Answer to Question #375 Submitted to "Ask the Experts"

1. Do we need a phantom to measure dose in mammography?

2. How to measure personnel doses from ¹⁴C and ³H?

3. How to know the dose to the fetus from EDE to pregnant mother radiation worker personal dose?

Do we need a phantom to measure dose in mammography?
If you are taking the measurement for compliance with regulatory standards, yes, you need a phantom. The Food and Drug Administration's Mammography Quality Standards Act specifies its dose limitation based on exposure measured in a 4.2 cm phantom with 50 percent glandular tissue content. The dose limit, by the way, is 300 mrem per single view. Some states have lower limits.

How to measure personnel doses from ¹⁴C and ³H?
¹⁴C and ³H emit low-energy betas that cannot be measured using standard personnel dosimeters (film badges, TLDs, etc.). This really is not a problem since they are not an external exposure hazard. They can, however, be internal exposure hazards. A urinalysis will indicate whether there was an uptake of these radionuclides (or actually, an uptake of any radionuclide). This consists of obtaining a urine sample and counting 1-10 ml on a calibrated liquid scintillation counter. You might want to check your radionuclide license to determine what your institution has told the Nuclear Regulatory Commission (NRC) or your state regulators. Your license may state that you will monitor everyone who works with any quantity of these materials, in which case a bioassay program needs to be implemented where urine samples are obtained from workers on a regular basis. But the license might state that only those who work with quantities greater than the annual limit on intake (ALI) will be monitored. The ALI for ¹⁴C is 2 mCi and for ³H is 80 mCi. If your institution never receives this amount of activity or doesn't allow labs to use more than these quantities, then a bioassay program might not be required. Even if it is not required, it is useful to be prepared to measure internal uptakes if an accidental ingestion or inhalation of beta emitters is thought to have occurred.

How to know the dose to the fetus from EDE to pregnant mother radiation worker personal dose?
Hopefully I've interpreted this question correctly for my answer. I've taken the question to be "If I'm a pregnant radiation worker, how do I figure out the dose to my baby from by radiation badge reading?" If you are exposed to x rays and wear your badge at the collar outside a lead apron, you could simply take that reading and multiply it by 0.05 for a 0.25 mm lead equivalent apron or multiply it by 0.025 for a 0.5 mm lead equivalent apron. The 0.05 and 0.025 represent the fraction of the x-ray field that passes through the lead apron, e.g., a 0.5 mm lead apron stops about 97.5 percent of the x rays so 2.5 percent get through. You can determine whether your apron is 0.25 mm or 0.5 mm lead equivalent by checking the tag.

So, let's say you work in fluoroscopy and your monthly badge reading averages 320 mrem. If you are wearing a 0.25 mm lead apron, this equates to an under-apron abdominal dose of 16 mrem (320 mrem x 0.05). For a 0.5 mm lead apron, it equates to 8 mrem (320 x 0.025). You could also take into account that your abdominal tissue protects the baby too, so the real dose would be lower—possibly half the dose to the abdomen surface. But I think the general calculation gives us a pretty good idea of fetal dose. If you work with gamma-emitting radionuclides, e.g., in nuclear medicine or a research laboratory, and you wear your badge on your torso, then the dose to the baby will be similar to the badge reading. Again, we are not accounting for abdominal tissue shielding, which has a tremendous dose-lowering effect for low-energy gamma emitters (¹²⁵I, ¹⁰³Pd, even ^99mTc). If you work with beta-emitting radionuclides (³H, ¹⁴C, ³²P, ³⁵S), the material would have to be taken internally (ingestion, inhalation, or absorption through the skin) for the baby to be exposed. In this case a urinalysis would need to be performed to calculate fetal dose. A reminder that the NRC dose limit to the fetus is 500 mrem during the gestation period. You might want to look at the Commission's Regulatory Guide - 8.13 Instruction Concerning Prenatal Radiation Exposure Web page.

Kelly Classic
Certified Medical Health Physicist