Principle of Radiation Protection- Avinesh Shrestha

Radiation protection is the science whose aim is to minimize the risks generated by the use of ionizing radiation. Briefly discusses The ICRP System of Radiological Protection, STRUCTURAL SHIELDING OF
IMAGING FACILITIES, APPLICATION OF INDIVIDUAL DOSE LIMTS, RADIATION EXPOSURE IN PREGNANCY, Diagnostic reference level, Personnel Protection in
Medical X-ray Imaging, Dose Optimization in CT, Radiation Protection in Nuclear Medicine.

1. Principle of Radiation Protection

2. INTRODUCTION Radiation protection is the science whose aim is to minimize the risks ( for people and environment ) generated by the use of ionizing radiation. Radiation Protection aims to reduce Stochastic effects and eliminate Deterministic effects of radiation The first sign that there was a limit to how much radiation exposure an individual could sustain without ill effects was known in the 1900s. It was not until the death of Clarence Dally (1865–1904), Thomas Edison’s assistant in the manufacture of X-ray apparatus, and the documentation of his struggle with burns, serial amputations, and extensive lymph node involvement, that medical observers took seriously the notion that the rays could prove fatal He died in 1904 from mediastinal cancer. Following this, Thomas Edison abandoned his research on X-rays . In 1928, the formal radiation protection standards were introduced with the formation of International Commission on Radiological Protection (ICRP)

3. The ICRP System Of Radiological Protection 8/9/2022 3

4. 5.3. Categories of exposure The Commission distinguishes between three categories of exposures: Occupational exposures ; defined by the Commission as all radiation exposure of workers incurred as a result of their work. Public exposures ; encompasses all exposures of the public other than occupational exposures and medical exposures of patients. Public is defined by the Commission as any individual who receives an exposure that is neither occupational nor medical Medical exposures of patients ; Radiation exposures of patients occur in diagnostic, interventional, and therapeutic procedures. A given individual may be exposed as a worker, and/or as a member of the public, and/or as a patient. 8/9/2022 4

5. Fundamental Principles and Methods of Exposure Control International Commission on Radiological Protection (ICRP) has formulated a set of principles (“ System of Radiation Protection ”) that apply to the practice of radiation protection (ICRP, 2007): Justification of practice Optimization of protection Application of individual Dose Limits

6. JUSTIFICATION OF PRACTICE Any decision that alters the radiation exposure situation, for example, by introducing a new radiation source or by reducing existing exposure, should do more good than harm , that is, yield an individual or societal benefit that is higher than the detriment it causes. No practice involving exposure to radiation should be adopted unless it produces at least sufficient benefit to the exposed individuals or to society to offset the radiation detriment it causes. If the exposure has no benefit it is not justified .

7. OPTIMIZATION OF PROTECTION Optimization of protection should ensure the selection of the best protection option under the prevailing circumstances, that is, maximizing the margin of good over harm . Thus, optimization involves keeping exposures as low as reasonably achievable (ALARA) , taking into account economic and societal factors. It means adjusting the quality and quantity of the radiation to the body habitus of the patient to use only the dose necessary for producing a study from which a diagnosis can be made with confidence.

8. OPTIMIZATION OF PROTECTION ALARA (As Low As Reasonably Achievable) concept Refers to the continual application of the optimization principle in the day-to-day practice Because of some risk, however small, exists from any radiation dose, all doses should be kept ALARA.

9. OPTIMIZATION OF PROTECTION ALARA vs ALADA The term ALADA (As Low As Diagnostically Acceptable) has been cited in the literature; concern that the dose rather than the diagnostic utility of the image may become the overriding metric of quality. This is not the intent of ALARA and it certainly can be applied to medicine if correctly interpreted, some see the distinction useful to keep the right balance on the elements that most affect patient care.

10. OPTIMIZATION OF PROTECTION ALARA vs ALADA Both the ALARA and the ICRP optimization principles imply using the lowest dose necessary to produce an examination result of appropriate diagnostic quality or, in the case of an image-guided intervention, to achieve the goals of the intervention . Therefore, reducing the examination dose to the point where important diagnostic information is lost or that results in the exam needing to be repeated is counterproductive and increases rather than decreases the overall risk to the patient . This is, in essence, the ALADA concept. In the United Kingdom (UK), ALARP-As Low As Reasonably Practicable (ALARP)

11. Methods of Exposure Control There are four principal methods by which radiation exposure to persons can be minimized: reducing the time (period) of exposure, increasing distance from the radiation source, shielding the source of the radiation, and controlling contamination of radioactive material. 8/9/2022 11 Cardinal Principle

12. Time Low exposure time, low radiation dose 100 200 300 mrem 100 mrem /hr 1 hour 2 hours 3 hours

13. DISTANCE Effective & Easy Inverse Square Law (ISL) Doubling distance from source, decreases dose by factor of four Tripling it decreases dose nine-fold More Distance = Less Radiation Exposure

14. DISTANCE Scattered radiation for diagnostic energy x-rays, a good rule of thumb is that at 1 m from a patient at 90° to the incident beam, the radiation intensity is approximately 0.1% to 0.15% of the intensity of the beam to the patient for a 400 cm 2 x-ray field area on the patient (typical field area for fluoroscopy). All personnel should stand as far away from the patient as practicable during x-ray imaging procedures and behind a shielded barrier or out of the room, whenever possible. The NCRP recommends that personnel should stand at least 2 m from the x-ray tube and the patient during radiography with mobile equipment (NCRP, 1989b). 8/9/2022 15

15. SHIELDING Materials “absorb” radiation Proper shielding = Less Radiation Exposure Shielding used where appropriate Significantly reduces radiation effects Lead, conventional gypsum board ,concrete block or brick

16. SHIELDING Due to the higher photon energies emitted from most of the radionuclides used in nuclear medicine, it is often not practical to shield the technologist from the radiation emitted from the patient Distance is the primary dose reduction technique. Imaging rooms should be designed to allow a large distance between the imaging table and the computer terminal where The use of a high atomic number material for shielding such as lead enhance photoelectric absorption thus reducing the thickness of the shielding compared to lower density materials. Placing shielding closer to a source of radiation does not reduce the thickness needed, but does reduce the mass of shielding necessary by reducing the area requiring shielding.

17. Lead apron, lead glasses, lead gloves Gonadal shielding Thyroid shielding Personnel shielding X-ray tube shielding Primary protective barrier (1.6 mm LE) Secondary protective barrier (0.8 mm LE) Viewing window – Lead glass (1.5 mm LE) Room shielding (Structural shielding) Protective housing Total filtration-2.5mm Al when operated above 70Kvp Proper light localized collimator

18. SHIELDING Lead free Aprons Metals like Tungsten, Tin, Antimony, Barium or Bismuth which have a smaller atomic weight as lead. When used together, Bismuth and Antimony provide similar qualities to lead. Lead-Free aprons have similar attenuation properties to the traditional lead but can weigh up to 15 - 40% less. (From 7kg reduced to 4kg) The real advantage in using lead free material is that it is not hazardous to the environment such as lead. 8/9/2022 19

19. Sources of Exposure The sources of exposure that must be shielded in a diagnostic or interventional x-ray room Scatter and leakage radiation are together called secondary or stray radiation. Primary radiation, also called the useful beam 8/9/2022 20

20. Sources of Exposure For radiation protection purposes scatter is considered as a separate radiation source with essentially the same photon energy spectrum (and penetrability) as the primary beam. The exposure due to leakage radiation is limited by FDA regulations 0.88 mGy/h (100 mR/h) at 1 m from the tube housing when the x-ray tube operated at the maximum allowable continuous tube current (usually 3 to 5 mA) at the maximum rated tube potential , typically 150 kV. 8/9/2022 21

21. Sources of Exposure The primary and secondary radiation exposure of an individual in an adjacent area to be protected depends primarily on the: amount of radiation produced by the source; distance between the patient and the radiation source; amount of time a given individual spends in an adjacent area; amount of protective shielding between the source of radiation and the individual; and distance between the source of radiation and the individual. 8/9/2022 22

22. Computation of X-ray Imaging Shielding Requirements the point of closest approach to the barrier is assumed to be 0.3 m for a wall , 1.7 m above the floor below, and, for transmission through the ceiling, at least 0.5m above the floor of the room above 8/9/2022 23

23. STRUCTURAL SHIELDING OF IMAGING FACILITIES Areas to be protected by shielding are designated as controlled and uncontrolled areas Shielding design goals, P, are amounts of air kerma delivered over a specified time at a stated reference point NCRP Report No. 147 recommends that the shielding design goal P for controlled areas be 5 mGy per year and that for uncontrolled areas be 1 mGy per year. 8/9/2022 24

24. STRUCTURAL SHIELDING OF IMAGING FACILITIES Shielding designed by these methods will keep the effective doses or effective dose equivalents received by workers in these areas much less than a tenth of the current occupational dose limits in the United States, will keep the dose to an embryo or fetus of a pregnant worker much less than 5 mGy over the duration of gestation, and will keep the effective doses to members of the public and employees, who are not considered radiation workers, less than 1 mSv per year . 8/9/2022 25

25. Computation of X-ray Imaging Shielding Requirements Terminologies Factors that must be considered for calculation of thickness of barrier Workload(W):it is the quantity of x rays generated per week Workload=mAs/patient X patient/ hr X hr /week=mAs/week kVP : measure of the penetrability. Constant operation assumed; 100 for general and 30 for mammography 8/9/2022 26

26. Computation of X-ray Imaging Shielding Requirements Use factor( U) :It is the fraction of time that the beam is directed at a particular barrier. floor and ceiling:1 and wall :1/4 Occupancy factor(T): Amount of time that the area will be occupied . T=1 means that the area will be occupied by the same individuals throughout the full workday. Distance (d) 8/9/2022 27

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28. Example Shielding Calculation Shielding thickness charts found in NCRP Report No. 147, which display the required shielding thicknesses for the various barriers as a function of N × T /( P × d 2 ), where, N is the number of patients per week, T is the occupancy factor P is the shielding design goal in mGy/ wk , and d is the distance in meters from the radiation source to the point to be protected. In abbreviated notation, this is NT / Pd 2 . Charts, which assume specific normalized workloads per patient ( W norm ) 8/9/2022 29

29. Example Shielding Calculation 8/9/2022 30

30. Example Shielding Calculation 8/9/2022 31

31. Example Shielding Calculation For the chest bucky wall primary radiation barrier with T = 0.2, P = 0.02 mGy/ wk , and d = 2.4 m, NT / Pd 2 = 120 × 0.2/(0.02 × 2.42) = 208 and, the lead thickness required is about 1.3 mm to achieve the shielding design goal 8/9/2022 32

32. Example Shielding Calculation Example Barrier Calculation for a CT Scanner For a CT scanner, all walls in the room are secondary barriers , because the detector array within the gantry is the primary radiation barrier. Three methods can be used to determine the shielding requirements based upon CTDIvol, based upon typical dose length product (DLP) values per acquisition, and based on the measured scatter distribution isodose maps provided by the manufacturer. 8/9/2022 33

33. Example Shielding Calculation Example Barrier Calculation for a CT Scanner produces a scatter air kerma of 63.8 mGy/ wk at 1 m from the gantry isocenter . barrier that is 3.3 m from the gantry isocenter to an uncontrolled corridor with P = 0.02 mGy/ wk occupancy factor T = 0.2 reduce the radiation to a level less than P/T = 0.02/0.2 = 0.1 mGy/wk . At 3.3 m, the unshielded air kerma is 63.8/3.3 2 = 5.86 mGy/ wk , and transmission factor,B through the lead must therefore be less than 0.1/5.86 = 0.017 Consulting Figure 21-15, this transmission factor requires a minimum of 1.0 mm lead. 8/9/2022 34

34. 8/9/2022 35 Secondary (scatter and leakage) radiation distributions for a 32-cm-diameter PMMA phantom, 40- mm collimation, 64-channel MDCT, 140 kV, 100 mA, 1 s scan. Actual secondary radiation levels must be scaled to the techniques used for the acquisition. A. Horizontal isodose scatter distribution at the height of the isocenter . B. Elevational isodose scatter distribution; the distribution is vertically symmetric above and below the level of the table.

35. APPLICATION OF INDIVIDUAL DOSE LIMTS A limit should be applied to the dose (other than from medical exposure and background radiation) received by an individual as the result of all practices to which he/she is exposed. Doses should not exceed specific values called “individual dose limits”. These dose limits are established in order to keep away from the “maximum risk level” so that no individual is exposed to a radiation that is judged to be unacceptable in any normal circumstance. The ICRP recommends that the last principle, limitation of maximal doses, not apply to medical exposure of patients or emergency situations. However, the other two principles do apply to medical exposure of patients and emergency situations.

36. 8/9/2022 37 Diagnostic Radiology Physics: a Handbook for Teachers and Students – chapter 24, 37 Recommended dose limits in planned exposure situations a (ICRP 103, 2007) Type of limit Occupational Public Effective dose 20 mSv per year, averaged over defined periods of 5 years e 1 mSv in a year f Annual equivalent dose in : Lens of the eye b 20 mSv 15 mSv Skin c,d 500 mSv 50 mSv Hands and feet 500 mSv – b this limit is a 2011 ICRP recommendation c The limitation on effective dose provides sufficient protection for the skin against stochastic effects d Averaged over 1 cm 2 area of skin regardless of the area exposed e With the further provision that the effective dose should not exceed 50 mSv in any single year Additional restrictions apply to the occupational exposure of pregnant women f In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year The BSS also adds stronger restrictions on occupational doses for “apprentices” and “students” aged 16 to 18 – namely dose limits of an: effective dose of 6 mSv in a year equivalent dose to the lens of the eye of 20 mSv in a year equivalent dose to the extremities or the skin of 150 mSv in a year These stronger dose limits would apply, for example, to any 16-18 year old student radiographers

37. GUIDANCE OR REFERENCE LEVELS Values of measured quantities above which some specified action or decision should be taken The ICRP recommends the use of Diagnostic Reference Levels (DRL) for patients (Report 73, 1996) Guidance dose levels are defined to serve as a reference for medical practitioners: if a level is exceeded some specified action or decision should be taken Guidance (reference) levels are not dose limits

38. RADIATION EXPOSURE IN PREGNANCY MEDICAL EXPOSURES Special consideration should be given to pregnant women because different types of biological effects are associated with irradiation of the unborn child As a basic rule it is recommended that radiological procedures of the woman likely to be pregnant should be avoided unless there are strong clinical indications There should be signs in the waiting area, cubicles and other appropriate places requesting a woman to notify the staff if she is or thinks she is pregnant For radiological procedures which could lead to a significant dose to an embryo or foetus , there should be systems in place to ascertain pregnancy status

39. FETAL RADIATION RISK There are radiation-related risks throughout pregnancy which are related to the stage of pregnancy absorbed dose Radiation risks are most significant during organogenesis and in the early fetal period somewhat less in the 2nd trimester and least in the third trimester 40 Less Least Most risk

40. RADIATION EXPOSURE IN PREGNANCY If, after consultation between the referring medical practitioner and the radiologist, it is not possible to substitute a lower dose or non-radiation examination, or to postpone the examination, then the examination should be performed Even then, the process of optimization of protection needs to also consider protection of the embryo/foetus Foetal doses from radiological procedures vary enormously, but clearly are higher when the examination includes the pelvic region At the higher end, for example, routine diagnostic CT- examinations of the pelvic region with and without contrast injection can lead to a foetal absorbed dose of about 50 mGy The use of a low-dose CT protocol and reducing the scanning area to a minimum would lower the foetal dose

41. RADIATION EXPOSURE IN PREGNANCY If a foetal dose is suspected to be high (e.g. >10 mGy) it should be carefully determined by a medical physicist and the pregnant woman should be informed about the possible risks It is, however, generally considered that for a foetal dose <100 mGy , as in most diagnostic procedures, termination of pregnancy is not justified from the point of radiation risks A fetal dose of 100 mGy has a small individual risk of radiation-induced cancer. There is over a 99% chance that the exposed fetus will NOT develop childhood cancer or leukaemia At fetal doses in excess of 100 mGy, there can be fetal damage, the magnitude and type of which is a function of dose and stage of pregnancy High fetal doses (100-1000 mGy) during late pregnancy are not likely to result in malformations or birth defects since all the organs have been formed.

42. OCCUPATIONAL EXPOSURE IN PREGNANCY A female worker should, on becoming aware that she is pregnant , notify the employer in order that her working conditions may be modified if necessary The employer shall adapt the working conditions in respect of occupational exposure so as to ensure that the embryo or foetus is afforded the same broad level of protection as required for members of the public , that is, the dose to the embryo or foetus should not normally exceed 1 mSv In general, in diagnostic radiology it will be safe to assume that provided the dose to the employee’s abdomen is less than 2 mSv , then the doses to the foetus will be lower than 1 mSv

43. Diagnostic reference level A Diagnostic Reference Level (DRL), is defined by the (ICRP) as: a form of investigation level, applied to an easily measured quantity, usually the absorbed dose in air, or tissue-equivalent material at the surface of a simple phantom or a representative patient. The ICRP recommends the establishment of diagnostic reference levels as a tool for optimizing the radiation dose delivered to patients in the course of diagnostic and/or therapeutic procedures. Used for optimisation of medical exposure , not for public and occupational exposure , Not to individual , a level set for standard procédure , standard sized patient, or phantom . Does not demarcate between good and bad practice, established for frequent and dose intensive procedures

44. Diagnostic reference level Main aim to avoid unnecessary dose not contributing clinical task by comparing Established levels with observed values. The purpose is advisory (not for regulatory or commercial purposes, and not linked to limits or constraints) a form of investigation level ( quality assurance) to identify unusually high levels, which calls for local review if consistently exceeded. Values BELOW DRLs may need optimization if the image quality is inadequate for clinical purposes. Values ABOVE DRLs require an investigation and optimization of X Ray system or protocols.

45. Personnel Protection in Medical X-ray Imaging 8/9/2022 47

46. X-ray Image Receptors Screen-film radiographic image receptors, now obsolete, had an inherent safety feature, but DR international exposure index standard for digital radiography is implemented This standard requires the imaging device vendor to estimate the radiation dose incident on the detector for a given exam ( e.g. , chest, abdomen, extremity) as an Exposure Index, EI, and to compare it to an accepted target Exposure Index, EIT, for that same exam. For immediate feedback to the technologist, a Deviation Index, DI, is generated as: DI = 10 log[EI/EIT] Acceptable DI values should be within –3.0 (underexposure of 50%) to +3.0 (overexposure of 100%). 8/9/2022 48

47. The following is a list of measures to be taken during a procedure to protect the patient: Increase SOD Select the lowest acceptable dose mode and fluoro pulse rate. Select the lowest acceptable digital magnification setting Remove the anti-scatter grid for small patients, for small body parts, and when the image receptor is far from the patient. Decrease OID. Collimate the x-ray beam aggressively. Use “virtual collimation,” which requires no radiation, to adjust the collimator. Minimize beam-on time. The operator should only activate fluoroscopy to see motion. Use last-image-hold feature. Minimize image recording. In cine and DSA, limit the number of imaging acquisition runs, duration of runs, and frame rate. Use fluoro -store as a low dose alternative when you can tolerate the increased random noise. 8/9/2022 49 Fluoroscopy and Intervention

48. The following is a list of measures to be taken during a procedure to protect the patient: Be aware that steeper beam angles increase the dose, due to the greater thickness of the body along the x-ray beam axis. Implement precautions to ensure, in steep oblique and lateral projections, that the patient's arm is not accidentally in the x-ray beam. Having an arm in the beam has been a contributing factor for some terrible tissue injuries Use more than one beam angle (“dose spreading”) if the dose to an area on the skin is approaching an amount that may cause injury. Assign a staff member to notify the operator when specified dose metric thresholds are reached. For the metric reference point air kerma ( K a,r ), recommended notification values are 3 Gy and every Gy thereafter ( NCRP, 2010a) If the procedure uses a biplane fluoroscopy system (two x-ray tubes and two image receptors), the notifications should be performed for both systems. 8/9/2022 50 Fluoroscopy and Intervention

49. The following is a list of actions to be taken by staff to protect themselves: Wear appropriate radiation protective apparel. scatter intensity is much higher on the x-ray tube side of the patient. Increase distance from where the x-ray beam intercepts the patient Support staff in the room should stand well back from the patient whenever they can. Use appropriate shielding Staff should keep their hands out of the primary x-ray beam, unless absolutely necessary. Wear dosimeters correctly. 8/9/2022 51 Fluoroscopy and Intervention

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57. 8/9/2022 65 Summary of Operational Factors That Affect Image Quality and Radiation Dose to the Patient and Staff

58. Dose Optimization in CT Reduce unnecessary phases. Thin slice, dose and resolution Tissues with inherently high tissue contrast may be acquired at low dose without loss of diagnostic value. These include lung and bone, CT-KUB Calculi, Lung nodule F/U. A kV of 140 could be selected for the most obese patients. Create separate abdominal protocols for obese patients. Reduced kV can also be employed, (80, 100 kvp ) Scan only the z -axis length you need. When possible, avoid radiation directly to the eyes. Vary tube current according to contrast phase. You may be able to use a lower mA for non-contrast phases. Use iterative reconstruction, if available. Use automatic tube current modulation, if available. Create low-dose protocols Compare these dose indices from individual scan protocols with DRLs and achievable doses. 8/9/2022 66

59. Dose Optimization in CT For cardiac CT protocols, substantial dose reduction can be achieved by the following strategies: Minimize the scan range. Use heart rate reduction. Prospective triggering results in lower dose than does retrospective gating. If using retrospective gating, employ ECG-gated mA modulation. Reduce tube voltage whenever possible to 100 kV or even 80 kV. 8/9/2022 67

60. Dose Optimization in CT The AAPM has recommended that the notification level be set higher than the relevant diagnostic reference level so that notifications are issued only for doses that are “higher than usual,” thereby warning the technologist that an acquisition parameter may be set incorrectly. The notification values recommended by AAPM are shown below (AAPM, 2011b). 8/9/2022 68

61. Radiation Protection in Nuclear Medicine The most effective method for reducing dose in nuclear medicine diagnostic procedures is to use radionuclides with short half-lives. A spectacular success in this area was the replacement of 131 I with its half-life of 8.1 days by 123I with a half-life of 13 hrs or 99mTc with half-life 6hrs. 131I was the only radionuclide used for thyroid function studies prior to 1960. In 1989 only 8% thyroid scans use 131I, the remainder being divided equally betweenn the 2 short lived radionuclides. The thyroid dose 650mSv from 2Mbq of 131 I was reduced to 39 mSv with the use of 10 MBq of 123 I and 2.6mSv with the us of 75MBq of 99m TC . Handling the radionuclide with care and without direct handling with hand.

62. Radiation Protection in Nuclear Medicine

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66. Patient protection Cardinal principles Technique selection Filtration Projection Image receptor Collimation Shielding Immobilization Equipment Public protection Information boards Restricted entry inside radiation area Regular radiation survey X-ray room design Radiation warning lamps and signs Professional Protection Patient protection Cardinal principles Use protective apparels Minimum fluoroscopy time Never hold patients Personnel monitoring ALARA SUMMARY

67. Conclusion As a radiology technologist we should; Always justify benefit to risk ratio while performing study Optimize the facilities and factors to reduce dose Apply ALARA principle i.e. time , distance and shielding. We should provide ; Shielding materials like lead apron Provide protection to attender Always try to take in one attempt Use proper technical factor.

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69. References Bushberg J. The essential physics of medical imaging. 3rd ed. Philadelphia, PA: Wolters Kluwer / Lippincott Williams & Wilkins; 2012. Bushong S. Radiologic science for technologists : physics, biology, and protection. 11 th ed. Elsevier Health Sciences; 2016. Kelsey C. Radiation biology of medical imaging. Hoboken, NJ: Wiley Blackwell; 2014. Hall. Radiobiology for the Radiologist. 8 th ed Wolters Kluwer; 2019. Forshier S. Essentials of radiation biology and protection. 2 nd ed Clifton Park, NY: Delmar; 2009. ICRP 73. Radiological Protection and Safety in Medicine. Annals of the ICRP, 26(2), 1996. Radiation Protection 118. Referral Guidelines for Imaging, European Commission, 2008. ACR Practice guideline for imaging pregnant or potentially pregnant adolescents and women with ionizing radiation. ACR Practice guideline for Diagnostic Reference Levels and achievable doses in medical x-ray imaging. Radiation shielding for diagnostic X Rays. BIR report (2000) Ed. D.G. Sutton & J.R. Williams NCRP Measurements, Report No. 147, “Structural Shielding Design for Medical X-Ray Imaging Facilities” Bethesda, MD 2004 8/9/2022 77

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