After surgery for rectal cancer dose is amount of

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Unformatted text preview: cells “see” many small treatments that are repaired between fractions ¡༊  Sublethal repair in NEOPLASMS is inefficient §༊  The cancer cells see only one big dose, because they cannot repair the damage between fractions §༊  This differential underlies the usefulness of radiation for cancer treatment ¡༊  If sequential doses of radiation are ‘timed” to correspond to when the cell is in G2- M, greater killing is observed ¡༊  Cells with high oxygen concentrations are killed more effectively by RT ¡༊  Mechanism thought to relate to the higher levels of oxygen free radicals ¡༊  The difference in radiosensitivity between hypoxic and aerated cells is known as the oxygen enhancement ratio (OER) This oxygen effect is very dramatic in vitro, and has real (but less dramatic) impact in clinical practice ¡༊  Variables: §༊  Sensitivity of cancer to radiation §༊  Normal tissue tolerance to radiation §༊  Patterns of spread of cancer §༊  Adequacy of dose and volume coverage ¡༊  Definitive – “main” treatment delivered to try to cure the cancer ¡༊  Neoadjuvant - treatment given before the “main” therapy §༊  Radiation before surgery for rectal cancer ¡༊  Adjuvant - treatment given after the “main” therapy §༊  Chemotherapy after surgery for rectal cancer ¡༊  Dose is amount of energy imparted to a volume (of tissue) by ionizing radiation ¡༊  Actual “energy” deposited in tissue is small §༊  A near- lethal dose of radiation to the whole body deposits a total energy equivalent to one sip of hot coffee ¡༊  Ionizing radiation most clinically useful §༊  Photons (x- rays) most commonly used §༊  Ionizing particles include electrons, neutrons, protons, and the nuclei of heavy atoms. Linear Accelerator (aka LINAC) Delivers radiation from outside of the patient, similar to a chest x-ray (but much higher energy!) ¡༊  Radioactive sources placed close to the targeted tumor/tissue §༊  Treat the target with high dose, while sparing normal tissue better than with external beam ¡༊  Brachytherapy only useful if source can be safely placed within the target ▪  ▪  ▪  ▪  Vagina, Uterus Breast Prostate Lung ¡༊  Image- guided §༊...
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