Patient Dosimetry in Interventional Procedures: what to measure and estimate, why and how?

Transcription

1 ICTP/IAEA Training Course on Radiation Protection of Patients September 2013, Trieste, Italy Patient Dosimetry in Interventional Procedures: what to measure and estimate, why and how? Renato Padovani Medical Physics Dpt, University Hospital, Udine, Italy

2 Contents Introduction: why radiation protection in IR? Angiography equipment performance assessment Patient dosimetry and reference levels Staff exposure, dosimetry and protection 2

3 Interventional Procedures Interventional procedures comprise fluoroscopically guided diagnostic and therapeutic techniques VASCULAR PROCEDURES Embolization Drug infusion (tumor catheter placement) Angioplasty (PTA, atherectomy, stent graft, placement) Cardiac intervention (PTCA, radiofrequency ablation) Transjugular intrahepatic portosystemic shunt (TIPS) NON-VASCULAR PROCEDURES Drainage & puncture Percutaneous needle biopsy Stent placement Coagulation therapy 3

4 Frequency and dose of procedures Large variations in frequencies and patient doses in PCI (Percutanous Cardiac Interventions) in Europe 4

5 Dose of procedures Large variations among centres/rooms and, usually, poor relationship between fluoroscopy time and patient dose (Percutanous Cardiac Interventions in Italy, 2012) Dose-Area Product (KAP). PTCA procedures (GISE study, 2012) 400 DAP vs FT Fluoroscopy (PCI), GISE time. Italy 2012 PTCA procedures (GISE study, 2012) Mean DAP (Gycm2) b b 24' 26 Mean fluorosocpy KAP (Gycm2) time (min) b b 13 13b 14 14b 15 15b b b 23 24' Centre no. Fluoroscopy Centre time no. (min) Patient Dosimetry in Interventional Procedures 5

6 Complex procedures may provide high skin dose Lesson learned: cumulative buildup of dose for steeply angled high-dose beam through large patient not recognized. Lesion required grafting 6

7 Staff: high exposure work area msv/h 1-5 msv/h 2-10 msv/h Annual cumulative ambient equivalent dose: msv IR room is a area of a hospital with the highest radiation risk 7

8 ICRP Report 85 (2001): Avoidance of Radiation Injuries from Interventional Procedures Cataract in eye of interventionalist after repeated use of old x ray systems and improper working conditions related to high levels of scattered radiation. 8

9 New dose limit for lens of the eye ICRP April 2011 Statement: The threshold for tissue reactions is now 0.5 Gy The recommended new occupational dose limit for the lens of the eye is 20 msv in a year. This recommendation has been incorporated into the interim version of the International Basic Safety Standards of the IAEA, published Nov

10 Contents Introduction Angiography equipment performance assessment Patient dosimetry and reference levels; complexity factors Staff exposure, dosimetry and protection 10

11 Which are the characteristics of an x-ray system specifically designed for interventional radiology? 11

12 Main characteristics of modern equipment Flat panel (FP) and image intensifier (II) imaging detectors Beam quality modulation: extra filtering Temporal beam modulation: pulsed fluoroscopy (grid controlled, temporal integration, etc). Spatial beam modulation: collimation (and virtual collimation), wedge filters, etc. Patient dose measurement, display and archive. Protective tools in the system. 12

13 Advantages of a FP vs. II Distortion-free images. FP II FP: Better contrast resolution. Larger dynamic range. Higher sensitivity to X-rays. Relative insensitivity to magnetic fields. 13

14 Reduction of Radiation Exposure with extra filtration Additional Cu filters can reduce the skin dose by more than 70%. Modern systems offer variable extra filtration (0.2 mm mm) that is automatically set according to patient weight and angulation of the C-arm. Automatic filter insertion keeps the skin dose as low as possible without degrading image quality. 14

15 Siemens AXIOM Artis dfa: CAREFILTER 15

16 Pulsed fluoroscopy Pulsed fluoroscopy can be used as a method of reducing radiation dose, particularly when the pulse rate is reduced. But pulsed fluoroscopy does not mean that dose rate is lower in comparison with continuous fluoroscopy!! Dose rate depends of the dose per pulse and the number of pulses per second. 16

17 Reduction of Radiation Exposure with virtual collimation Radiation-free collimation. Manipulation of diaphragms and movement of patient table in Last Image Hold (LIH)... no fluoroscopy required. 17

18 Antiscatter grid Antiscatter grid shall be used to improve image quality for adult patients. For paediatrics and slim patients, the antiscatter grid must be easily removable according to IEC standards. 18

19 Patient dose monitoring Patient dose measured or computedm displaied and archived: Dosimetric indications inside the interventional room. DICOM Report: Radiation Dose Structured Report (RDSR) IHE REM Profile 19

20 Operational Modes Requirements for image quality are not the same throughout the procedure and for the different procedures: Various operational modes are available Each mode delivers a different level of dose and image quality. 20

21 Example of Acquisition Mode (Siemens, Axiom Artis) Acquisition rates: 0.5, 1, 2, 3, 4, 7.5, 10, 15, 30 images/s User selectable Entrance dose at FP: 0.10, 0.12, 0.14, 0.17, 0.20, 0.24, 0.36, 0.54, 0.81, 1.20, 1.82, 2.40, 3.60 mgy/image (max/min=36!!) X-Ray pulse width: 3 to 200 ms enlarged automatically according to tube load Automatically added filtration: 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.9 mm Cu 21

22 Example of Acquisition Mode (Siemens, Axiom Artis) Each protocol: 4 acquisition modes 3 fluoroscopy modes Siemens, Axiom Artis 22

23 Automatic Exposure Control (AEC) The Automatic Exposure Control modifies exposure parameters (kv, ma, pulse width, added filtration) as a function of: Protocol selected Imaging mode selected: fluoro (Low, Normal, High), cineangiography, Digital Subtraction Angiography (DSA) Field Of View (FOV) Pulse rate Patient body absorption Patient dose is a function of all these parameters 23

24 Angiographic equipment set-up Entrance surface air kerma rate in image acquisition (cine) modes Large variability in equipment set-up and performance: - Dose rates: - cine low: ratio max/min 4 - cine normal: ratio max/min 4 SENTINEL survey (2007) 24

25 Assessment of equipment performance Measuring entrance dose and image quality Test object to measure image quality, at the isocenter Flat ionisation chamber to measure phantom entrance air kerma (K e ) Ionisation chamber: sensible to backscatter Semiconductor detector: shielded on the back 25

26 Ke = Ki * BF Patient Dosimetry in Interventional Procedures 26 Ionisation chamber: sensible to backscatter Semiconductor detector: shielded on the back

27 Entrance Air Kerma Measurement Angiographic system with the experimental arrangement to measure entrance phantom dose 27

28 Example: Entrance air kerma rates in fluoroscopy 28

29 Dose Quantities Useful quantities for QA and patient risk evaluation: Cumulative Air Kerma-Area product P KA (KAP or DAP) for Quality assurance: comparison with reference values Stochastic risk assessment Cumulative Air Kerma CK at the Interventional Reference Point To monitor skin exposure (to prevent skin injuries) 29

30 Cumulative Air Kerma at IRP CK is the incident air kerma (Ki) accumulated at the interventional reference point (IRP). The IRP is a point representative of the position of the patient s skin at the entrance of the X-ray beam during an interventional procedure. The IRP is located along the central ray of the X-ray beam at a distance of 15 cm from the isocentre in the direction of the focal spot. IRP 30

31 Image Quality Assessment 31

32 Example: Phantom image quality in image acquisition modes 0.10 µgy/image

33 Contents Introduction Angiography equipment performance assessment Patient dosimetry and reference levels; complexity factors Staff exposure, dosimetry and protection 33

34 Calibration of KAPmeters & CK indicator KAPmeter measures air kerma-area product P KA (KAP or DAP) KAP and CK on some systems are calculated QC protocol should include the check of the KAP/CK meter calibration Dose Management in Interventional Radiology 34

35 KAP and CK calibration Performed using a ionisation chamber positioned with a jig under the coach. For constancy check tests, PKA (KAP) and CK calibration factors should be preferably performed without the couch and mattress attenuation. 35

36 Example: Survey results on KAPmeter calibration Survey on the calibration of KAP meters (Sentinel, 2007) Mean: 0,83 SENTINEL survey (2006) 36

37 Computed KAP 5.0 DAP variations (%) PHILIPS Allura FD 20 Percentage (%) Cine FOV 15 Cine FOV 7 Cine FOV 10 Cine FOV 19 Fluoro 15 pps Fluoro 30 pps DSA 3 fps DSA 6 fps DSA 1 fps -6.0 kv IEC EN : Tolerance for DAP meter calibration: 35% New DAP meters (calculated values) maximum deviation from the reference point ( 80 kv, field of view 15 cm) is 5%

38 Patient dosimetry Patient doses (KAP, CK) available from: the header of DICOM format images the DICOM MPPS data The DICOM RDSR (Radiation Dose Structured Report) and, private reports QA protocol should include periodic data collection Data should be compared with Reference Levels and/or Local Reference Levels Patient Dosimetry in Interventional Procedures 38

39 DICOM header for QA DICOM header included dosimetric and technical information of procedure But there are important differences in the implementation of DICOM header on the different x- ray systems: Some data are stored in private Tags Total fluoroscopy time and total KAP are stored in different Tags at the end of the procedure DICOM TAG (, ) Value (0018,1110) Distance source to detector 1050 (0018,1111) Distance source to patient 820 (0018,1150) Exposure time (0018,1150) X-ray tube current 500 (0018,1152) Exposure (cumulative dose?) (0018,1154) Average pulse width 120 (0018,115E) Image Area Dose Product 4000 (0018,1162) Intensifier size 320 (0018,1510) Positioner Primary Angle 0 (0018,1511) Positioner Secondary Angle -10 (0020,0011) Serie number 2 (0028,0008) Number of frames 21 (, ) Dose Management in Interventional Radiology 39.

40 Private dose reports Siemens provides Exam protocol : fhe file is added to DICOM images the file is not a standard DICOM Dose Management in Interventional Radiology 40

41 Private dose reports Ospedale Santa Maria Misericordia Nome paziente: XXXXXXXXXXXX Data e ora esame: 28 settembre Data di nascita: mercoledì 20 aprile 1949 Sesso paziente: Maschio Medico incaricato: xxxxxxxxxxxxxxxx Esame CORO + CCDX Tempo fluoroscopia cumulativo: 4:44 mm:ss DAP cum. (fluoroscopia): mgycm2 DAP cum. (esposizione): mgycm2 DAP totale: mgycm2 Air Kerma cum.: mgy Numero tot. seriog. acquisite: 13 Numero tot. imm. acquisite: 1027 Numero tot. imm. esposiz. acquisite: 1019 Philips: patient dose report printed/mailed 41

42 Role of Reference Levels in IR For fluoroscopically guided interventional procedures the ICRP has stated that in principle, DRLs could be used for dose management, but are difficult to implement because of the very wide distribution of patient doses. (ICRP 120, Radiation Protection in Cardiology). Patient dose reports should be produced at the end of procedures, archived and recorded in the patient s medical record. If dose reports are not available, dose values should be recorded in the patient s medical record together with the procedure and patient identification. Patient dose audits (including comparison with diagnostic reference levels) and reporting are important components of the QAP. 42

43 Patient dose: PTCA procedures PTCA: FT: median values in a range from 5 to 13 minutes (factor 2.5) KAP: median values in a range from 35 to 85 Gycm2 (factor 2.5) 43

44 Reference levels in cardiac procedures SENTINEL recommendation (2007) Reference levels have been derived as the rounded values of the 3rd quartile of the frequency distributions 44

45 RLs vs complexity of PTCA procedures IAEA CRP study, 2006 Determinants for complexity of procedures identified Procedures grouped according to the level of complexity (CI - Complexity Index) Reference levels assessed as a function of CI Dose Management in Interventional Radiology 45

46 Preliminary RL for Interventional Radiology Procedure Lower limb angiography Total KAP Gy cm 2 Ft min No of frames Carotid angiography Cerebral arteriography Hepatic embolization Peripheral therapeutic procedures Nephrostomy

47 Monitoring skin exposure To reduce the probability of skin injuries: periodic monitoring is required and, for new equipment and procedures a trigger value in term of CK to IRP, or KAP, should be adopted to alert interventionalists Procedure repetition on the same patient has to be taken into account A follow-up protocol should be introduced for patients could have received high skin doses 47

48 Monitoring of skin dose in high dose procedures Skin dosimetry methods: Cumulative dose at IRP Large area detectors (radiochromic films) periodically on sample of patients Relationship between CK or KAP and Peak Skin Dose 48

49 Example: Monitoring of skin dose in high dose procedures (IAEA survey) Radiochromic films used to measure patient peak skin dose (PSD) in a sample of 392 interventional procedures In 52 procedures (7.4%) the PSK> 2 Gy, in 15 > 4 Gy; maximum PSK was 6.6 Gy, 38 PTCA, 6 RF ablation, 1 neuro-embolisation and 6 hepatic 39 occurred at two hospitals! 49

50 Contents Introduction Angiography equipment performance assessment Patient dosimetry and reference levels; complexity factors Staff exposure, dosimetry and protection 50

51 Variability of staff exposure in cardiac procedures Doses to interventionalists have high variability. How personal monitoring is performed? Are personal dosimeters properly used? Is the exposure optimised? 51

52 Operation dose quantities for staff monitoring Personal dose equivalent Hp(d) Used for comparing with regulatory requirements such as dose limits, dose constraints, intervention levels. Hp(10) for effective dose estimation Hp(0.07) for skin, hand dose (lens of the eye) Hp(0.3) for the lens of the eye ICRU Report 47, 51, 57 52

53 Personal dosimetry ICRP Report 85 (2001) Paragraph 66: The high occupational exposures in interventional radiology require the use of robust and adequate monitoring arrangements for staff. A single dosimeter worn under the lead apron will yield a reasonable estimate of effective dose for most instances. Wearing an additional dosimeter at collar level above the lead apron will provide an indication of head (eye) dose. 53

54 Double Dosimetry algorithms E = α H u + β H o DD algorithm without TS α β Remarks Wambersie and Delhove (1993) 1 0,1 Rosenstein and Webster (1994), NCRP Report 122 (1995) 0,5 0,025 Niklason et al. (1994) 1 0,06 H o -->H o -H u Swiss ordinance (1999) 1 0,1 McEvan (2000) 0,71 0,05 Franken and Huyskens (2002) 1 0,1 Sherbini and DeCicco (2002) 1 0,07 Von Boetticher et al. (2003), Lachmund (2005) 0,65 0,074 Clerinx et al. (2007) 1,64 0,075 54

55 ISEMIR (IAEA) 2012: Recommenda5ons of the Working Group on Interven5onal Cardiology on occupa5onal doses to the lens of the eye in Interven5onal Cardiology Elements of a monitoring protocol should include the following: The use of double dosimetry (over-apron at neck level and underapron at chest/waist level); The use of ambient dosimeters (such as at the C-arm) in identifying the lack of compliance in wearing personal dosimeters and to help to estimate occupational doses when personal dosimeters have not been used; The use of active dosimeters to identify means for improving radiation protection practice. Improved methodologies to assess lens doses need to be developed, including when lead glasses are worn. Patient Dosimetry in Interventional Procedures 55

56 Italian survey on eye doses in IR/IC Annual doses over the protective apron (2010) Maximum/Mean Eye Lens Annual Dose/Hospital Eye lens dose (msv/year) CI CI CI CI CI EF EF EF IN IN IN IN NI NI RI RI RI RI RI RI TE TE TE max Mean 5 hospitals - CI: int. cardiologist, EF: electrophysiologist, IN: nurse, NI: neuroradiologist, RI: int. radiologist, TE: radiographer Large fraction of interventionalists are receiving lens doses over the recently recommended ICRP limit AGENAS/ISS/AOUD

57 Factors affecting eye lens dose assessment 57

58 Eye protectors in IR Ceiling suspended protective screen Reduce doses to the head by factors of depending on the diligence of staff in their use Impossible to use in some procedures Eye protectors (protective glasses) Characteristics Efficacy Attenuation coefficients 58

59 Effect of protective eyewear Radiation direction Dose transmission factor (%) Frontal, left lens Frontal, right lens Below, left lens Below, right lens When radiation is coming from below, great difference between models due to the gap created between the eyewear and the cheek and the nose T. Geber, Eye Lens Dosimetry for Interventional Radiology, Rad. Meas. 46 (2011) Design and individual fit is decisive. Design should minimise gaps 59

60 Active dosimeters: Ambient vs. Operator Dose A dosimeter on C-arm can measure cumulative scatter radiation that can be correlated with operator doses Example of cumulative scatter dose on C-arm and cumulative Hp(10) over the apron to a interventional cardiologist, measured with active dosimeters; dose rates of single acquisitions are also reported (Udine, 2012) 60

61 Conclusions IR using fluoroscopy guide is a growing radiological practice implying high patient and staff doses Frequently procedures are performed by nonradiologists outside radiological departments with low RP knowledge High dose procedure can be performed in surgical rooms with inappropriate mobile equipment Because staff and patient exposure are correlated, patient and staff protection should be part of the same safety programme. A robust QA programme should be implemented. 61