Liều giới hạn liệu bức xạ cho bệnh nhân (Radiation dosimetry)

Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Dosimetry Radiation Dosimetry of the Patient – Chapter 24, Bushberg Kalpana Kanal, Ph.D., DABR Lecturer, Diagnostic Physics Dept of Radiology UW Medicine ¬ Radiation dosimetry is primarily of interest because radiation dose quantities serve as indicators of the risk of biologic damage to the patient ¬ The biologic effects of radiation can be classified as either deterministic (non(non-stochastic) or stochastic ¬ Deterministic or nonnon-stochastic effects are believed to be caused by cell killing ¬ if a sufficient number of cells in an organ or tissue are killed, killed, its function can be impaired a copy of this lecture may be found at: http://courses.washington.edu/radxphys/PhysicsCourse04http://courses.washington.edu/radxphys/PhysicsCourse04-05.html Kalpana M Kanal, Ph.D., DABR Dosimetry Dosimetry ¬ Deterministic or nonnon-stochastic effects ¬ effects include terratogenic effects to the embryo or fetus, skin damage and cataracts ¬ a threshold can be defined below which the effect will not occur ¬ for doses greater than the threshold dose, the severity of the effect increases with the dose ¬ to assess the likelihood of a deterministic effect on an organ from an imaging procedure, the dose to that organ is estimated Kalpana M Kanal, Ph.D., DABR A stochastic effect is caused by damage to a cell that produces genetically transformed but reproductively viable descendants ¬ cancer and hereditary effects of radiation ¬ probability of a stochastic effect, instead of its severity increases increases with dose ¬ No dose thresholds below which the effects cannot occur ¬ The NRC’s radiation dose limits described in Chapter 23 are intended to limit the risks of stochastic effects and to prevent the nonnon-stochastic effects Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR ¬ Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Dosimetry Dosimetry ¬ ¬ Entrance Skin Exposure ¬ The radiation exposure incident on a patient is the entrance skin skin exposure ¬ Skin doses are easy to measure but they are poor indicators of patient risk ¬ They not take into account the exposed area, penetrating power of the xx-ray beam, or the radiosensitivity of the exposed region ¬ DoseDose-Area Product (DAP) ¬ Product of patient entrance skin exposure and crosscross-sectional area of the xx-ray beam (exposed area) ¬ Units are in mGymGy-cm2 or mradmrad-cm2 ¬ Used in fluoroscopy At diagnostic energies, the ff-factor (roentgen(roentgen-toto-rad) conversion is close to 1.0 so that dose is numerically equal to exposure Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR Dosimetry Dosimetry ¬ ¬ Radiation Dose ¬ Radiation dose is defined as the absorbed energy per unit mass but but this says nothing about the total mass of tissue exposed and the distribution of the absorbed energy ¬ Would you prefer to receive a dose of 10 mGy to the whole body or or 20 mGy to the finger? ¬ The 10 mGy whole body dose represents about 1,000 times the ionizing energy absorbed for a 7070-kg person with a 35 g finger Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR Imparted energy ¬ the total amount of energy deposited in matter is called the imparted energy (Joules), (Joules), is the product of the dose (Gray) and the mass (Kg) over which the energy is imparted ¬ assume each 11-cm slice of a head CT scan delivers a 30 mGy dose to the tissue in the slice ¬ If the scan covers 15 cm, the dose is still the same, however the imparted energy is approx 15 times that of a single slice (you also have to consider scatter from adjacent slices, about 1010-25%) Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Dosimetry Dosimetry ¬ The disadvantage of imparted energy is that it does not account for the different sensitivities of the exposed tissue to biologic biologic damage ¬ Effective dose is used for comparing risk of stochastic effects ¬ E (Sv) = ∑wT x HT ¬ has shortcomings, wT were developed from epidemiologic data and incorporate significant uncertainties ¬ Kalpana M Kanal, Ph.D., DABR Organ Doses ¬ It is possible to estimate organ doses from a given entrance skin exposure (ESE) ¬ Organ doses are substantially lower than skin dose ¬ For AP projections, projections, the embryo dose will be between 1/3rd and 1/4th the ESE (in the direct beam) ¬ For PA projections, projections, the embryo dose will be about 1/6th of the ESE (in the direct beam) ¬ For LAT projection, projection, the embryo dose will be about 1/20th of the ESE (in the direct beam) Kalpana M Kanal, Ph.D., DABR 10 Dosimetry ¬ → Comparing ESE is useful for assessment of equipment performance and calibration, when a comprehensive analysis of effective dose is unnecessary → → ← ← c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 797 ← c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 59 Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 11 Kalpana M Kanal, Ph.D., DABR 12 Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Risk Risk ¬ The International Commission on Radiological Protection (ICRP) estimates the risk of fatal cancer for exposures to adults of working age to be 0.004 deaths per Sv or 0.0004 per rem ¬ this translates to cancer death per 2,500 people receiving an effective dose of 10 mSv (1 rem) ¬ Because of the linear, nono-threshold assumption used in risk estimates, risk is presumed to be proportional to the effective dose Risk is proportional to the effective dose ¬ there would be a in 25,000 chance that a fatal cancer would result from an effective dose of mSv (0.1 rem), or ¬ a in 500 chance of a fatal cancer from an effective dose of 50 mSv (5 rem) ¬ ¬ The ICRP estimates the risk to be two or three times higher for infants and children and substantially lower for adults older than 50 years of age Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 13 14 Risks Effective Dose (mSv) Risk of Fatal Cancer (per million) Equivalent to Number of Cigarettes Smoked Chest Radiograph 0.04 1.6 12 29 Skull Exam 0.1 4.0 29 71 Mammography 0.1 4.0 29 71 Thoracic Spine 1.0 40.0 292 714 Pelvis 1.1 44.0 321 786 Abdomen 1.2 48.0 350 857 CT Head 1.8 72.0 526 1286 Lumbar Spine 2.1 84.0 613 1500 Intravenous Urography 4.2 168.0 1226 3000 CT Pelvis 7.1 284.0 2073 5071 CT Abdomen 7.6 304.0 2219 5429 CT Chest 7.8 312.0 2277 5571 Barium Enema (with fluoro) 8.7 348.0 2540 6214 Typical Absorbed and Effective doses Procedure → → → → c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 798 Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 15 Kalpana M Kanal, Ph.D., DABR Equivalent to Number of Highway Miles Driven 16 Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Interventional Radiologic Procedures Radiographic Procedures Geometry for measuring the output free-in-air of a radiographic system c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 799 ← c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 801 Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 17 18 Radiographic Procedures Radiographic Procedures Geometry for measuring the output free-in-air of a radiographic system when phototiming is used c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 804 c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 802 Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 19 Kalpana M Kanal, Ph.D., DABR 20 Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Effective Dose Comparison with Chest PA Exam Procedures Eff Dose (mSv) Equivalent no of chest xx-rays Approx period of background radiation Chest PA 0.02 days Pelvis 0.7 35 months Abdomen 1.0 50 months CT Chest 400 3.6 years CT Abdomen or Pelvis 1010-20 500 4.5 years Question ¬ Assuming the skin entrance dose from a single slice CT study is rad, the dose for a 10 slice examination would be approximately _ rad and the imparted energy would be rad (ignore scatter) A 5, 15 B 15, D 50, E 5, 50 Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 21 22 Question ¬ Question The skin entrance exposure from a CT slice is 2.0 R Ten contiguous slices are taken, then dye is injected and 10 slices are repeated The total entrance skin exposure is about _ R ¬ The national average ESE for a normal 23 cm thick A/P abdomen film with a 400 speed screenscreen-film system is: A 13 mR B 150 mR C 300 mR D 850 mR E 3000 mR A 2.0 B 2.2 D 5.0 E 20.0 You have to consider scatter 25% of R = 0.5 So 2.5 per scan is the rad exp For two scans, 2.5*2 = 5.0 Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 23 Kalpana M Kanal, Ph.D., DABR 24 Radiation Dosimetry – Chapter 24 Bushberg Diagnostic Imaging Physics Course 10 February 2005 Question ¬ Question Match the exposure or dose with the appropriate item: A 15 mR B 40 mR C R D 10 R E 50 mrem ¬ CT head scan ESE Lateral chest ESE 10 fluoro (thin patient) Monthly limit for a pregnant technologist CT head scan ESE – to R typical Lateral chest ESE – 1010-15 mR for PA to times for Lateral 10 fluoro (thin patient) – 1-2 R/min for thin patient Monthly limit for a pregnant technologist – 0.5 mSv or 50 mrem Kalpana M Kanal, Ph.D., DABR Kalpana M Kanal, Ph.D., DABR 25 Kalpana M Kanal, Ph.D., DABR Match the exposure or dose with the appropriate item: A 15 mR B 40 mR C R D 10 R E 50 mrem 26