Extra-osseous localization of 99Tcm-Sn pyrophosphate

99Tcm-Sn pyrophosphate bone scans of 250 patients referred for skeletal metastatic survey were analysed to determine the frequency of abnormal extra-osseous localization and the various pathological causes. Twenty-six patients demonstrated abnormal extra-osseous concentration. There were three false positives. Sixty-five per cent of the extra-osseous lesions concentrating pyrophosphate were malignant (carcinoma of lung and breast, metastatic hepatic carcinoma, chondrosarcoma) and the remainder were benign lesions, e.g. sarcoidosis, soft-tissue calcification, post-surgical and irradiation sites. An incidental finding was the unusual frequency of accumulation of pyrophosphate in various joints, especially the knees and shoulders in asymptomatic patients above 70 years of age.