Skin metastases of malignant tumors arise principally when the diagnosis of the primary cancer has been previously established, and cutaneous metastases from internal malignancies are an infrequent, although not totally rare, phenomenon [
1]. In contrast, breast cancer is very common in women and its metastases frequently involve skin, with cutaneous findings in about one quarter of breast cancer patients [
2].
Cutaneous metastases of carcinomas are encountered in 0.7-9.0% of all patients with cancer in general [
3]. In the main, skin metastases occur long after the diagnosis of cancer, however, in some cases they may be the first sign of clinically silent visceral malignancies. The location of skin metastases depends on the location of the primary malignancy, the mechanism of the metastatic spread, and the gender of the patient. Cutaneous metastases can vary in size and clinical appearance dependent upon the type of primary malignancy. Some skin metastases may mimic benign dermatological conditions such as cutaneous cysts, hemangiomata, herpes zoster eruptions, alopecic patches, and erysipelas [
3].
In 2010 Fernandez-Flores investigated 78 cutaneous biopsies from 69 patients and identified six histological patterns of cutaneous metastasis: nodular, diffuse, infiltrative, intravascular, bottom heavy, and top heavy [
1]. The majority of the patients were between 60 and 80 years of age. The most frequent anatomical location of the metastases was the abdomen. As to the primary tumor, breast carcinoma was the most common in females. In 18% the origin of the primary tumor was unknown and in all the cases investigated there had been no clinical suspicion of metastasis [
1].
In breast carcinoma in particular there is a wide range of clinical presentation of skin metastases. Most metastases are observed on the chest wall; less common sites include scalp, neck, upper extremities, abdomen and back [
3]. In general, eight specific clinical patterns associated with cutaneous breast cancer are known: cancer en cuirasse [
4], inflammatory metastatic carcinoma (carcinoma erysipelatodes) [
2,
5], carcinoma teleangiectaticum [
4,
6], alopecia neoplastica [
7,
8], Paget's disease [
9,
10], breast carcinoma of the inframammary crease [
11], metastatic mammary carcinoma of the eyelid with histiocytoid histology [
12], nodular metastases [
13,
14], and mucinous adenocarcinoma metastatic to the skin [
2]. Skin metastases from breast carcinoma can also be present in a zosteriform distribution when occurring at the sides of the abdomen [
13,
15]. Metastatic nodules are primarily caused by hematogenous spread, whereas inflammatory carcinomas and carcinoma en cuirasse are caused by lymphatic spread [
7]. In a case of cancer en cuirasse the fibrotic response is induced by the invading cancer with infiltrating tumor cells that resemble single files [
2]. This leads to the formation of a chest wall that resembles a metal breastplate of a cuirassier (a mounted cavalry soldier) [
2,
4]. In a case of Paget's disease, tumor cells infiltrate the epidermis directly with a typical pagetoid spreading [
7,
16]. Alopecia neoplastica presents as oval plaques or patches on the scalp that may be confused clinically with alopecia areata [
7,
16]. Breast carcinoma metastases of the scalp usually manifest as cutaneous nodules, although they also manifest less commonly as alopecia neoplastica.
Tracking the differentiation from primary cutaneous malignancies can be challenging due to the ability of the tumor cells to mimic specific dermal structures. Although most skin metastases show morphologic and immunohistologic features of the primary malignancy, they can also mimic other dermatological patterns on histology.