EARLY DETECTION OF MELANOMA METASTASES WITH RADIOIODINATED METHYLENE-BLUE

Melanin synthesised in melanoma cells presents a unique target to whic h the treatment can be selectively addressed, provided the pigment is recognised by a suitable drug. Methylene blue (MTB) possesses a high a ffinity for melanin and, therefore, accumulates preferentially in mela noma cells. Since not directly toxic to the tumour, MTB serves as a ca rrier for radioisotopes and, once taken up by melanoma cells, acts as a selectively localised source of radiation. Hence, radioderivatives o f the compound can be used for both diagnosis and therapy of dissemina ted melanoma. Eleven patients with confirmed metastatic melanoma and o ne with a recent local recurrence were studied using radioiodinated (i odine-123 or iodine-131) MTB and a gamma camera. Biopsies of cutaneous lesions were taken to determine directly the compound uptake in tumou rs, This first clinical investigation concerning the diagnostic potent ial of radioiodinated MTB in patients with disseminated melanoma confi rmed the existence of approximately 80% of internal lesions previously identified by routine methods and, additionally, enabled detection of unknown secondaries in 6 of 12 patients studied. There were no false- positive gamma camera images regardless of whether I-123 Or I-131 was used. I-131 proved to be more suitable than I-123 for detecting melano ma metastases with radioiodinated MTB. Hazy images of the lesions trea ted with external beam radiation and/or some drugs suggest that initia l radio- and chemotherapy might affect MTB uptake in melanoma metastas es and reduce the clarity of the scintigrams obtained from a gamma cam era. However, small, untreated internal lesions that cannot be visuali sed easily with the standard diagnostic methods are revealed with I-13 1-MTB regardless of their localisation. It is concluded that use of ra dioiodinated MTB in conjunction with gamma camera or positron emission tomographic imaging might prove to be a useful and accessible tool fo r the detection of early melanoma dissemination.