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Radiation Exposure and Potential

Risks to the Conceptus -

Radiation Exposure and Potential

Risks to the Conceptus -

Louis K. Wagner, Ph.D.

Department of Diagnostic and Interventional Imaging

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Note: This presentation will focus solely on methods of estimating absorbed dose and potential risks to a conceptus from diagnostic radiations.

While doses from radiation therapy are very important, such circumstances are unusual and have to be planned with care. This presentation will not address these issues.

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Resources on Subject

Conflict of interest statement: If enough of you buy this book, I might be able to take my wife out to dinner once or twice this year.

Expect revisions this year

2012

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What am I really trying to do?

When assessing dose to a pregnant patient, the real goal is to assess radiation

risk to the conceptus for the purpose of influencing medical care.

The perspective is different for pre-examination assessment versus post-

examination assessment but the calculational methods are the same.

Pre- versus Post- Assessment

• Pre-– Goal is to ensure radiation dose will be

acceptably small should the patient be pregnant.

• Post-– Goal is to determine what actually happened.

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Risks and gestation age

Mechanisms for effects:

Mis-repaired sub-cellular changes – stochastic in nature, include cancer and heritable genetic effects.

Cell death – detectable effects due to cell death have a practical threshold below which the effect is not observed, include all forms of malformation.

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From: Russell and Russell. J Cell Physiol Suppl 1 43: 103, 1954

From: Brent and Gorson. Curr Probl Radiol 2: 1, 1972

Image withheldCourtesy: Wm J Schull

9From ACR Guidelines – applies ONLY to X rays!

Potential Risks at Doses under 200 mGy

Risk Probable single-dose threshold (mGy)

Vulnerable Postconception age

Cancer ~ 0 All stages??

Early Termination ~100 <2wks (postconception)

Malformation ~100 >2, <9 wk

Cretinism I-131 After thyroid function commences ~ 8 wk

Small head size > 50 >2, <16 wk

IQ deficit ~100 >7, <16 wk

Severe mental retardation

~150 >7, <16 wk

Hypothesis:

Doses from dental x-rays during pregnancy deliver doses to the thyroid, hypothalamus, or pituitary gland sufficient to alter function and result in low-birth-weight infants.

Doses were ~1.2 mGy to thyroid.

2004

Epidemiology is simply not capable of detecting radiation effects following such trivial doses

Literature review misleading with little if any relevance to low-dose exposure of adult thyroid gland High doses in childhood damage the infantile uterus – this is

cause of lbw spinal x-rays and low birth weight children are most likely due

to effects of and severity of scoliosis, not radiation A-bomb survivor studies do not support their findings association between radiation exposure and autoimmune

thyroid disease is far from established

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periodontal disease has been linked to preterm births and low birth weight

Offenbacher S, Katz V, Fertik G, Collins J, Boyd D, Maynor G, McKaig R and Beck J 1996 Periodontal infection as a possible risk factor for preterm low birth weight J. Periodontol. 67 (Suppl) 1103–13JeffcoatMK, GeursNC, ReddyMS, GoldenbergRL and HauthJC 2001 Current evidence regarding periodontal disease as a risk factor in preterm birth Ann. Periodontol. 6 183–8Jeffcoat M K, Geurs N C, Reddy M S, Cliver S P, Goldenerg R L and Hauth J C 2001 Periodontal infection and preterm birth: results of a prospective study J. Am. Dent. Assoc. 132 875–80

Medical literature cited to support their findings involved doses orders of magnitude higher and irradiated the conceptus directly

Hundreds of animal studies results were not referenced and refute their findings

No detailed clinical study of 31 growth retarded babies to determine if maternal disease, genetic or other factors would explain the results.

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What is the magnitude of the risk for induced cancer?

Rough estimate of risk is 1 extra mortality and up to 4 cancer occurrences for every 1000 fetuses exposed to 10 mGy.

Put another way, expose 1000 pregnant patients to single-phase CT of the pelvis and of these about 992 will likely demonstrate no effects of radiation, 8 might develop cancer later in life, 2 might die of it.

Source for this data is BEIR VII data and Wakeford and Little (2003).

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Lou Wagner’s Rules for Management of Dose to a Conceptus

• Every effort should be extended to avoid cumulative doses, especially acute doses, to a conceptus that might exceed 100 mGy.

• Cumulative doses of 50 - 100 mGy are in a very worrisome gray zone for effects and such doses should rarely occur.

• Doses less than 50 mGy are not associated with malformations, but do carry risk of induced neoplasm – meaning: keep benefit/risk AHARA by maintaining high standards on diagnostic value with doses ALARA.

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Dose to Conceptus from Conventional Radiography

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Patient in early pregnancy undergoes AP KUB radiography. The technique was 70 kVp, 35 mAs,112-cm SID, The patient measures 23 cm thick over the uterus and the cassette is 5 cm below the table top. What is the dose to the conceptus? The machine output at 70 kVp is 4.2 mR/mAs at 102 cm and the HVL = 2.9 mm Al at 80 kVp.

Notes:

1)HVL of 2.9 mm Al at 80 kVp is about 2.5 mm Al at 70 kVp. (Went to tables in my book for that piece of data.)2)4.2 mR/mAs is 0.037 mGy (air kerma)/mAs3)We will assume conceptus is 6 cm below surface.

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Output measurements (O) in units of mR/mAs or mGya/mAs at some source-to-reference point distance (SRP) as a function of kVp. For our KUB example, output measured at 102 cm.

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SRPSSE

d

T

SCD

Our KUB:

SCD = 90 cmSRP = 102 cmSSE = 84 cmd = 6 cmT = 23 cm

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SRPSSE

KSE = ORP x (SRP/SSE)2 x mAs

Calculation of entrance air kerma, K

Note: K(mGya) = 8.76 mGya/R x X(R)

Our KUB:KSE = 0.037 mGy/mAs x (102/84)2 x 35 mAs = 1.91 mGy

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Calculation of entrance skin dose, ESD

ESD(mGyt) = KSE(mGya) x f X B(field size, beam quality)

f ~ 1.06 mGyt/mGya

B ~ 1.3 to 1.5 (typically)

Our KUB:

ESD = 1.91 mGy x 1.06 x 1.37 = 2.77 mGy tissue dose.

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Calculation of entrance skin dose, ESD

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Dose to Conceptus from Conventional RadiographyCalculation of conceptus dose, CD

CD(mGyt) = ESD(mGyt) x P(field size, beam quality, d, SCD)/100

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Image withheld

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0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

%

Depth

Dose

Depth in cm

Percent Depth Dose for 4-mm HVL in Al

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If conceptus depth unknown, range of measurement is: 21% to 63%!

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Dose to Conceptus from Conventional RadiographyCalculation of conceptus dose, CD

CD(mGyt) = ESD(mGyt) x P(field size, beam quality, d, SCD)/100

Our KUB:

CD = 2.77 mGy x 36/100 = 1.0 mGy

Image withheld

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SCD (Data Table)

dSCD (Actual)

P (Table) = Pll x ((SCD(Table)-d)/SCD(Table))2

Pll = P (Table) x (SCD(Table) /(SCD(Table)-d))2

P (Actual) = Pll x ((SCD (Actual)-d)/SCD(Actual))2

P (Actual) = P (Table) x (SCD(Table) /(SCD(Table)-d))2 x ((SCD (Actual)-d)/SCD(Actual))2

d

Correction of Tables for Inverse-Square Effect

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50 cm

6 cm80 cm

P (Table) = Pll x ((50-6)/(50))2

Pll = P (Table) x 1.29

P (Actual) = Pll x ((80-6)/80)2

P (Actual) = P (Table) x 1.29 x 0.856 = P(Table) x 1.10

6 cm

Empirical data geometry

Actual patient geometry

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Very dependent on d. The HVD (half-value depth) is about 4 – 5 cm for conventionally filtered beams.Somewhat dependent on HVL for same kVp and field size. (PROBLEM!!!)A little dependent on field size for same kVp and HVL.Not very dependent on kVp for same field size and HVL.

HVL FOV

kVp

No data for heavy filtration

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How to estimate technique factors for AEC

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Lateral:

Use Rosenstein or do depth dose as before

Oblique (not usually a critical factor but can be in unusual cases):

Use previous techniques with estimates of distances, or

Correct AP or PA mGyt/mGya in Rosenstein for conceptus depth. Interpolate between AP (or PA) and lateral normalized uterine doses for a reasonable figure. I typically estimate Oblique as closer to AP or PA than to lateral because of oval geometry of most abdomens. Note that the SED changes as well as the kVp and mAs.

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Dose to Conceptus from Fluoroscopy

Calculation is same as before but the depth of the conceptus is often much deeper than 12 cm, usually ~ 16 cm.

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Scatter dose from projection radiography

d

EFD

CD ~ ESD x PED (EFD, d) x FOV area / 1500 cm2

EFD = edge of field distance

2.5 cm from field edge


10 cm from field edge

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Scatter dose from projection radiography

CD ~ ESD x PED (EFD, d) x FOV area / 1500 cm2

EFD = edge of field distance

Example: Cardiac patient with infection at pacemaker leads. Entrance dose to heart area ~ 2 Gy with EFD of 20 cm, d of 13 cm and FOV area of 325 cm2.

CD ~ 2 Gy x 0.007 x 325/1500 = 0.003 Gy or 3 mGy



10 cm from field edge

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Errors in Estimate of Absorbed Dose

• Technique, especially mAs

• Depth of conceptus

• Undisclosed retakes or lost views

• Bone, air, fat effects

• Body contour

How I handle it:•Report defines the examinations used in the estimate •Calculate best estimate, then give possible upper limit value.

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Re: Patient SL (153 lbs 5'4" tall) who underwent a BE examination in room 2 at Confidential Medical Center on 4 Nov 2011. (Note: technical data regarding radiation output and performance of this equipment is provided in the annual physics report on the machine.) The following is my report on the likely exposure to the conceptus of the above patient resulting from radiographic examinations received at Confidential Clinic in the Confidential Medical Center.The information on the study that was provided to me is as follows:

1. One KUB image on 29 October 2011 On 04 November 2011-2. Three barium AP radiographs3. One AP decubitus4. One PA decubitus5. One AP scout6. One AP bladder7. Two lateral digital spots8. Two oblique digital spots9. Two upper abdomen digital spots10. Five minutes of PA fluoroscopyIf you are aware of any discrepancies in this summary of image acquisitions, please let me know.

Report Example

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I used a barium film technique of 20 mAs at 100 kVp for AP views. I then translated this data into lateral and oblique dose factors. The digital spot estimates are an extracted from the KUB data. I estimated an entrance skin exposure rate of 4R/min for the fluoroscopy.

Based on this, I estimate the dose to be 5.4 rad (54 mGy). Given consideration for error, the dose could have been in the range of 7.2 rad.

If you have any questions regarding this dose estimate please feel free to ask.

Report Example – Page 2

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Absorbed dose to conceptus from CT

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Tutorial on CTDI (computed tomography dose index)

• Intended to be a standard absorbed dose reference.

• There are multiple variations on and definitions of CTDI.

• Take away: The weighted CTDI, CTDIw, when corrected for pitch (CTDIvol)renders a resonable estimate of an adult dose but may be very misleading for small patients.

Example:CTDIvol body: 19 mGyConceptus dose estimate: 23 mGy

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Patient Measures 20 cm thick by 34 cm wide

This phantom is 33-cm in diameter

CTDI at center underestimates dose because:

1. Phantom too big2. Acrylic density is 1.19 gm/cc3. Not enough scatter (Scan length is

only 140 mm)CTDI as originally defined may underestimate dose by factor of 4!

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CT Dose Estimate Techniques

CTDI reference dose model

Dose = Dref x [ F(0) + (T / 10mm) x i F(zi) ]Dose = Measured value x [correction factors]

Technique of Felmlee et al., AJR, 154, 185-190,1990. 44

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0-15 5 10-10 15-5

10-mm Contiguous Scans spanning 31 cm with conceptus in center scan

Dose = Dref x [ Fd(0) + 2 x (T / 10mm) x i=1,15 F(zi) ]

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Reference Dose“CTDI” absorbed dose to center of 16-cm diameter PMMA head phantom measured with a 10-cm CT pencil-type chamber.

Reference absorbed dose in mGyt = 0.93mrad/mR x CT readout in mR x Correction factors x 100mm/nominal beam width in mm

Typical results: 20-mm nominal beam width; 360 mR for 120 kVp, 250 mAs/rot; dosimeter correction factor 1.96 renders 3.28 rad or 32.8 mGyt.

Note: this applies to clinical beam collimations of 20 mm only.

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Dose = Dref x [ F(0) + (Ti / 10mm) x i F(zi) ]

Dose = 32.8 mGyt x [ F(0) + (10mm/ 10mm) x i F(zi) ]Dose = 32.8 mGyt x [ 0.29 + 2 x 0.22] where i = 1 to 15Dose = 23.9 mGyt

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Note: for calculations, contiguous scans or pitch of 1.0 can be simulated as contiguous 10-mm scans.

Example: conceptus in center slice of 15 rotation 20-mm wide beam scan of pitch 1. (Note: essentially same as 31 10-mm wide scan.) Conceptus at 6-cm depth in abdomen.

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Dose = 24 mGyt

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Restrictions:Applies to ~20 cm thick patient and 6-cm deep conceptus, contiguous scans (pitch 1) and 20-mm slices with conceptus in center slice of 15 rotations.

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What if conceptus is located in slice 3 of 31 10-mm contiguous scans?

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Dose = 32.8 mGyt x [ F(0) + (10mm/ 10mm) x 2i=1,2 F(zi) +i=3,28 F(zi) ]

= 32.8 mGy x [0.29 + 0.164 + 0.138 + (0.003)]= 20 mGy

Scatter from these slices calculated independently

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What if conceptus is located 4 cm from first scan of 31 10-mm contiguous scans?

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Dose = 32.8 mGyt x [ (10mm/ 10mm) x i=4,34 F(zi)]= 32.8 mGy x [0.110 + (0.008)]

= 4 mGy

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Correction for pitches different from 1.0:

Dose = Contiguous or pitch-1 dose / pitch

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Example:CTDIvol body (GE): 19 mGyConceptus dose: 23 mGy

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Computed tomography dose:

CT scan of 33-cm solid-water body phantom

CT scan of 33-wk prenancy

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Computed tomography dose:

Notes on dose in late pregnancy:1. An average dose is suitable2. Accuracy is less critical because no deterministic

effects3. Use of CTDIvol is reasonable, but consideration

for additional scatter must be given.

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Auto-dose-control scanning• Based on noise control; technique varies

with body section• MUST know how it is used at your facility –

requires active review of patient data• Study images

– Locate image of the center of conceptus– Note mAs for that image– Note variation in mAs for adjacent images– Note depth of conceptus– Apply modified Felmlee method making

adjustments to scatter factors as mAs varies, if necessary.

• Prospectively – must disable or change auto-dose parameters for low-dose techniques 56

Doses from Radionuclides

Use ICRP tables on conceptus or uterine doses from radionuclides, ICRP Publication # 53

1. Doses in mGy /MBq admininistered2. Be careful about specific fetal organ

doses, i.e., Iodine3. Be careful about selecting the proper

radionuclide4. Be careful about funtionality of organs5. Always use a healthy level of error.

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Doses from Radionuclides Doses from PET-CT in Pelvis

FDG:

• 0.022 mGy/MBq to adult uterus

• 15 mCi = 444 MBq

• Dose to uterus = 12.2 mGy.

CT:

Typical CT dose 20 mGy

Total dose = 32 mGy, max est. < 50 mGy58

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Some “typical” dosesExamination “Typical” Conceptus Dose

(mGyt)

2-view Chest series < 0.01 mGy

4-view Lumbar spine series

5 – 20 mGy

AP Pelvis/PA Pelvis 1 – 3 mGy/ 0.1 – 0.5 mGy

CT – Abdomen/Pelvis 15 – 30 mGy

20 mCi Tc-99m DTPA 5 – 10 mGy60The End

Artistic design: Dan Klepac

The Department of Diagnostic and Interventional Imaging

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