Photoscanning of bone lesions utilizing strontium 85.

Abstract

The metabolism of strontium 85 in man has been extensively studied by Bauer and colleagues (1, 2). Significant localization in bone of strontium 85 occurs in fractures, metastatic cancer, eosinophilic granuloma, chondromas, osteomyelitis, and Paget's disease. Quite recently, Gynning et al. (3) reported the usefulness of skeletal strontium-85 localization for detection of carcinoma metastatic from the breast. We should like to call attention here to the feasibility of photoscanning selected skeletal areas which harbor localized strontium 85. Technic Strontium 85 was obtained from Oak Ridge National Laboratory. The isotope was in an acid solution of approximately one normal and had a specific activity greater than 500 millicuries per gram. Purity was of the order of 98 per cent, with less than 1 per cent strontium 89. The stock solution was prepared by first neutralizing with 3 per cent sodium hydroxide and diluting the strontium 85 in physiologic saline to 20 microcuries per milliliter. This solution was then passed through a Millipore bacterial filter3 to insure sterility. Twenty microcuries were given intravenously to selected patients under twenty years of age, and 50 to 60 microcuries were utilized in indicated adult cases. At least twenty-four hours were allowed for localization of the isotope. Areas to be scanned were selected either on the basis of x-ray changes or by probe counting over suspected bone areas. Scanning was performed with a Picker magnascanner equipped with a 3 × 2-in. thallium-activated sodium iodide crystal and a 19-hole, coarse-focus collimator (4). With an 80-kev window about strontium 85, single 0.51 Mev gamma, count rates over positive areas of the skeleton averaged 500 counts per minute above surrounding bone tissue. Satisfactory scans were made at a scanning speed of 16 to 20 cm. per minute with a count per minute range differential setting (4) of 10 per cent and a film density of 100. Appropriate markers were placed on the photoscan and, with lead overlays of these identifying marked areas, a roentgenogram was made for superimposition (Figs. 1 and 2). Comment Strontium 85 has a half-life of sixty-five days. It decays by electron capture to metastable Rb85, which emits a single gamma photon of 0.51 Mev and becomes stable Rb85. There are no beta or alpha emissions and hence the bone radiation dose is small. Of the administered dose, approximately 55 per cent is excreted by the fifth day (2). The remaining strontium 85 becomes permanently fixed in the skeleton. Correcting for 50 per cent excretion in the first five days, one may estimate the total whole-body dose to a standard man at 0.326 rads, with a bone dose of approximately 2.28 rads (5). The rather energetic 0.51 Mev gamma of strontium 85 presented no unusual collimation problems, as the shielding on the probe of the equipment utilized was adequate. Preliminary work indicates that photoscanning of bone lesions is practical, desirable, and informative.