Lens of Eye Dose Assessment for Nuclear Medicine and PET Technologists

Background

In 2021, the Canadian Nuclear Safety Commission (CNSC) adjusted the equivalent dose limit for the lens of the eye to 50 mSv in a one-year period based on a recommendation made by the International Commission on Radiological Protection (ICRP). This is a significant reduction from the previous limit of 150 mSv in a year. The CNSC has suggested a review of workplace hazards to determine if additional safety practices need to be implemented to protect the lens of the eye. Situations that pose an increased risk were outlined by the CNSC, such as those subject to non-uniform exposures to the eye and those exposed to weakly penetrating radiation. Non-uniform fields can occur in situations when the trunk of the body is shielded but not the eyes and when head is closer to the source than the body is. For example, when viewing or preparing a syringe containing medical isotopes for nuclear medicine (NM) and PET procedures. The goal is to examine exposure to the lens of the eye and compare to doses recorded by whole-body (WB) dosimeters.

Methods

A total of 8 workers in NM and PET were assigned Mirion Type 27 (LiF:Mg, Ti TLD100 chip) eye dosimeters to wear for approximately 2 quarters (6 months). These were worn with previously assigned personal WB and ring extremity TLDs. Staff were selected based on having the highest WB equivalent doses from CNSC licensed activities. The dosimeters were worn near the eye on the front of the face. Workers were provided safety glasses with eye dosimeters attached or with a secondary option of securing the dosimeters to their medical masks at approximately equal distance from the lens of the eye. It is worth noting that the staff rotate through PET and NM locations. In the first quarter, a single badge per wearer was assigned for both locations. In the second quarter badges were assigned per location (2 per wearer) and dose results summed. The number of procedures performed by the workers was collected and considered.

Results

Theoretical doses were conservatively calculated to include exposure from WB dosimeters. This was considered extra dose incurred per quarter from non-uniform fields (when head closer to the source than torso WB dosimeter, at 50 cm distance). It was assumed that each staff may spend a maximum of 10 seconds at a 10 cm source to eye distance per procedure. This occurs when transferring stock upon receipt to the L block, leaning around the L-block for better view when drawing up a dose, or brief exposure during patient dose administration via IV line. Staff approximately 5 foot in height or shorter noted issues being able to view activity being drawn up in syringe through the L-block and have utilized a stool. Similarly, tall staff are potentially exposed when viewing over the L-block. However, 10 seconds of preferentially unshielded eye exposure is a conservative estimate for each procedure performed and shielding factors other than the syringe/vial are not considered with these calculations.

Mirion eye TLD results for part of Q1 and Q2 2024 confirmed the estimates for the staff monitored. The 3 highest quarterly exposures were 2.48 mSv, 2.15 mSv, and 1.97 mSv. Conservatively extrapolating to 4 quarters, dose results for the lens of the eye are slightly below 10 mSv annually and well within the 50 mSv/year lens of eye dose limit. When comparing 2024 Q2 eye TLD results total and 2024 Q2 WB TLD results total (mSv), one can see that WB results align well with lens of eye dose results. For wearers 001, 002, 003, 004, and 008 the difference between WB and eye dose readings only ranges from 0.07- 0.64 mSv. Wearers 005, 006, and 007 results are inconclusive, as 2 PET and 1 NM eye TLDs were lost or not processed. For those not involved with PET isotopes, there is a lower dose risk. This is evident in the 2024 Q2 eye TLD results, as the highest exposed individuals had majority of their dose coming from PET location contribution (2.08 mSv, 1.59 mSv, and 1.72 mSv, respectively).

Conclusion

Based on literature review along with the calculated estimates, the NM and PET staff are not expected to exceed the CNSC dose limit of 50mSv/year. It is worth noting the additional finding of PET procedures contributing a higher exposure to the eye. Since measured and calculated doses align for both eye and WB, it can be recommended that WB dosimeters are representative of the lens of eye dose as well.