Radiation safety restrictions in the radionuclide treatment of canine osteosarcoma

Objectives Veterinary radionuclide therapy imposes radiation safety burdens that are similar to those of human treatments. The durations of quarantine and of restricted contact after release that would be needed to comply with regulatory dose limits were assessed for Sm-153 complexed with the bone-seeking chelant DOTMP and Y-90 as a permanent interstitial implant, which are being investigated for the treatment of canine osteosarcoma.

Methods The exposure rate at 1 meter from six dogs was measured at 3, 24, 48, 72 and 96 hours after the administration of 1-2 mCi/kg of Sm-153-DOTMP and fitted by a single exponential function. A software phantom consisting of soft tissue irradiated by a bone-like source of Sm-153 or Y-90 at various distances was simulated in Monte Carlo software (GATE, v. 6.2) to estimate the radiation absorbed dose from prolonged contact with a treated patient.

Results The exposure rate from Sm-153-DOTMP, extrapolated back to time zero, was 0.0195 mR/mCi-hr at one meter. The effective half-life was 46.16 hours. By a NUREG 1556 calculation with a 25% occupancy factor at one meter, a dog administered up to 308 mCi need be quarantined for only three hours (for urinary clearance) to impart a cumulative dose below 1 mSv. The estimated surface dose rate at a simulated bone tumor from Sm-153 is 1.154 mGy/mCi-hr. To keep the worst case surface dose to the owner from 4.24 mCi in the tumor below 50 mSv (which is 1/10^th of the annual occupational limit), the pet should sleep alone for three days. The estimated surface dose rate from Y-90 is 33 mGy/mCi-hr and thus the pet should sleep alone for four weeks after a 43 mCi treatment. Alternatively, the pet should sleep alone for three days and then wear a 1 cm thick water-equivalent dressing for four weeks when near people.

Conclusions Radiation safety quarantines that are based on a number of physical half-lives might be overly conservative and unnecessarily burdensome in these situations.

Research Support NCI R44CA150601 and The Gabriel Institute