[Intl_DxMedPhys] Out of Field Fetal Dose Calculations

Tue Jun 3 15:49:12 EDT 2025

Unless we have hundreds of repeats, I don’t think we come close to any fetal dose limits for not contiguous fetal irradiation events in ct, even touching the fetal with edge of SFOV you will need many more than is clinically indicated for any scan we would have in clinic. I would be careful of knowing how the scanner does helical overscanning (not all scanners have hard shutter collimators)

Below I share relevant sections of my book (figures) on this.

-stick

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Timothy P. Szczykutowicz, Ph.D., DABR
Associate Professor
Departments of Radiology, Medical Physics and BME
University of Wisconsin Madison
Cell# 1-716-560-7751<tel:(716)%20560-7751>
Office# 1-608-263-5729
he/him/his

From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu> On Behalf Of Adam Springer via Intl_dxmedphys_wd_osu_list
Sent: Tuesday, June 3, 2025 2:29 PM
To: Rebecca Milman <milman at gmail.com>
Cc: Gretchen Raterman Bell via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu>
Subject: Re: [Intl_DxMedPhys] Out of Field Fetal Dose Calculations

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Hi, Rebecca. I believe the ACR practice parameters for imaging pregnant and potentially pregnant patients does not suggest doing dose estimates outside the field for routine CT, radiography and diagnostic fluoro. And I use that approach to avoid
Hi, Rebecca.

I believe the ACR practice parameters for imaging pregnant and potentially pregnant patients does not suggest doing dose estimates outside the field for routine CT, radiography and diagnostic fluoro. And I use that approach to avoid wasting efforts for those modalities. But fetal doses outside the beam can still be concerning in interventional fluoro, depending on the procedure. I did one estimate for an ESAK of 8,000 mGy. All I knew was it was outside the field, no idea how far. I estimated well over 100 mSv with the factor Gretchen mentioned below. I believe it was third-trimester, so still not a risk of birth defects, but reportable in my state.

Gretchen,

I’m not an expert, but NCRP 174 indicates there is no risk of birth defects below 100 mSv at any stage of pregnancy. That threshold may be much higher. And fetal doses over 50 mSv are reportable in Louisiana. If a conservative estimate is below that, I’m not sure a more accurate estimate is worth the extra effort. I assume the factor 2.5 cm outside the beam is conservative for fetuses larger than 5 cm (~11 weeks.) If it’s smaller, you could evaluate whether the dose in the field has risks or is reportable and decide whether more effort would make any difference in the patient’s care or reporting requirements. You could also report the in-field dose as a maximum, i.e. you know the actual dose outside the field was less.

I look forward to learning from others’ perspectives, especially if any of my assumptions are wrong.

~Adam

=========================
Adam C. Springer, MS, DABR®
Medical Physicist
KLS Physics Group, LLC
124 Killgore Rd
Ruston, LA 71270
318-272-8292

On Jun 3, 2025, at 14:01, Rebecca Milman via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>> wrote:

When I was a resident, I was told a story about Lou Wagner getting a phone call once about a fetal dose estimate (either for fluoro or radiography). His first (and I think only) question was whether or not the fetus was in the imaging field
When I was a resident, I was told a story about Lou Wagner getting a phone call once about a fetal dose estimate (either for fluoro or radiography). His first (and I think only) question was whether or not the fetus was in the imaging field of view. (I take no responsibility for the veracity of this story...)

I may be the outlier here, but if the fetus isn't in the imaging field of view, the dose will be lower than where there are any known effects. Fetal dose estimate is important (saying this so people won't accuse me of hating babies...) but whether it's 10 mGy or 0.36 mGy just doesn't matter and won't have any effect on clinical decision-making. It is also worth reporting any fetal dose estimate as a range since it's not known with much accuracy.

Rebecca.

Rebecca Milman, Ph.D.
University of Colorado Anschutz Medical Campus

On Tue, Jun 3, 2025 at 12:06 PM Gary via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>> wrote:
It is not modern, but the Felmlee CT fetal dose paper might be a start.  It gave dose to a point near the conceptus but at n cm outside the scan range.  Your IR procedure would have a pretty different kV, but I would guess that an estimate based on CT would be an upper limit.

From the paper, the dose fraction at 1 cm outside a 10 cm wide scan range is 0.2, while the dose fraction at the center of the same field would be 0.55 so that you could perhaps take the fetal dose to be 0.2 / 0.55 times whatever the dose is at depth in the middle of your IR field.  So if the dose to the tissue in field is 1 mGy, the fetal dose at 1 cm out of field should be pretty close to 0.36 mGy.

---
Thanks,
Gary Isenhower

On Tue, 3 Jun 2025 11:43:05 -0500, Gretchen Raterman Bell via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>> wrote:

>Salutations!
>
>I am currently working on a fetal dose estimation for an IR neph tube
>procedure where the docs did a great job of keeping the fetus out of
>the FOV.  But when looking at a past radiograph with the tubes in
>place (yes, this is a multi-step fetal dose calc), it appears that the
>fetal anatomy would have to be right outside the FOV for the IR
>procedure.  Currently, my only reference would be Wagner, Fig. A-6,
>which caps out at 2.5 cm for the closest approach of the rad field.
>While I do a literature search, I figured I'd ask the gurus here if
>anyone happens to have a more modern paper for out of field fetal dose
>calcs?  One that perhaps addresses ~ 1 cm or less outside FOV?
>
>Thank you, thank you, thank you, to anyone who has said  paper.
>
>Gretchen R. Bell, M.S., DABR
>Diagnostic Imaging Physicist
>Ochsner Medical Center
>(504)842-8506
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