Hard beta and gamma emissions of 124I Impact on occupational dose in PET/CT

Aim: The hard beta and gamma radiation of I-124 can cause high doses to PET/CT workers. In this study we tried to quantify this occupational exposure and to optimize radioprotection. Methods: Thin MCP-Ns thermoluminescent dosimeters suitable for measuring beta and gamma radiation were used for extremity dosimetry, active personal dosimeters for whole-body dosimetry. Extremity doses were determined during dispensing of I-124 and oral administration of the activity to the patient, the body dose during all phases of the PET/CT procedure. In addition, dose rates of vials and syringes as used in clinical practice were measured. The procedure for dispensing I-124 was optimized using newly developed shielding. Results: Skin dose rates up to 100 mSv/min were measured when in contact with the manufacturer's vial containing 370 MBq of I-124. For an unshielded 5 ml syringe the positron skin dose was about seven times the gamma dose. Before optimization of the preparation of I-124, using an already reasonably safe technique, the highest mean skin dose caused by handling 370 MBq was 1.9 mSv (max. 4.4 mSv). After optimization the skin dose was below 0.2 mSv. Conclusion: The highly energetic positrons emitted by I-124 can cause high skin doses if radioprotection is poor. Under optimized conditions occupational doses are acceptable. Education of workers is of paramount importance.