Paediatric patient dose surveys

1. Paediatric patient dose surveys Colin Martin and David Sutton

2. Overview Introduction 1. Chest and abdominal radiographs 2. Dynamic procedures

3. Why are kids special? higher risk than for adults often different techniques less data available only performed occasionally in some hospitals lack of optimisation

4. Radiation risk in children Greater chance for expression of radiation induced effects Greater sensitivity for some cancers High frequency for some examinations Lack of cooperation and optimisation

5. Large numbers of images can be taken of new born babies K. Smans, Leuven

6. Sometimes over 100 images can be taken of new borns C. Maccia, Paris

7. Some Paediatric Exams Water soluble contrast studies Mict Cystogram for reflux Intussusception Non-accidental injury assessment Hips for displasia

8. Fewer patients • Paediatric exams only represent 1-2% of those in UK database (despite extra efforts to collect data) • Only paediatric hospitals have sufficient patients for full survey • Other hospitals may have only a handful of patients even for commonest exams

9. X-ray equipment for paediatrics Generators Filtration (additional copper) Paediatric anti-scatter grids Automatic exposure control Low dose fluoroscopy KAP meters with resolution of 0.1 mGy cm2 The Ideal is to use dedicated equipment. Factors to consider include:

10. 1. Paediatric Radiography • Simple examinations • Assess ESD or KAP • Large range in patient size

11. Best Practice Guidelines High speed screen/film systems Avoid use of anti-scatter grids where possible Additional filtration - copper High kV-short exposure techniques Gonad protection Dedicated equipment Trained staff Cook et al, 1998

12. Use of Grids • Use of an anti-scatter grid can double the dose • Children’s bodies are small • so there is less scattered radiation • grids are often unnecessary

13. New born child can be placed directly on cassette for radiograph

14. Influence of grid on doseEffect of introduction of grid for larger children

15. Poor collimation exposes sensitive organs unnecessarily

16. Good collimation for neonatesAchieved by positioning lead/rubber drapes on incubator Poor radiation protection Good radiation protection

17. Copper filtration:Removes low energy photons that are absorbed in superficial tissues Spectra of 80 kV X-ray beams with and without 0.2 mm copper X-ray photons removed Reduces skin dose Increase in mAs required Maintains good contrast Exposure adjusted to give similar dose at the image receptor

18. Dose quantities Radiography ESD : measured with TLD or calculated from tube output KAP : If KAP meter fitted Fluoroscopy KAP : measured with KAP meter

19. Dosimetry Problems low readings (KAP meters/TLD) give poor precision TLD may be visible or cause practical difficulties calculations of dose complicated by variable patient size small sample sizes uncertainties in effective dose and risk factors

20. Exposure parameters & tube output measurements Need tube output (as a function of kV) from the medical physics report Need kV, distance and mAs values for exposure Uncertainties: Accuracy of the distance data Field size

21. Paediatric Data Collection Exam Patient data essential age Height & weight  thickness Focus to Skin Distance Use of grid Filtration

22. Why is size a problem ? Continuous size distribution (neonate to adult) Establishment of DRLs and comparisons with reference levels need to be meaningful Conflict arises between sample size and variability arising from patient size

23. Increase in KAP with patient sizeNote that vertical axis is a log scale Courtesy T. Kiljunen, STUK, Finland

24. Possible Approaches Age banding Patients grouped together to provide sufficient data Retrospective data analysis Correction of data using effective attenuation coefficients

25. Size correction using effective attenuation coefficients Corrections can be made to data to enable patients with a range of ages/sizes to be used Equate to a patient of standard thickness ‘s’ FESD = eµ(s-d) Where  = effective attenuation coefficient (derived from measurements of entrance and exit doses) with varying phantom thicknesses – fixed kV d = thickness of patient for which data collected Hart et al (2000): NRPB-R318

26. h Equivalent cylinder diameter • Patient diameter can be equated to a standard cylinder of the same length and weight, if other data not available • Equivalent diameter = 2√[Weight/( Height)] Density = ( 1 g cm3)

27. Size correction for KAP Similar corrections can be made to KAP data FKAP = eµ(s-d)s2/d2  = effective attenuation coefficient S = thickness of standard patient d = thickness of patient for which data collected Incorporates FSD correction Field size correction for KAP Hart et al (2000): NRPB-R318

28. Age ranges • 0 – 6 months  newborn • 7 m – 23 m  1 y old infant • 2 – 7 y  5 y old • 8 y – 12 y  10 y old

29. National patient dose survey • Collect data from hospitals • Input data into database • Calculate mean KAP or ESD • Compare data from all centres

30. National SurveyDistribution of doses for paediatric chest • Identify the 3rd quartile dose for use as DRL • 75% below DRL; 25% above need optimisation 3rd Quartile - 70 mGy

31. Setting DRL based on ESD data for agreed age band Third Quartile DRL 70 mGy

32. UK Paediatric Radiographic DRLs

33. Regional patient dose survey • Compare data from all centres • For non-paediatric hospitals it may be necessary just to collect standard factor data for some examinations • Investigate possible reasons for high doses above DRL • Produce individual report for each centre • Liaison with individual hospitals is necessary to ensure optimisation of protection

34. Issues with survey data … • Include patient weight • 20+ patients if possible • Reset KAP meter after each examination • Quote KAP in correct units • Often large differences between centres

35. National DRL Neonatal dose data from factors used in District General Hospitals • Hospitals rarely carry out examinations and factors are not optimised

36. Patient dose survey report

37. Reasons for high DAP values- Radiographic • Low patient numbers • Patient weight not given • Using a low kV • KAP quoted in wrong units • KAP meter not reset after each patient/examination • Different techniques used

38. 2. Dynamic examinations Less frequent KAP measurements Effect of the weight? More complicated to calculate effective doses

39. UK Paediatric Fluoro DRLs

40. DAP v age for MCU BJR 72:763-772 (1999) Courtesy of F. Schultz

41. Reasons for high DAP values • Patient weight not given • Low patient numbers • High pulse rate • Old equipment • Large variation in screening times • Protocols not being followed

42. Summary Paediatric dosimetry is not straightforward Dose measurement/calculation methodologies need to be well thought out A consistent approach to patient size correction is needed Dose quantities need to be appropriate for task

43. Any Questions?