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

BACKGROUND: 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). PATIENTS AND METHODS: 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.
RESULTS: 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.
CONCLUSIONS: Survival was longer in OPM patients. This may reflect an intrinsically superior host-tumour interaction.