Estimation of Derived Air Concentration of I-131 in a High Throughput High-Dose Radioiodine Therapy Facility

Objectives: Iodine-131 (I-131) is a widely used radionuclide in nuclear medicine for treating hyperthyroidism and thyroid cancer, but its volatility as elemental iodine poses inhalation risks during handling, especially in high-dose units. The International Commission on Radiological Protection (ICRP) provides Derived Air Concentration (DAC) limits to guide monitoring, with an inhalation Annual Limit of Intake (ALI) as per ICRP Publications. This study details I-131 volatilisation chemistry and stabilisation via excipients (e.g., ethylenediaminetetraacetic acid [EDTA], sodium thiosulfate) and pH control (7.5–9.0), estimates DAC, and assesses airborne concentrations during routine high-throughput operations (5–10 patients/week, total 37–74 GBq liquid I-131 per session). The novelty of this work lies in providing practical, chemistry-driven strategies for preventing airborne contamination and optimising sampling protocols while maintaining exposure levels well below DAC limits (<15%), supported by a detailed discussion of the physical characteristics of I-131 along with its historical evolution and development of its radiological protection framework. Material and Methods: Airborne I-131 was sampled over four consecutive weeks in a month and then on a random occasion as routine mandatory radiation protection monitoring in a 20 m 2 area dose-administration room at a high-dose therapy unit (one patient at a time), using a Staplex® high-volume sampler with potassium iodide/triethylene diamine-impregnated activated charcoal cartridges (flow: 50–100 L/min; volumes: 1.55–18.3 m 3 ). Activities were measured via a Geiger–Müller counter (Nucleonix GC-602A), with efficiency (0.21%) calibrated using an I-131 standard in replicated cartridge geometry (background-subtracted, ±5% accuracy). Weekly workloads included three to six high-dose therapies (total 24,825–29,346 MBq) plus 3–13 whole-body doses (~74 MBq), mostly liquid (87% high-dose administrations). Results: Measured concentrations of airborne I-131 were 64 Bq/m 3 , 74 Bq/m 3 , 33 Bq/m 3 , 61 Bq/m 3 , and 65 Bq/m 3 , which is well below the prescribed DAC limit, with the maximum at 13.8% and minimum at 6.1% of the DAC limit. Lower uptake occurred during the week where ~50% of I-131 was used in capsule form (week 3), reflecting reduced volatilisation versus liquid. Conclusion: Engineering controls (fume hood with high-efficiency particulate air [HEPA]/charcoal filtration) kept airborne I-131 below DAC limit, validating safety in high-volume therapy. Routine monitoring remains essential to detect transients from exhalation or preparation, ensuring staff protection.