Fetal Dose Calculations and Impact on Patient Care

Transcription

1 Fetal Dose Calculations and Impact on Patient Care Matt Hough, MS, DABR, DABMP Florida Hospital Diagnostic Medical Physics and Radiation Safety

2 Resource ACR-SPR Practice Parameter for Imaging Pregnant or Potentially Pregnant Adolescents and Women with Ionizing Radiation (Revised 2013) Includes Overall guidance Appendices with detailed Risks Sample Policy Sample Patient History Form Sample Consent

3 Estimated Fetal Dose Ranges for Diagnostic Procedures All non-abdominal imaging Negligible scatter dose to fetus Abdominal Primary Exposure General X-ray and Fluoro Well below 20 mgy CT (Single Phase) Usually less than 35 mgy (15-20 mgy) CT (Multi Phase) Multiple of single phase doses Fluoroscopically Guided Interventional procedure Can be well above 100 mgy

4 ACR-SPR Practice Parameter for Imaging Pregnant 2013

5 Prospective Scenario Screening and Pregnancy Testing Only for high risk procedures? What if you have a negative test? What if you have a positive test? Avoid multi-phase CT and FGI if possible Is there an alternate procedure? ACR Appropriateness Criteria If procedure must be done Radiologist should Consult a Qualified Medical Physicist for dose estimate Counsel and Consent Patient

6 Retrospective Scenario Pregnancy discovered during/after procedure Radiologist should Consult a Qualified Medical Physicist for dose estimate Counsel Patient s ordering physician? Counsel Patient

7 Qualified Medical Physicist Role QMP Dose Estimation Where do we start? What options do we have? Communicating risk associated with that Dose estimate How do we communicate our estimate to the Radiologist? Do we ever speak directly to patients?

8 CTDI is Computed Tomography Dose Index A standard parameter used to measure Scanner Radiation Output in a reproducible manner There are different types CTDI 100, CTDI W, CTDI VOL There are different sizes of CTDI phantoms Adult Body: 32 cm diameter Adult Head, Ped Body, Ped Head: 16 cm diameter CTDI VOL is the one dose metric required to be reported by scanner

9 What affects Scanner Radiation Output? CTDI VOL Avg = mgy CTDI VOL Min = mgy CTDI VOL Max = mgy

10 CTDI is not Actual Patient Dose!

11 How do we estimate Organ and Fetal Doses in CT? Different approaches to the same problem Direct Measurement Ion Chamber in Rando Phantom (Felmlee Method) Monte Carlo Simulation Voxelized Patient Phantom (Angel Method) Stylized Mathematical Phantom (Radimetrics one vendor) Other methods available

12 CT Fetal Dose Estimation - Felmlee Methodology Felmlee, et Al Estimated Fetal Radiation Dose AJR 154: , January 1990

13 CT Fetal Dose Estimation Important Notes Scanners from 1980s - Felmlee Methodology Data is averaged over multiple scan parameters (e.g. kvp) Phantom based Overestimate for large patient, underestimate for small patient What CTDI? Not scanner reported CTDI vol CTDI 100 for 16 cm phantom Have to back-calculate based on scanner specific measurements Can be TCM corrected manually Can be pitch corrected

14 CT Fetal Dose Estimation - Angel Methodology Angel, et Al Radiation Dose to Fetus Radiology Vol 249 Number 1 October 2008

15 CT Fetal Dose Estimation Important Notes - Angel Methodology Monte Carlo based calculation GE LightSpeed 16 Source Model 120 kvp only Patient based but not patient specific? Dose estimate based on Mother s Perimeter and Fetal Depth as modeled from study s sample Provides Fetal Dose per 100 mas Can be TCM corrected manually Can be Pitch corrected manually

16 CT Fetal Dose Estimation - Radimetrics Methodology

17 CT Fetal Dose Estimation - Radimetrics Methodology Important Notes Monte Carlo based calculation Stylized mathematical model of patient anatomy Scan based but not patient specific Provides Uterus and Fetal Dose for 1 st, 2 nd, and 3 rd trimester patient/fetal models Dose is scaled based on slice specific CTDI VOL Automatically TCM corrected Automatically Pitch corrected Automatically kvp corrected

18 First look data

19 Correcting for model limitations

20 What does this show? QMP must understand model limitations and how to correct for them When accounting for model limitations, there seems to be good agreement Radimetrics can be used as a surrogate for other methods to quickly ascertain a fetal dose estimate

21 How do we communicate risk Know your audience Radiologist (knows something about radiation and dose) Ordering Physician (probably knows very little) Patient (probably knows very little and is very scared) Remain in a positive tone Compare estimated dose to threshold for deterministic effects based on fetal age Note potential for stochastic effects

22 Conclusions Institutions should have detailed Policies ACR-SPR is a great resource Benefit versus Risk should always be taken into consideration The majority of diagnostic imaging procedures should not pose deterministic risks If there is a high risk procedure, QMP involvement is important Account for any limitations in Dose estimations Risk should be communicated with a positive tone