A Comparison of Er-169 and Sn-117m Radiation Synoviorthesis Using Monte Carlo Simulation

Objectives The distribution of the radiation absorbed dose in the synovial membrane from a new radionuclide for radiation synoviorthesis, Sn-117m, was simulated and compared to that of a currently used radionuclide, Er-169, in order to determine whether or not significant differences in their dose distributions should be expected.

Methods The model of the synovial joint proposed by Johnson (Med Phys 20(3):747-754, 1993) was constructed in the GATE Monte Carlo software (Phys Med Biol 49:4543-4561, 2004). It consists of layers of 5 mm of bone, 1 mm of articular cartilage, 0.44 mm of joint capsule, 0.30 mm of phagocytic lining cells or intima, and 5 mm of synovial tissue or subintima. GATE source definitions for Er-169 and Sn-117m were derived from the ICRP 107 (Ann ICRP 38(3):1-96, 2008) emission data. Three configurations of each radionuclide were simulated: (1) all activity was located at the interface between the capsule and the synovial lining, (2) the activity was uniformly distributed within the synovial lining and (3) the activity was uniformly distributed within both the capsule and the synovial lining. The spatial resolution of the simulations was 0.04 mm in the plane of the layers and 0.01 mm perpendicular to the layers. Two billion events were simulated for each of the six scenarios. The dose per event was normalized by the emissions per disintegration to yield dose per disintegration and then divided by the activity per unit surface area and multiplied by 1.443 times the physical half-life to give the dose per unit administered activity on the assumption that all of the activity is retained in the original distribution.

Results For administered activities of 1.0 MBq/cm^2 of Er-169 and 0.84 MBq/cm^2 of Sn-117m, the dose distributions overlapped very closely. The peak dose to the capsule was about 87 Gy when the activity was confined to the lining. The peak dose to the capsule and synovial lining cells was about 40 Gy when the activity was distributed in both. The greatest absolute differences in dose in those two configurations were slightly more than 1 Gy. In the regions without radioactivity, Sn-117m deposits a larger, but still small dose of less than 1.1 Gy. The full width at half maximum of the dose from the plane source was 0.14 mm. The half maximum points of the dose distributions of the distributed sources were at the faces of the sources.

Conclusions When the administered activity of Sn-117m is set to 84% of that of Er-169, the dose distributions of the two radionuclides are nearly identical in the high dose regions. In the low dose regions, the extra dose deposited by Sn-117m compared to Er-169 is only 1-3% of the peak dose.