Answer to Question #3843 Submitted to "Ask the Experts"

Our PET (positron emission tomography) department is currently a mobile unit. We are starting to use a PET/CT (computerized tomography) scanner and have increased our patient load from 14 a day to 30 a day. They also want to increase the dose from 15mCi to 20mCi (¹⁸F FDG [fluoro-D-glucose]). Our badges are already reading higher than recommended and nobody seems to be bothered by that fact and that this will continue to grow due to patient load and the increase in activity. There will be two additional techs for a total of five now. Our RSO (radiation safety officer) doesn't have any other readings to compare our badges with, so we want to know how much is too much. My current badge readings are 365 mrem, DDE (deep dose equivalent); 346 SDE (shallow dose equivalent); and 340 mrem hand. I always use my syringe shields and try to stay away from the patient as much as possible.

I appreciate your concerns and understand that you are anxious about the impact of the proposed increase in FDG dose and the higher patient volumes. Let’s analyze the current situation and the proposed scheme. Your concerns may be valid and warrant further attention by management, or they maybe alleviated by a review and explanation of the exposures and expected changes.

Let’s discuss your current "badge readings" of 365 mrem DDE, 346 SDE, and 340 mrem hand. I do not know what wear period these readings correspond to. For this discussion, I will assume a wear period of one month. For further understanding, let me pause to explain what these terms listed in your dosimeter report mean. "DDE" is the "deep dose equivalent," the amount of radiation energy that was able to penetrate your skin barrier and is assumed to be absorbed by deep-seated tissue or organs (located within the trunk of the body) (Shapiro 1990). "SDE" is the "shallow dose equivalent," the dose to the outer layer of skin; unable to penetrate the skin. "Hand" dose reflects the dose to your extremities that are in closer contact with the source (FDG, patients) and which are not shielded by lead aprons. The reason that your dose report contains these three different categories is because each has different regulatory limits for exposure (based upon the degree of sensitivity for radiation injury).

Federal and state regulations (10 CFR 20.1201 a-c), and if such oversight is not required, most experts in the field recommend that staff exposures to ionizing radiation are managed such that they are less than:

* 5,000 mrem per year for DDE

* 50,000 mrem per year for SDE or the extremities.

Your current exposure of 365 mrem per month for DDE results in 4,380 mrem per year. The SDE of 346 mrem per month equates to a yearly exposure of 4,152 mrem per year. Hence, your current exposures are within regulatory and recommended dose limits. Are they "as low as reasonably achievable" (ALARA)? This question would need to be answered by the hospital RSO or his/her designee. There have been several recent articles in radiation protection journals that explore technician doses from patients and offer suggestions for reductions (search the links of the Health Physics Society and the Society of Nuclear Medicine). It is possible that additional precautions, changes in handling procedures or patient flow, could achieve a reduction in staff exposure.

In terms of workload, currently 14 patients are scanned per day by three technicians. In a span of five days (one work week), 70 patients are scanned, for an average of about 23-24 patients per technician. Under the proposed scheme of 30 patients per day, 150 patients will be scanned in one workweek by five technicians, for an average of 50 patients per day. Hence, the current workload per technician will essentially double.

A doubling of the workload for any employee can be a daunting prospect and raises concerns beyond simply exposure to staff. It is hard to predict if this doubling of patient load will yield a direct corresponding doubling of staff exposure. It does seem prudent that management would want to review this proposed patient volume increase, model the anticipated incremental staff exposure (based on current operating parameters), and closely monitor the changes. Certainly, management would need to address any situation in which the staff exposures approach the regulatory limit or recommended guidelines.

In terms of the proposed increase in FDG dose, from 15 mCi to 20 mCi, it could be that the existing syringe shielding and other protective equipment and barriers will effectively handle this increase in activity. It may also impact patient scanning times and modify staff exposure. This is difficult to predict and should be monitored in a few index cases by the RSO and his/her designee. I also recommend for your further reading the Benatar et al. reference below on some experiences of others in this area.

Lisa L. Coronado
Jamey West, MD

Reference
Shapiro J. Radiation protection. 3rd ed. 1990: 237-238, 272.

Benatar NA, Cronin BF, O’Doherty MJ. Radiation dose rates from patients undergoing PET: Implications for technologists and waiting areas. Eur J Nucl Med 27:583-589; 2000.