NCRP 122 two-dosimeter formula: 1.5 x waist + 0.04 x collar gives effective dose equivalent

The formula is real, but it’s narrower than older versions of this page claimed. It estimates a different quantity from effective dose, it applies when you wear an apron, and ARRT® doesn’t list it.

What is the formula?

For a badge under the apron at the waist and a badge outside the apron at the collar:

Effective dose equivalent = 1.5 x waist + 0.04 x collar

The NRC credits this to NCRP Report 122 in Regulatory Guide 8.40, which also warns that it overestimates if the apron gives little shielding. We read it there. We didn’t open NCRP 122 itself. The 2010 CIRMS presentation traces the same formula to a 1989 paper by Webster, and says its use in fluoroscopy met criticism for substantial errors.

Example: collar 3.0 mSv, waist 0.1 mSv. That’s (1.5 x 0.1) + (0.04 x 3.0) = 0.15 + 0.12 = 0.27 mSv.

Under-apron readings are usually far lower than collar readings. Sherer reports a difference of about 30 times in one 0.5 mm apron test.

What other methods exist?

Badges	Method	Source
Two	0.5 x waist + 0.025 x collar, for effective dose, only in conditions similar to those it was developed for	CIRMS 2010
Two	0.89 x waist + 0.11 x collar	ANSI/HPS N13.41, per NRC RG 8.40
One, at the collar	0.3 x collar, if the reading exceeds 25% of the limit	NRC RG 8.40
One, at the collar	Collar divided by 5.6	NCRP 122, per CIRMS and HPS
One, at the collar	0.1 x collar	Bushong

These give different answers from the same badge. The NRC says it doesn’t regulate the use of x-ray equipment for medical procedures, and its guide says a worker’s x-ray dose must be considered under 10 CFR Part 20 if that person is also engaged in NRC-licensed activity. If a state has jurisdiction and has approved a method, the NRC guide says it will accept that method. Where no state method applies, NRC licensees may use the guide’s listed methods. Under 10 CFR 20.1201(c), the deep-dose reading otherwise stands in for effective dose unless the NRC has approved a method. NCRP Report 102 says that if two dosimeters are worn, the radiation safety officer needs to adopt a method for combining the two doses, so ask yours which one your facility uses.

An earlier version of this page said the 1.5 shows that torso organs are more sensitive, and that ARRT expects this formula. We found no source for either claim.

Who needs two badges?

We found no general rule that sets this. Sherer says some facilities prefer two for lengthy interventional work. The NRC rule (20.1502) requires monitoring for adults likely to exceed 10% of the limits, and for declared pregnant workers likely to exceed 1 mSv (0.1 rem) during the pregnancy.

For a declared pregnant worker, many facilities add a badge at the waist. The NRC limit is 5 mSv (0.5 rem) to the embryo-fetus for the entire pregnancy. For x-ray work it rests on the deep-dose equivalent, not this formula. The rule also counts any dose from radionuclides.

What should you learn for the exam?

The 2027 outline lists dosimeter types (TLD, OSL, DIS), proper use and dose limits, and the 2022 outline lists types without naming them. For 2027 they cite NCRP Report 180 and 21 CFR, and SI units are primary. Learn those first, starting with the Radiation Protection guide and the Radiation Protection chapter. Know the formula’s meaning in case your program teaches it. Then drill the Radiation Protection practice set.