Survival of patients with visceral metastatic melanoma from an occult primary lesion: A retrospective matched cohort study

Abstract

Malignant melanoma presents as metastatic disease without an apparent primary in about 4% of cases. These are referred to as occult primary melanoma (OPM). It is not known whether these represent de novo malignant transformation in non-cutaneous sites or the disappearance of an unrecognised primary, perhaps on an immunological basis. We hypothesised that OPM might have a superior prognosis compared to patients with similar disease extent from a known primary lesion (KPM). We performed a retrospective cohort survival study of 146 patients with OPM and visceral metastases treated at the Sydney Melanoma Unit between 1983 and 1996. A control group of patients with KPM was matched for age, sex and site of visceral metastases. Survival was measured from the date of diagnosis of visceral metastases. Patients with OPM had a median survival of 233 days, significantly longer than the 176 days for those with KPM (P = 0.024; logrank test). Multivariate analysis allowing for simultaneous or prior involvement of lymph nodes, subcutaneous tissues or bone, and site of visceral involvement showed a significantly superior survival for OPM (hazard ratio (HR): 0.72; 95% confidence interval (CI): 0.55-0.93). A small part of the effect was explained by treatment, but models allowing for this still showed a significantly longer survival. Survival was longer in OPM patients. This may reflect an intrinsically superior host-tumour interaction.