1. David Sutton or Colin Martin But Borrowed from Jerry Williams
IAEA/RCA DRL Kampala
What is Dose ?
David Sutton or Colin Martin
But Borrowed from Jerry Williams
Kampala

2. Why Measure Radiation Dose ?
There is a detriment associated with ionising radiation.
We want to reduce that detriment to a level that is As Low As Reasonably Achievable.
Kampala

3. An amount of radiation ……….
What is Dose?
An amount of radiation ……….
Kampala

4. What does dose tell us?
Quantity related to clinical outcome - effects of radiation
Measurable (assessable) quantity
Related to equipment performance
Kampala

5. Dose: What can we measure?
Energy imparted:
temperature rise
Ionisation
Kampala

6. Radiation quantities (dose)
Physical
Biological
Kerma
Energy transferred per unit mass
Unit: J kg-1 or gray (Gy)
Absorbed dose
Energy absorbed per unit mass
Unit: gray (Gy)
Equivalent dose
Absorbed dose multiplied by radiation weighting factor and summed over all radiation types
Unit: sievert (Sv)
Effective dose
Equivalent dose weighted for tissue sensitivity and summed over all tissues
Kampala

7. KERMA & ABSORBED DOSE
Kampala

8. KERMA Kinetic Energy Released per unit Mass (to Medium; to Matter)
Amount of energy transferred from the photon beam to the material (ΔEtr) in a small mass of the material (Δm)
Unit
joule/ kg (J/kg)
gray (Gy)
Kampala

9. Measurement in practice
Kampala

10. Field instrument calibration
Traceable to primary standard
Energy dependent because not air equivalent
Kampala

11. …..but the body is not composed of air
So we measure air kerma
…..but the body is not composed of air
National Protocol for Patient Dose measurements in Diagnostic Radiology:
Entrance surface dose defined as absorbed dose to air …….
Kampala

12. Dose measurement Ionisation chamber Measures air kerma Uncertainties
In presence of chamber and phantom
Uncertainties
Energy dependence of calibration factor (±4% to +/- 10%)
Dependence of temperature & pressure (±3%)
Angular dependence (non-isotropic)
Influence of measuring device
Overall measurement uncertainty: ±5% to ±15%
(Not including systematic uncertainty in dosimetry standard)
Kampala

13. Solid state (diode) dosimeter
Energy dependence
Use of dual detectors with energy compensating filters
Lead backed
Back scatter rejection
New transparent detectors
Kampala

14. Measurement quantities for dose audit
Kampala

15. Measurement quantities for dose audit: Radiography and Fluoroscopy
Entrance Surface AK or Dose (ESAK orESD)
Air Kerma
Includes back scatter
Assessment:
Calculation from X-ray factors
TLD
Units: mGy
Incident Air Kerma
As above,no BSF
Air Kerma Area Product (KAP)
Calibrated in scatter free geometry
Undercouch systems: Calibrated above table top
Assessment:
KAP meter
Calculated from X-ray factors
Units: µGy m2 = 10 mGy cm2 =1 cGy cm2 = 0.01 Gy cm2
IAK rate (mGy/min)
Cumulative IAK
Kampala

16. ESD, IAK and KAP BSF is about 1.3
BSF data from Aoki and Koyama, 2002, PMB, 47, 1205
Kampala

17. Measurement quantities for dose audit CT scanning
Kampala

18. CT Dose Index (CTDI)
Kampala

19. CTDI and Dose Length Product (DLP)
Kampala

20. Doses in mammography
Kampala

21. Mean Glandular Dose Average absorbed dose to glandular tissue
Assumes uniform distribution of breast tissue
K – Entrance air kerma at surface of the breast ( no backscatter)
g – conversion factor from entrance air kerma to MGD
c – age dependent factor to correct from assumption of 50% glandular/ 50% adipose breast composition
s – spectrum correction factor (normalised to Mo/Mo)
Kampala

22. Effective dose Measurement of Risk
Kampala

23. Organ Absorbed Dose
Radiation
weighting factors wR
Equivalent dose
Tissue
weighting factors wT
Effective Dose
Kampala

24. Equivalent dose
ICRP 103 factors (2007)
Kampala

25. Effective Dose
Remainder: Adrenals; Extrathoracic region; Gall bladder; Heart; Kydneys; Lymphatic nodes; Muscle; Oral mucosa; Pancreas; Prostate; Small intestine; Spleen; Thymus; Uterus/ cervix.
ICRP 103 factors (2007)
Kampala

26. Radiation Doses (mSv) Dose limit (worker) Natural bgd per year10
Natural bgd per year 1
Dose limit (public)
0.1
Min reading TLD dosimeter
Return flight New York
0.01
Natural bgd per day
One way flight London
0.001
Natural bgd per hour
Kampala
0.0001

27. Weighting factors ICRP 60 – ICRP 103
Annual dose from natural background radiation
Average =2.2 mSv
Max = 7.4 mSv
What if wT for α’s = 10 or 30?
Kampala

28. Effective dose Measure of risk Good for comparison of techniques
CT colonoscopy vs Ba Enema
Dose from a medical procedure and another source
Cannot give risk to individual
NOT GOOD FOR DRLS
Kampala

29. Summary Requirements for dosimetric quantities:
Related to clinical outcome
Measurable
Ionisation is easily measured quantity
For dose audit 5 to 20% measurement uncertainty acceptable
Quantities used for dose audit: ESD, KAP, CTDI, DLP, MGD
Effective is not relevant to dose audit
Kampala