Metastasis to the skin is an uncommon presentation of internal malignancy. Cutaneous metastases are far more likely to be seen in a patient with a diagnosis of previously metastatic disease. The frequency of cutaneous metastasis is dependent on the primary tumor. Almost all types of internal malignancy have been reported to metastasize to the skin; however, a few types of cancers account for the bulk of cutaneous metastases. The distribution of the metastases is also dependent on the original tumor. The most common form of skin metastasis is from an underlying, previously metastatic melanoma.
Clinical Findings: Most cutaneous metastases mani- fest as slowly enlarging, dermal nodules. They are almost always firm and have been shown to vary in coloration. Some nodules eventually develop necrosis, ulcerate, and spontaneously bleed. Skin metastasis can occur as a direct extension from an underlying malignancy or as a remote focus of tumor deposition. Although skin metastasis often arises in the vicinity of the underlying primary malignancy, the location of tumor metastases is not a reliable means of predicting the primary source. The scalp is a common site, probably because of its rich vascular flow.
Sister Mary Joseph nodule is a name given to a periumbilical skin metastasis from an underlying abdominal malignancy. This is a rare presentation that was first described by an astute nun at St. Mary’s Hospital at the Mayo Clinic. This has been described to occur most commonly with ovarian carcinoma, gastric carcinoma, and colonic carcinoma.
Melanoma metastases are usually pigmented and tend to occur in groups. Cutaneous metastasis from melanoma can manifest with the rapid onset of multiple black papules and macules that continue to erupt. As the tumors progress, patients can develop a generalized melanosis. This is a universally fatal sign that occurs late in the course of disease. It is believed to be caused by the systemic production of melanin with deposition in the skin.
Breast carcinoma is another form of malignancy that frequently metastasizes to the skin. Breast carcinoma tends to affect the skin within the local region of the breast by direct extension.
Pathogenesis: The exact reason why some tumors metastasize to the skin is unknown. This is a complex biological process that is dependent on many variables. Metastases are likely to be dependent on size, ability to invade surrounding tissues (including blood and lymphatic vessels), and ability to grow at distant sites far removed from the original tumor. This is an intricate process that depends on the production of multiple growth factors and evasion of the patient’s immune system.
Histology: The diagnosis of cutaneous metastasis is almost always made by the pathologist after histological review. Each tumor is unique, and the histological picture depends on the primary tumor.
Treatment: Solitary cutaneous metastases can be surgically excised. The risk of recurrence is high, and adjunctive chemotherapy and radiotherapy should be considered. Palliative surgical excision can be under-taken for any cutaneous metastases that are painful, ulcerated, or inhibiting the patient’s ability to function. The prognosis for patients with cutaneous metastasis is poor. The overall survival rate for multiple cutaneous metastases has been reported to be 3 to 6 months. The length of survival is increasing now because of improved treatments.