Practical exercise: Effective dose estimate in CT

Transcription

1 Practical exercise: Effective dose estimate in CT TRAINING COURCE PROGRAM May 2011, Sofia, Bulgaria Virginia Tsapaki Medical Physics Dpt Konstantopoulio General Hospital

2 Dose DataMed1 project In order to assess population exposures from medical radiology in terms of the collective or per caput effective dose it is necessary to estimate representative mean effective doses (E), for each type of x-ray examination that makes a significant contribution to the collective dose in a country 2

3 CT exams in the TOP20 list of the report 154 CT head CT neck CT chest CT spine CT abdomen CT pelvis CT trunk 3

4 4

5 focused to organ of interest 5

6 Summary of Dose Metrics Slice Plane Air dose Rotation Axis CTDI CT Dose Index DLP Organ Dose Effective Dose 6

7 CT Dose Index in multislice CT CTDI vol = CTDI w / pitch CTDIvol was introduced to allow for variations in exposure in the z direction when the pitch was not equal to one 7

8 Dose Length Product (DLP) DLP = CTDI vol ( Exposure _ length) 100 mgycm = 10 mgy CTDI 10 cm 300 mgycm = 10 mgy CTDI 30 cm Dose in one rotation x exposure length (mgycm) DLP : A convenient index for total dose 8

9 Effective dose (E) E = Σ w T.H T E : effective dose wt : weighting factor for organ or tissue T HT : equivalent dose in organ or tissue T The E,, is defined in ICRP 60 [ICRP, 1991], ICRP 103 [ICRP, 2007b] and ICRU 51 [ICRU, 1993]. It is the sum over all the organs and tissues of the body of the product of the equivalent dose, H T, to the organ or tissue and a tissue weighting factor, w T, for that organ or tissue 9

10 Effective dose Calculation from organ doses Calculation from DLP using conversion factors. (European Guidelines) 10

11 CTDI pencil chamber method A specially designed ion chamber with a shape of a pencil. 11

12 12

13 Dose Length Product (DLP) DLP = CTDI vol ( Exposure _ length) 100 mgycm = 10 mgy CTDI 10 cm 300 mgycm = 10 mgy CTDI 30 cm Dose in one rotation x exposure length (mgycm) DLP : A convenient index for total dose 13

14 A first order effective dose estimate (for mean population dose purposes) E = E DLP DLP Region of Body E/DLP Conversion Factor msv.mgy -1. cm -1 Head Neck Chest Abdomen Pelvis EUR16262 Guidelines on Quality Criteria for CT ( 14

15 European Guidelines for Multislice Computed Tomography ttp:// The document includes both adult and paediatric conversions coefficients 15

16 Important for paediatric exams When using conversion coefficients for children one must be aware that these coefficients have been obtained for a 16 cm CT dose phantom, whereas the CT console indicator will provide DLP or CTDI assuming the use of the 32-cm diameter body phantom. In pediatric examinations, the figures displayed in the CT console should be multiplied by a factor of 2 for children and of 3 for infants in order to give a realistic estimate of the patient's dose. 16

17 Organ doses NRPB Jones DG and Shrimpton P Nomalised Organ Doses for x-ray computed tomography calculated GSF using Monte Carlo techniques NRPB SR 250 NRPB Chilton Oxon 1993 Zankl M, Panzer W and Drexler G The Calculation of Dose from External Photon Exposures using Reference Human Phantoms and Monte Carlo Methods Part IV: Organ Doses from CT Examinations GSF-Bericht 30/91 GSF- Forschungszentrum fur Umwelt und Gesundheit, Institut fur Strahlenschutz, Neuherberg, Germany

18 Organ doses Anthropomorphic phantoms (Rando phantom):tlds placed in specific positions in the phantom for organ dose measurement 18

19 Software methods CT-Expo [ ImPACT CT Patient Dosimetry Calculator ( 19

20 ImPACT CT Patient Dosimetry Calculator Version /08/2010 Introduction This spreadsheet is a tool for calculating patient organ and effective doses from CT scanner examinations. It makes use of the NRPB Monte Carlo dose data sets produced in report SR250 (link at bottom of page). SR250 provides normalised organ dose data for irradiation of a mathematical phantom by a range of CT scanners. As SR250 was produced in 1993, it does not include data for more modern scanners. To overcome this problem, the ImPACT CT scanner dose survey was carried out by physicists in the UK and Europe. This work, provides a method for 'matching' the dose distribution of newer scanners to scanners included in SR250. The matching results are included in this spreadsheet. As new scanners are introduced, their matches will be included in updates to this spreadsheet. More details can be found on the dose survey page on the ImPACT website (link below) The results produced by the CTDosimetry spreadsheets have been checked against those produced by CTDOSE, produced by John Le Heron, the standard software used to calculate doses from the NRPB SR250 datasets. The two methods produce identical results for a range of scans and scanners, with the exception of small differences between the doses calculated for muscle and for the 'remainder' organs. These differences are present despite an apparent similarity in calculation method, and are typically 1-2%. Installation The system should work on any PC with Microsoft Excel 2000 or above. It has not yet been tested on a Apple computer, but it is anticipated that it should work on a Mac. Installation is fairly simple, and only requires the SR250 data sets (MCSET01.DAT to MCSET23.DAT) to be present in the same directory as this spreadsheet. (SR250 is sold by the NRPB - see link below) Macros are used on this spreadsheet for a variety of purposes. Depending on your version of Excel, and macro options, the security level may have to be switched to 'medium' (select 'Tools' -> 'Options' -> 'Security' -> 'Macro Security'), and/or ImPACT added to your trusted Macro sources. This spreadsheet has been checked for macro viruses, and the logic and calculations been tested extensively, however ImPACT accept no responsibility for loss or damage incurred as a result of its use. Worksheets CTDosimetry.xls consists of 12 worksheets Introduction Scan Calculation Paediatric Phantom Scanners MatchData Collimation MonteCarloData Doses DoseCalculations Selections Version Provides an introduction and instructions for use The data entry and results sheet Information on relative doses to adult and paediatric patients Allows interactive selection of the scan range used for dose calculation using a diagram of the phantom used to generate SR250 Provides data on CT scanner models, including CTDI in air and phantom, as well as the scanner matching data Gives data required to perform the scanner matchings in the Scanners worksheet Lists relative CTDI values at different collimations for a range of CT scanners. These values are more useful for multi-slice scanners, as the CTDI can vary considerably over the range of available collimations Contains the unformatted SR250 data set. Contains the formatted dose data from the SR250 data set that is currently loaded. Performs the organ dose calculations, and calculation of remainder organ doses etc. Provides data for the drop down selection boxes in the ScanCalculation worksheet, and performs calculations for 'remainder' organ doses Details changes made in each version, from version 0.99e onwards 20

21 Using CTDosimetry.xls To calculate doses using CTDosimetry.xls, the user must enter a number of parameters relating to the scanner and the scan series The following four selections, made in the top left box on the ScanCalculations worksheet define the Monte Carlo data set that is used: Manufacturer Scanner k V Scan Region Tube current Rotation time Spiral pitch mas/rotation Effective mas Collimation Select the scanner manufacturer from the drop down list Select the scanner model or scanner model group for the drop down list. Choose the appropriate scan kv. Choose head or body. The Monte Carlo data set that is used for this combination of scanner, kv and body part is displayed in the cell marked 'Data Set'. The data set that is currently loaded is displayed below. If these do not match, no dose is calculated. To load the appropriate data set, and enable dose calculation, press the 'Update Data Set' button. Scan and patient data is entered in the box on the top right of the ScanCalculations worksheet. The x-ray tube current. Note that this should be the actual scanner ma, and not the 'effective mas' displayed on some multi-slice scanners The scanner tube rotation time The scanning pitch (table travel per rotation/total collimated slice width). For axial scanning, (couch increment)/(collimated slice width) should be used The total mas per gantry rotation. Do not enter data in this box - it is calculated automatically. The mas/per rotation divided by the spiral pitch. This is a calculated value that provides a basis for comparison of spiral protocols with different pitches The total nominal x-ray beam width along the z-axis, selected from a range of possible 21 values in the drop down box. This determines the relative CTDI compared to the reference (usually 10 mm) collimaiton.

22 Zoom In Zoom Out 42,

23 I mpact CT Patient Dosimetry Calculator version 0.99m, 1/07/2002 Scanner Model: Acquisition Parameters: Manufacturer: GE ma 340 ma Scanner: GE QX/i, LightSpeed, LightSpeed Plus Rotation time 0.8 s kv: 120 mas / Rotation 272 mas Scan Region: Body Collimation 5 mm Data Set MCSET19 Update Data Set Slice Width 10 mm Current Data MCSET19 Pitch 1.35 Scan range Rel. CTDI Look up 1.26 at selected collimatio Start Position -5 cm Get From Phantom CTDI (air) Look up 34.5 mgy/100mas End Position 45 cm Diagram CTDI (soft tissue) 36.9 mgy/100mas Patient Sex: f nctdi w 12.8 mgy/100mas Organ w T H T w T.H T Remainder Organs H T Gonads Adrenals Bone Marrow (red) Brain Colon Upper Large Intestine Lung Small Intestine Stomach Kidney Bladder Pancreas Breast Spleen Liver Thymus Oesophagus (Thymus) Uterus Thyroid Muscle Skin Bone Surface CTDI w (mgy) 34.7 Remainder CDTI v ol (mgy) 25.7 Remainder DLP (mgy.cm) Total Effective Dose (msv)

24 Calculate 2. Scan Range Scan Range Data (Slice Positions) 1. Age Group Gender Get Values Scan Range z L from z- to z+ [cm] Adult male female 0 3. Scanner Model Scanner Data for Scan Region "Body" Manufacturer Demo nctdi w U ref P B,H k CT k OB L Scanner Spiral Scanner [mgy/mas] [kv] [cm] 0, ,42 0,80 1,00 0,0 4. Select mode Body mode for head/neck region Spiral mode -0,0070 3,35 0,00 0,00 5. Scan Parameters Please Enter Actual Settings: U I t Q el Q N * h col TF h rec p Ser. [kv] [ma] [s] [mas] [mas] [mm] [mm] [mm] 0 1,0 6. Results Dose Values per Scan or per Series* Tissue or H T per Series Remainder H T per Series CTDI w CTDI vol DLP w * E * D uterus * Organ [msv] Organs [msv] [mgy] [mgy] [mgy*cm] [msv] [msv] Thyroid 0,0 Brain 0,0 0,0 0,0 hrec? hrec? n.a. Breasts 0,0 Thymus 0,0 Child/Baby: all CTDI and DLP values refer to 16cmhead phantom! Oesophagus 0,0 Spleen 0,0 Lungs 0,0 Pancreas 0,0 Dose Values per Examination Liver 0,0 Adrenals 0,0 DLP w E D uterus Stomach 0,0 Kidneys 0,0 [mgy*cm] [msv] [msv] Colon 0,0 Small intest. 0,0 Ser? Ser? n.a. Testicles 0,0 Upp. large int. 0,0 Effective dose E refers to ICRP 60 Ovaries 0,0 Uterus 0,0 Please note: Bladder 0,0 All organ doses H T are based on conversion coefficients for stand- Bone marrow 0,0 Misc. H T per Series ard patients (ADAM, EVA, CHILD, BABY) and serve for information Bone surface 0,0 24 [msv] purposes only (in particular organs outside the scan range)! Skin 0,0 Eye lenses 0,0

25 Dose Reference Level (DRL) values are reported by various national or international organizations for abdomen CT DRL CTDI DLP Abdomen EUR SC Abdomen EC MDCT Abdomen UK SC Abdomen UK MDCT Abdomen Germany MDCT Abdomen IAEA Abdomen USA NEXT Survey 30* 740 * Data from USA are not DRL but 75th percent of data distribution. They are put in this table for comparison purposes. 25

26 Table 1. Effective dose in various exams. Examination Effective dose (msv) Abdomen radiography 0.7 Chest or Abdomen CT phase CT liver study 15.0 CT guided biopsy 23.0 CT guided RF ablation 35.0 Repeated CT guided RF ablation CT coronary angiography 10.0 Coronary angiography 8 Thallium heart scan 35 CT urography 14.8 CT colonography 5.1 Neonatal abdomen CT year old abdomen CT 11.1 Annual natural background radiation

27 Thank you for your attention 27