Hi, Dr. Zoomie! I have radiation dosimetry to measure my dose. But I hear radiation safety folks talk about “dosimetry” as being a bunch of calculations. What gives? Can you do dosimetry without having dosimeters?
The National Council on Radiation Protection and Measurements (NCRP) makes an odd claim in their report on Guidance for Emergency Response Dosimetry – something to the effect that dosimeters are not required to perform dosimetry. Which seems like an odd statement because I wear my dosimetry every day that I work with my radioactive sources. If I don’t have my dosimeter, how in the world can I do dosimetry? I’ve got a lot of respect for the NCRP – but this seems strange. And it turns out NCRP addresses this as well…and I should’ve guessed the answer.
NCRP points out that there’s the field of biodosimetry – using the body’s response to radiation to help determine how much dose a person received. Biodosimetry can include drawing a blood sample to look for chromosomal damage caused by radiation exposure, tracking the drop in white blood cells (called lymphocyte depletion) following radiation exposure, pulling teeth to study the properties of electrons in the tooth enamel, the length of time it takes a person to begin vomiting, and more. According to NCRP “Dosimetry is defined as the science or technique of determining radiation dose. Strictly speaking, involving measured quantities, but also used informally to mean “dose assessment,” (i.e. involving measurements and/or theoretical calculations).” What this means is that “dosimetry” is a process – the process of determining how much radiation a person (or a part of a person) was exposed to. This process will usually involve the use of dosimeters, but they’re not necessary. Here are a few examples that might be of interest.
Case #1: A health physicist I worked with at a university a few decades ago had a badge reading of over 50 rem one month and we needed to find out if he really received that high a dose or if it was erroneous. We had to do some dosimetry in order to check on the dose recorded by his dosimeter. We used a few different methods:
- We sent a blood sample to a lab to analyze it for chromosomal damage indicative of high levels of radiation exposure,
- The blood sample was also analyzed for blood cell counts to look for a drop consistent with exposure to high levels of radiation,
- And we traced his movements based on the work he’d recorded doing for the month, measuring the dose rates and calculating his exposure.
None of these showed any evidence of high radiation exposure and his dose record was adjusted accordingly.
Case #2: A thyroid cancer patient needed to be given some I-131, which would be absorbed by her thyroid, delivering enough radiation dose to destroy the cancerous gland. The nuclear pharmacist carefully measured out the right amount of I-131 to deliver the dose prescribed by the physician.
In this particular case the dose was taken orally in the form of a capsule and the patient vomited onto the floor, including some of the I-131. The physician needed to know how much I-131 was in the vomit so he could calculate how much less I-131 was administered than was prescribed. By measuring radiation levels from the vomit we were able to calculate this and the nuclear pharmacist was able to make up the “missed” dose to make sure the patient’s cancer was properly treated.
Case #3: A decontamination technician accidentally contaminated his skin with tritiated water, which enters the body easily. We needed to find out how much tritium had entered the body and the dose to the technician. Since tritium emits a low-energy beta the only way to measure the amount of tritium in the body was to take a urine sample and to measure the amount of tritium in the urine. With this information we were able to calculate the tech’s radiation exposure from the tritium intake.
Case #4: I was working to develop a technique for calculating a person’s radiation exposure from breathing in airborne radioactivity. We were able to develop a procedure for putting a Geiger probe on a person’s upper back to count the radioactivity in their lungs and then to convert the count rate into a radiation dose from the inhaled radioactivity.
Case #5: The medical physicists at a hospital I worked at would calculate exactly how much time a patient needed to be exposed to the beam from a linear accelerator to deliver the dose to destroy a tumor.
These are all examples of dosimetry and none of them involve using dosimeters. And there are more as well. What they all have in common is that all are ways to figure out how much radiation a person received. They are all different ways to do dosimetry – and none of them involve using dosimeters.