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Classification of a Tumour in the Popliteal Fossa by Radioimmunoscintigraphy 11 Years after Resection of a Malignant Melanoma

Radioimmunodetection of melanoma is increasingly employed since monoclonal antibodies labelled with radionuclides are now available. In clinical practice radioimmunoscintigraphy is aimed particularly at detecting and classifying recurrencies and lymphatic involvement (1-4). However, less data are available concerning changes of histopathologic findings over time after primary resection, and ability of delayed metastases to bind the antibody. This report regards a 58-yr-old woman who had resection of a malignant melanoma of the lower left limb in 1975 (histologically typical spindle cell pattern, Fig. 1a). Local recurrencies occurred in 1979 (Fig. 1b) and in 1981 (Fig. 1c). Histological findings gradually changed to a mucoid clear-cell pattern (Fig. 1c). In Spring 1986, she developed pain in her left kneejoint and presented a palpable mass in the popliteal fossa which was first imaged by X-ray computed tomography (Cl', Fig. 2). It revealed a dense, relatively homogeneous and sharply delineated tumour (arrow). Since a secondary of the melanoma was suspected, a radioimmunoscintigraphy was performed, 6 h after i.v. injection of 1 mg 99m-Tc labelled F (ab') 2 fragments of the 225.28S monoclonal antibody (555 MBq) (SORIN" Saluggia, Italy). Gamma camera images, obtained in posterior and left-lateral projections, revealed high accumulation within the mass (Fig. 3, arrows), exceeding the remaining radioactivity within the blood vessels. Some radioactivity was also found in the synovia (small arrows). To separate clearly intravasal from intratumoral uptake, knee joints were again imaged after i.v. injection of 555 MBq 99m-Tc human serum albumin (HSA) 24 h later. HSA is known to remain in the blood for several hours. Five hours after injection (Fig. 4), the blood pool of the left knee joint was elucidated, revealing only a small increase of blood volume within the mass (arrow). Thus, an additional positive tumour uptake of the antibody could be confirmed. The tumour was surgically removed. Histological findings (Fig. 1d) indicated a clear cell sarcoma. Therefore, employing the same 225.28S antibody in indirect immunoperoxidase staining technique, the specimen was processed to prove positive antibody reaction. The usual controls and histochemical reaction with antimouse IgG were negative. It was of interest to note that only a few cells showed specific staining after application of an antibody concentration of 1:10 (Fig. 1e).