Nonmyeloablative iodine-131 anti-B1 radioimmunotherapy as outpatient therapy

The expected effective dose equivalent to an individual from contact with ¹³¹l anti-B1 radioimmunotherapy (RIT) patients released immediately after therapeutic infusion was estimated. Methods: Effective dose equivalents were calculated retrospectively using data acquired on 46 patients treated with ¹³¹l anti-B1 RIT as inpatients. Effective dose equivalents to members of the public were estimated using the method published in the Nuclear Regulatory Commission (NRC) Regulatory Guide 8.39, assuming the administered activity, the patient-specific effective half-life, the 0.25 occupancy factor, and no photon attenuation. Effective dose equivalents also were estimated using ionization chamber dose rates, measured immediately after therapeutic infusion and integrated to total decay based on the measured effective half-life. Results: For the whole-body treatment absorbed dose limit of 75 cGy (75 rad), the administered ¹³¹l activity ranged from 2.1 to 6.5 GBq (56 to 175 mCi), and the measured dose rate at 1 m ranged from 70 to 190 μSv/hr (7 to 19 mrem/hr). The total-body effective hafl-life for these patients ranged from ~40 to 88 hr. Using the NRC method and not accounting for the attenuation of photons, the mean dose equivalent to the public exposed to an ¹³¹l anti-B1 patient discharged without hospitalization was 4.9 ± 0.9 mSv (490 ± 90 mrem). The range was 3.2-6.6 mSv (320 to 660 mrem), where 48% of patients would deliver a dose to another individual that is <5 mSv (500 mrem) (i.e., 48% of the patients would be allowed to return home immediately following the infusion). Using the measured dose rate method, the mean dose equivalent to an individual exposed to the same RIT patients was 2.9 ± 0.4 mSv (290 ± 40 mrem). The range was 2.0-3.7 mSv (200-370 mrem), where 100% of the estimated effective dose equivalents were <5 mSv (500 mrem). Conclusion: Based on calculated and patientspecific exposure data, outpatient RIT with nonmyeloablative doses of ¹³¹l should be feasible for all patients under current NRC regulations. Implementing outpatient RIT should make the therapy more widely available and more convenient and should lower patient care costs without exceeding accepted limits for public exposure to radiation.