Beta-particle dosimetry of various radionuclides used in the treatment of rheumatoid arthritis was estimated using Monte Carlo radiation transport simulation coupled with experiments using reactor-produced radionuclides and radiachromic film dosimeters inserted into joint phantoms and the knees of cadavers. Results are presented as absorbed dose factors (cGy-cm2/MBq-s) versus depth in a mathematical model of the rheumatoid joint which includes regions of bone, articular cartilage, joint capsule, and tissue (synovium) found in all synovial joints. The factors can be used to estimate absorbed dose and dose rate distributions in treated joints. In particular, guidance is provided for those interested in (a) a given radionuclide's therapeutic range, (b) the amount of radioactivity to administer on a case-by-case basis, (c) the expected therapeutic dose to synovium, and (d) the radiation dose imparted to other, nontarget components in the joint, including bone and articular cartilage.
Using the EGS4 Monte Carlo code, absorbed dose rate factors were estimated for four radionuclides of interest in radiation synovectomy, an intra-articular radiation therapy to treat rheumatoid arthritis. The treatment consists of the injection of a beta-emitting radionuclide into the joint capsule in order to eliminate diseased synovium through irradiation. The radionuclides investigated are 32P, 90Y, 165Dy, and 198Au. Calculations reveal the absorbed dose factor (cGy cm2/MBq s) as a function of distance (mm) in an EGS4 model of the rheumatic joint. The model incorporates bone, articular cartilage, joint capsule, and tissue (synovium) components found in all synovial joints, with dimensions in the model corresponding to dimensions typically found in larger joints, e.g., the knee, shoulder, or hip. Results are compared with previous, analytical approaches to beta dosimetry in radiation synovectomy. In addition, radiation backscatter due to the presence of bone is investigated and determined to have a negligible enhancement effect on absorbed dose to synovium.
Absorbed dose profiles are presented for radionuclides of frequent or potential use in radiation synovectomy, an intraarticular technique aimed at treating rheumatoid arthritis through direct and highly selective irradiation of inflamed synovium. The radionuclides investigated were 198Au, 165Dy, 32P, 186Re, 90Y, and 166Ho. These profiles reveal the absorbed dose imparted per unit activity of injected radionuclide (Gy/mCi) as a function of penetration distance (mm) to major components of the arthritic synovial joint. Their usefulness is twofold: they can be employed to select the radionuclide best suited to achieving the proper depth of penetration in tissue (approximating the thickness of the inflamed synovium) and to determine, a priori, the necessary dose of radioactivity (delivering an absorbed dose sufficient to effectively treat all the diseased tissue). The extent of radiation of other synovial joint components, such as bone and articular cartilage, and how the advancing rheumatoid arthritis can be expected to alter the extent and pattern of absorbed dose penetration in the joint are also discussed.
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