Shedding light on cutaneous metastases

Until now, a systematic approach to the common dermoscopic appearance of cutaneous metastases (CM) from various primary malignancies has been lacking. The article by Danica Tiodovic1 in this issue finally sheds light on this topic and provides important information not only on dermoscopy but also on the localization of metastatic disease in the skin. CM are not uncommon. They occur in approximately 5% of internal malignancies, and as many as 10% of patients with known metastatic cancer develop CM.2 They are a poor prognostic sign with a median survival of less than 1 year after presentation in most studies, with better survival in both melanoma and breast cancer.3 Recognition is important because they are often the first sign of tumour recurrence or may be the first sign of internal malignancy, especially in lung cancer. Verremmen reported that in 18.6% of patients with CM from internal malignancies, they were the first presentation of a previously clinically silent primary malignancy, 80% of which were lung tumours.4 The most common CM from internal malignancies are from breast cancer in women, followed by bronchus and lung, then gastrointestinal cancers, while in men, bronchus and lung are followed by gastrointestinal cancers.4-6 Clinically, they present as single or, more frequently, multiple skin-coloured, blue-brown, black or reddish-purple cutaneous or subcutaneous firm, painless, hard nodules. Ulceration, especially in breast cancer, may occur over time but is usually not the first presenting sign. Clinical appearance may give a clue to the originating cancer, so pigmented metastases are in the vast majority of cases from melanoma in 97.8%,1 while CM from renal cell carcinoma are often red or purple and may mimic hemangiomas or pyogenic granulomas.2, 3 Most clinical variants have been described in metastatic breast cancer, which is not surprising since 23.9% of patients with metastatic breast cancer developed CM in one large study.2 As for the localization of CM, more than 70% are locoregional, for example, melanoma metastases are often located near the primum and along regional lymphatic vessels. In absolute numbers, most CM develop on the trunk (39%), especially for breast cancer. Relative to body surface area, the head and neck were the most common sites of cutaneous metastasis, accounting for 32% of all distant metastases compared to 9% of body surface area.1 Another common site of cutaneous distant metastasis relative to body size is the umbilicus (0.5% of body surface area, rule of palm), with approximately 4% of CM developing there. They are called Sister Mary Joseph nodules and have been associated with gastrointestinal tumours and gynecologic malignancies.2-4 Melanoma, as a cutaneous malignancy, exhibits the most promiscuous metastatic pattern involving virtually all body sites.1 CM have been difficult to recognize due to their variable clinical presentation with wide differential diagnoses that can mimic benign skin lesions including erysipelas, herpes zoster, impetigo, erythema annulare centrifugum, radiation dermatitis, morphea, or pyogenic granuloma or malignant lesions including basal cell carcinoma, Merkel cell carcinoma or primary melanoma.5, 6 Dermoscopy of non-pigmented CM is not statistically different for various metastatic tumours, including melanoma.1 However, a common dermatoscopic pattern was identified that may help differentiate CM from other skin proliferations: structureless white combined with linear serpiginous vessels were found in 76% of cases, rising to 87% of cases when amelanotic melanoma CM were excluded. Not surprisingly, the majority of melanoma CM were pigmented, with structureless blue and brown being the most common patterns with vascular structures. Differentiation from blue nevi may still be difficult, but as the authors note, the presence of vascular structures on dermoscopy, recent onset and history of melanoma helps in differentiation.1 Recognition of CM is important in cancer follow-up in order to initiate treatment in a timely manner and a targeted cutaneous examination especially at the site near the primary tumour, second the head and neck area for all metastatic disease, the thoracic wall especially for breast cancer and the umbilicus for malignant gastrointestinal tumours. None. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.