo of vascular change and loss of parenchymal cells. may never be completely repaired, permanent late effects in normal tissues
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