Answer to Question #8895 Submitted to "Ask the Experts"

My question has to do with the use of an ASM 990 digital survey meter with a 489-110D GM pancake probe. The energy response of the 489-110D probe at the ¹²⁵I is expressed as ratio of 3.5 and 4, indicated over Actual. Meter/probe combination is calibrated by our health physics consultant at ¹³⁷Cs where the probe's energy response is one. Is it appropriate when surveying an ¹²⁵I implant patient for exposure rate at one meter to divide meter readings by 4 for reporting purposes?

Your rationale for estimating the exposure rate or dose rate at one meter is generally correct. I reviewed the energy response characteristics for the 489-110D GM probe at this Fluke website. There are a couple of points that you should probably note.

The first is that the energy hash mark shown for ¹²⁵I on the energy response curve is at about 35 keV, at which point the Indicated/Actual value is shown as about 3.5. Iodine-125 emits some gamma photons at 35.5 keV, but most of the detectable photons are x rays with an average energy of about 28 keV. The weighted average energy value for all the measurable photons is 28.4 keV. It would probably be more appropriate, then, to use the Indicated/True value at this energy; this value appears to be about 2.8. Because some photon scattering will occur inside the patient, there may be some reduced-energy photons that contribute to the reading at one meter from the patient, and the Indicated/True value could be a bit lower, but at one meter from the patient the scatter contribution is reduced compared to measurements made much closer to the patient.

The second consideration refers to what quantity is being measured with the instrument. As you know, the ASM 990 instrument may display exposure units (e.g., mR h^-1) or equivalent dose units (mrem h^-1 or mSv h^-1). I suspect the energy dependence curve at the site above refers to exposure measurements. If this is the case, it would be appropriate to divide the exposure rate reading from ¹²⁵I by 2.8 to estimate the actual exposure rate at the measurement point. The conversion to another dose quantity, such as effective dose, would require multiplying the corrected exposure rate by another factor that reflects the effective dose per unit exposure (this factor would be approximately 0.3 mSvR^-1 at ¹²⁵I energy for a front-to-back exposure, based on Fig. B.16 that appears in ICRP Report  43, "Determination of Dose Equivalents from External Radiation Sources – Part 2." A similar value may be interpreted from data in Table A.17 of ICRP Report 51). At the ¹³⁷Cs energy (662 keV), the effective dose per unit exposure factor would be about three times larger than the ¹²⁵I value.

I hope this addresses your question.

George Chabot, PhD, CHP