Detection of Intraocular Tumors with the Use of Radioactive Phosphorus

Abstract

At the present time the most positive means of identifying malignant tissue is histologic examination. This is the method of choice and can be used in most situations. In the case of intraocular tumors, however, histologic confirmation is possible only after enucleation. It was therefore considered of great importance to utilize the selective localization of certain radioactive isotopes in tumors to aid in the diagnosis, in vivo, of such intraocular neoplasms. It was known from earlier work (1–6) that various tagged materials are selectively taken up by rapidly proliferating tissue and that the increased radioactivity can be detected in vivo by specialized counting procedures. Application of this general method to the problem of identification of intraocular tumors was a logical extension. The problem was essentially one of differentiating between fluid detachment of the retina and detachments resulting from underlying malignant tumors. Although it is often possible to differentiate between these conditions by means of the usual diagnostic procedures (transillumination) and established criteria (presence of a retinal tear), many times such means are inadequate. It is in these cases that the present test has proved most valuable. Since intraocular tumors are known to be adjacent to the outer surface of the eye, it is possible to use a beta emitter for their identification. The advantage in using beta rays is that the measurement obtained by counting directly over the suspected tumor is essentially unaffected by the radioactive material in the surrounding normal tissue. Thus, the masking which might occur with a gamma emitter is avoided. P³² was chosen for the study because it had been shown by other investigators (1, 3, 6) to be taken up in tumor tissue in considerably higher concentrations than in normal tissue. It emits beta particles which have adequate penetration (8 mm. maximum in tissue)² and it has a half-life of sufficient length (14.3 days) to allow ease of handling and scheduling of patients. Method The method used at the present time is essentially the same as that reported by the authors in an earlier paper (7). Counting Apparatus: Measurements over points anterior to the equator of the eye and points slightly posterior to the equator are made with a small end-window Geiger counter (Fig. 1).³ Preliminary work has been started on the development of a scintillation counter, which should allow counting directly over areas on the posterior half of the globe. In Vivo Counting Procedure: 1. The patient is prepared by instillation of 1 per cent tetracaine hydrochloride or 4 per cent cocaine hydrochloride into the conjunctival sac. 2. A thorough ophthalmoscopic examination is carried out to determine the exact location and extent of the suspected tumor. 3. After a check has been made to determine that the counting apparatus is operating properly, 500 microcuries of P³² (in sterile isotonic saline) is injected intravenously.⁴