Thin regressing malignant melanoma: significance of concurrent regional lymph node metastases

We attempted to clarify the prevailing controversy regarding the significance of regression in thin (less than 0.76 mm) primary melanomas. Of 7540 patients with cutaneous melanomas treated at the Sydney Melanoma Unit, 28 first presented with a thin primary lesion and concurrent regional lymph node metastases (stage II). Major differences in tumour histology existed between these patients and stage I patients with thin lesions that subsequently recurred. Regression was present in all 28 lesions in stage II patients. In 61 stage I patients ultimately developing a recurrence, 67% of lesions displayed regression. Significantly, however, in 735 stage I patients ultimately not developing a recurrence, 61% of lesions also displayed regression. Why regression occurs so frequently in thin lesions which never recur is unclear. Our results suggest that the histology of thin primary melanomas may be influenced by the presence or absence of metastases in patients at that time.