Radioactive phosphorus uptake testing of choroidal lesions

32p testing for suspected ocular malignancy has been performed during the past three years at the Mayo Clinic in a manner similar to that described by Shields, Sarin, Federman, Mensheha-Manhart, and Carmichael (I 974). Until recently our technique differed, however, in that we administered the 32p orally rather than intravenously. A dose of 750 ,tCi of 32p was given to the patient orally on an empty stomach two to six days before testing, as suggested by Ruiz and Howerton (I975). The Gass marker is used for localizing the boundaries of the suspected tumour to the scleral surface (Gass, 1966). Radioactive counts are then recorded alternately over the tumour site and at a specifically marked control site usually situated on the opposite side of the globe. We use the radioactivity detection probe and scaler (Ruiz, 1972).* Four measurements are generally recorded over each site, although more are taken if there is inconsistency in counts or if the differences between counts over control and tumour site are not obviously disparate. The percentage uptake is then determined by the formula: percentag'difference = (CtCc)/Cc x I00, in which Ct is the average count over the suspected lesion and Cc is the average count over the control site. We consider that the test is positive if the percentage uptake over the tumour is at least 75 per cent greater than that over the control site.