Radiation Protection in Paediatric Radiology

1. Radiation Protection in Paediatric Radiology Understanding Radiation Units L02

2. Educational Objectives At the end of the programme, the participants should become familiar with the following: Why is it important to measure radiation dose in children? How radiation dose can and should be expressed? Understand the radiation quantities and units used in diagnostic radiology. Radiation Protection in Paediatric Radiology L02. Understanding radiation units 2

3. Answer True or False The same amount of radiation falling on the person at level of breast, head or gonads will have the same biological effects. Effective dose can be easily measured. Diagnostic reference levels are not applicable to paediatric radiology. Radiation Protection in Paediatric Radiology L02. Understanding radiation units 3

4. Contents • Dose descriptors outside the patient’s body. • Dose descriptors for effects that have threshold (deterministic effects) • Dose descriptors to estimate stochastic risks • Diagnostic reference levels • Dose descriptors and units for staff dose assessment Radiation Protection in Paediatric Radiology L02. Understanding radiation units

5. Introduction Several quantities and units are used in the field of diagnostic radiology to measure and describe radiation dose Some can be measured directly while others can only be mathematically estimated Radiation Protection in Paediatric Radiology L02. Understanding radiation units 5

6. Two types of radiation effects Stochastic effects Where the severity of the result is the same but the probability of occurrence increases with radiation dose, e.g., development of cancer There is no threshold for stochastic effects Examples: cancer, hereditary effects Deterministic effects Where the severity depends upon the radiation dose, e.g., skin burns The higher the dose, the greater the effect There is a threshold for deterministic effects Examples: skin burns, cataract Radiation Protection in Paediatric Radiology L02. Understanding radiation units 6

7. Hot Coffee – Energy contained in a sip Excess Temperature = 60º - 37 = 23º 1 sip = 3ml 3x 23 = 69 calories Radiation Protection in Paediatric Radiology L02. Understanding radiation units

8. Radiation Dose X-rays Lethal Dose= 4Gy LD 50/60 = 4 Gy For man of 70 kg Energy absorbed = 4 x 70 = 280 J = 280/418= 67 calories = 1 sip Energy content of a sip of coffee if derived in the form of X-rays can be lethal Radiation Protection in Paediatric Radiology L02. Understanding radiation units

9. Dose of Radiation • Radiation energy absorbed by a body per unit mass. Radiation Protection in Paediatric Radiology L02. Understanding radiation units 9

10. Dose Quantities and Radiation units - Dose quantities external to the patient’s body. - Dose quantities to estimate risks of skin injuries and effects that have threshold. - Dose quantities to estimate stochastic risks. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

11. Why so many quantities?Radiation dose is a complex topic • 1000 Watt heater giving off heat (IR radiation)- unit is of power which is related with emission intensity • Heat perceived by the person will vary with so many factors:distance, clothing, room temperature • As can be seen with the example of heat, the energy transformation is a highly complicated issue • This is the case with X-rays- radiation can’t be perceived Radiation Protection in Paediatric Radiology L02. Understanding radiation units

12. Basic Radiation Quantities Used to quantify a beam of X or γ-rays There are: Quantities to express total amount of radiation. Quantities to express radiation at a specific point • Radiation at a specific point • Photon fluence • Absorbed dose • Kerma • Dose equivalent • Total radiation • Total photons • Integral dose Radiation Protection in Paediatric Radiology L02. Understanding radiation units 12

13. Exposure: X Exposure is a dosimetric quantity for measuring ionizing electromagnetic radiation (X-rays & Ɣ-rays), based on the ability of the radiation to produce ionization in air. Units: coulomb/kg (C/kg) or roentgen (R) 1 R = 0.000258 C/kg Radiation Protection in Paediatric Radiology L02. Understanding radiation units

14. KERMA KERMA (Kinetic Energy Released in a Material): Is the sum of the initial kinetic energies of all charged ionizing particles liberated by uncharged ionizing particles in a material of unit mass For medical imaging use, KERMA is usually expressed in air SI unit = joule per kilogram (J/kg) or gray (Gy) 1 J/kg = 1 Gy Radiation Protection in Paediatric Radiology L02. Understanding radiation units

15. Absorbed dose, D, is the mean energy imparted by ionizing radiation to matter per unit mass.SI unit = joule per kg (J/kg) or gray (Gy).In diagnostic radiology, KERMA and D are equal. Absorbed dose: D Harold Gray Radiation Protection in Paediatric Radiology L02. Understanding radiation units

16. Mean absorbed dose in a tissue or organ The mean absorbed dose in a tissue or organ DT is the energy deposited in the organ divided by the mass of that organ. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

17. Now things get a little more complicated ! Radiation Protection in Paediatric Radiology L02. Understanding radiation units 17

18. Radiation Dose Quantities • Primary physical quantities are not used directly for dose limitation • The International Council on Radiation Protection (ICRP) has defined values for dose limits in occupational exposure Radiation Protection in Paediatric Radiology L02. Understanding radiation units

19. Radiation Dose Quantities Equivalent Dose: • Accounts for the type of radiation • Different radiation types have different level of biologic damage per unit absorbed dose Radiation Protection in Paediatric Radiology L02. Understanding radiation units

20. Radiation Weighting Factors, wR (Source: ICRP 103) Radiation Protection in Paediatric Radiology L02. Understanding radiation units

21. Equivalent Dose : HT,R The absorbed dose in an organ or tissue multiplied by the relevant radiation weighting factor : where DT,R is the average absorbed dose in the organ or tissue T, and wR is the radiation weighting factor for radiation R. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

22. Radiation Quantities and Units Equivalent dose (Unit = sievert, Sv ) Compares the biological effects for different types of radiation, X-rays, Ɣ-rays, electrons, neutrons, protons, α-particles etc. For X-rays, Ɣ-rays, electrons : absorbed dose and equivalent dose have the same value Gy = Sv. Rolph Sievert Radiation Protection in Paediatric Radiology L02. Understanding radiation units 22

23. Detriment Radiation exposure to different organs and tissues in the body results in different probabilities of harm and different levels of severity. The combination of probability and severity of harm is called “detriment”. Effective dose reflects the combined detriment from stochastic effects due to the equivalent doses in all the organs and tissues of the body. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

24. Effective Dose: ET Effective dose takes into account the organ specific radio-sensitivity to develop cancer and hereditary effects from radiation Unit = sievert, Sv Radiation Protection in Paediatric Radiology L02. Understanding radiation units 24

25. Effective Dose: ET A summation of the tissue equivalent doses, each multiplied by the appropriate tissue weighting factor: where HT is the equivalent dose in tissue T and wT is the tissue weighting factor for tissue T. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

26. Tissue Weighting Factors, wT • The organs have different weighting factors, wT. • These factors are published in ICRP 103 (2007) and have been changed over the years due to increased knowledge. Radiation Protection in Paediatric Radiology L02. Understanding radiation units 26

27. The weighting factors sum up to 1.0. They are relative and compares one organ with the other. They are the same for children and adults! Tissue Weighting Factors Radiation Protection in Paediatric Radiology L02. Understanding radiation units 27 27

28. Data is primarily taken from knowledge derived from studying the Japanese population exposed to atomic bombs in Hiroshima and Nagasaki On going research has changed the weighting factors from 1990 (ICRP 60) to 2007 (ICRP 103). Tissue Weighting Factors Radiation Protection in Paediatric Radiology L02. Understanding radiation units 28

29. Multipliers of the equivalent dose to an organ or tissue to account for the different sensitivities to the induction of stochastic effects of radiation. Tissue Weighting Factors *ICRP 103 **Remainder Tissues (14 in total): Adrenals, Extrathoracic (ET) region, Gall bladder,Heart, Kidneys, Lymphatic nodes, Muscle, Oral mucosa, Pancreas, Prostate, Small intestine, Spleen, Thymus, Uterus/cervix.. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

30. Effective Dose (E) Dose to lungs times their weighting factor; DL x wL + Dose (mean absorbed dose) to gastrointestinal tract times their weighting factor; DGI x wGI + ....(summation over organ after organ) = Effective dose where T stands for tissue Radiation Protection in Paediatric Radiology L02. Understanding radiation units 30

31. Effective Dose (E) We can compare different paediatric imaging procedures through their different effective doses, E. Radiation Protection in Paediatric Radiology L02. Understanding radiation units 31

32. Radiation Quantities and Units used in Diagnostic Radiology Incident air kerma Entrance surface air kerma Air kerma-area product Air kerma-length product Dosimetric quantities for CT Dosimetric quantities for interventional radiology Radiation Protection in Paediatric Radiology L02. Understanding radiation units 32

33. Incident Air Kerma Measured Free in Air on the central beam axis at the focal spot to surface distance. Only primary beam is considered, that is, no scatter contribution. Unit: joule/kg or gray (Gy) Radiation Protection in Paediatric Radiology L02. Understanding radiation units

34. Entrance Surface Air Kerma (ESAK) ESAK measured on the surface of the patient or phantom where X-ray beam enters the patient or phantom. Includes a contribution from photons scattered back from deeper tissues, which is not included in free in air measurements. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

35. Entrance Surface Air Kerma (ESAK) If measurements are made at other distances than the true focus - to - skin distance, doses must be corrected by the inverse square law and backscatter factor incorporated into the calculation. References: Dosimetry in Diagnostic Radiology: An International code of practice, TRS 457, IAEA, 2007 Phys. Med. Biol. 43 (1998) 2237-2250. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

36. Kerma in X-ray field can be measured using calibrated: Ionization chamber Semiconductor dosimeter Thermoluminescent dosimeter (TLD) Dose Measurement Radiation Protection in Paediatric Radiology L02. Understanding radiation units 36

37. Kerma-Area Product: KAP The kerma - area product (KAP) is defined as the kerma in air in a plane perpendicular to the incident beam axis, integrated over the area of interest. This is the dose related quantity measured and displayed on all modern X-ray equipment excluding CT. KAP meter Radiation Protection in Paediatric Radiology L02. Understanding radiation units

38. Kerma-Area Product: KAP The KAP (Gy·cm2) is constant with distance since the cross section of the beam is a quadratic function which cancels the inverse quadratic dependence on dose . KAP remains constant along the beam axis as long as it is not measured close to the patient/phantom surface which introduces backscatter. Radiation Protection in Paediatric Radiology L02. Understanding radiation units

39. KAP = K x Area the SI unit of KAP is the Gy·cm2 Kerma-Area Product: KAP d1=1 Area = 1Dose = 1 d2=2 Area = 4Dose = 1/4 Radiation Protection in Paediatric Radiology L02. Understanding radiation units 39

40. Kerma-Area Product: KAP d1=1 Area = 1Dose = 1 d2=2 Area = 4Dose = 1/4 KAP is independent of distance from the X-ray source, as: • Air Kerma decreases with the inverse square law. • Area increase with the square distance KAP is usually measured at the level of the tube diaphragms Radiation Protection in Paediatric Radiology L02. Understanding radiation units 40

41. This is a picture of a KAP meter which measures the kerma area product Radiation Protection in Paediatric Radiology L02. Understanding radiation units

42. Example of a dose display during fluoroscopy or cine runs with dose rate as shown Radiation Protection in Paediatric Radiology L02. Understanding radiation units 42

43. In paediatric radiology KAP may be used for: Diagnostic reference levels (DRLs) By use of conversion factors, it can be converted to skin dose and/or effective dose Kerma-Area Product Radiation Protection in Paediatric Radiology L02. Understanding radiation units 43

44. Dosimetric Quantities for CT Computed Tomography Dose Index (CTDI) - determined using scan protocol parameters. -useful for comparison of different scanners. Dose-Length Product (DLP) - measure of dose to patient - used to estimate effective dose Radiation Protection in Paediatric Radiology L02. Understanding radiation units

45. CT and Risk KERMA (in phantom) CTDI (dose in phantom per slice) Length of scan and pitch DLP Effective dose Risk Radiation Protection in Paediatric Radiology L02. Understanding radiation units

46. Measurement of Dosimetric Quantities in CT Pencil ionisation chamber with active length of 100 mm. Measurements free-in-air or in standard dosimetry phantom. Alternatives: TLD, solid state detectors. CTDIVOLshould be displayed on the console, reflecting the conditions of operation selected (IEC, 2003) Radiation Protection in Paediatric Radiology L02. Understanding radiation units

47. Dose Indicators in Interventional Radiology • For quality assurance purposes • To estimate the probability of occurrence of stochastic effects use: Kerma-air product rate (KAP, PKA) Radiation Protection in Paediatric Radiology L02. Understanding radiation units

48. Dose Indicators in Interventional Radiology • For quantifying the threshold and severity of deterministic effects use: • Maximum skin dose (MSD) • Cumulative dose (CD) to Interventional Reference Point (IRP) • In a complex procedure skin dose is highly variable Radiation Protection in Paediatric Radiology L02. Understanding radiation units

49. In some procedures, patient skin doses approach those used in radiotherapy fractions Maximum skin dose (MSD) or peak skin dose is the maximum dose received by a portion of the exposed skin. Interventional Procedures: Skin Dose Radiodermatitis in the rightarm. 7 year-old patient. Photograph taken 4 months after radiofrequency ablation. Surce: ICRP 85 Radiation Protection in Paediatric Radiology L02. Understanding radiation units 49

50. Cumulative Dose to Interventional Reference Point* • IRP is located 15 cm from the isocentre towards the focal spot • The air kerma accumulated at a specific point in space relative to the fluoroscopic gantry (IRP) during a procedure • Cumulative dose does not include tissue backscatter and is measured in Gy. • Cumulative dose is sometimes referred to as cumulative air kerma *IRP Radiation Protection in Paediatric Radiology L02. Understanding radiation units