Melanoma is a serious type of skin cancer that originates in the pigment-producing cells called melanocytes. While most melanomas appear as irregular, dark spots on the skin’s surface, the disease can manifest in various ways. Subcutaneous melanoma refers to an aggressive and deep form of this cancer that has significantly invaded the subcutaneous fat layer beneath the skin. This subtype presents a unique challenge in diagnosis and treatment because it tends to grow downward rather than across the skin’s surface.
Defining Subcutaneous Melanoma
Subcutaneous, meaning “beneath the skin,” describes a melanoma that has progressed deeply into the hypodermis, the layer of fat and connective tissue below the dermis. This presentation is commonly associated with nodular melanoma (NM) and amelanotic melanoma (AM). Nodular melanoma is characterized by a rapid vertical growth phase, meaning the tumor invades deep tissues quickly. This inherent vertical growth means the cancer is often deep by the time it is first detected.
Many deep-seated lesions are amelanotic, meaning they lack the typical dark pigment of melanin. The absence of pigment causes the lesion to appear pink, red, or skin-colored, making it easy to mistake for a benign mole or other non-cancerous growth. This deceptive appearance often leads to a delayed diagnosis, contributing to the aggressive nature of this presentation. Pathologically, subcutaneous melanoma is defined by deep dermal or hypodermal invasion, where cancer cells have entered the subcutaneous fat.
Clinical Presentation and Diagnosis
Identifying subcutaneous melanoma is difficult because it frequently does not follow the traditional ABCDE criteria (Asymmetry, Border irregularity, Color variation, Diameter greater than 6mm, and Evolving). Instead of a flat, changing mole, patients typically present with a firm, raised lump or nodule beneath the skin. This lump is often painless but rapidly increases in size and elevation. The lack of pigment in amelanotic variants further complicates visual detection, requiring a high degree of suspicion from clinicians.
The diagnostic process begins with a biopsy deep enough to include the entire lesion and allow for accurate depth measurement. An excisional biopsy, which removes the entire growth along with a small margin of surrounding tissue, is preferred over a superficial shave biopsy. Pre-operative imaging, such as high-frequency ultrasound (HFUS), is increasingly used to non-invasively measure the tumor’s thickness. This measurement accurately estimates the Breslow thickness, aiding in surgical planning and staging.
Fine-needle aspiration (FNA) uses a thin needle to withdraw cells, primarily for assessing suspicious lymph nodes to check for cancer spread. For the primary tumor, a deep punch or excisional biopsy remains the standard, allowing the pathologist to fully analyze the lesion’s architecture. Obtaining an accurate tumor thickness measurement is the most important information for determining the subsequent treatment plan and predicting the patient’s long-term outlook.
Staging and Prognosis
The severity of subcutaneous melanoma is determined using the American Joint Committee on Cancer (AJCC) TNM staging system. The most powerful factor influencing the T-stage and prognosis is the Breslow thickness, a measurement from the skin’s surface to the deepest point of tumor invasion. Because subcutaneous melanomas are defined by deep growth, they are often classified as T3 (2.01 to 4.0 mm thick) or T4 (greater than 4.0 mm thick) upon initial diagnosis. The presence of ulceration, a breakdown of the skin overlying the tumor, is an adverse factor that increases the T-stage classification and worsens the prognosis.
For most melanomas thicker than 0.8 millimeters, a Sentinel Lymph Node Biopsy (SLNB) is recommended. This procedure involves injecting a tracer near the primary tumor site to identify the first draining lymph node. If cancer cells are found in this sentinel node, the cancer is classified as Stage III, which significantly impacts the prognosis and necessitates additional systemic treatment. Given the deep nature of subcutaneous melanomas, lymph node involvement is a frequent finding.
Treatment Approaches
The initial treatment for localized subcutaneous melanoma is Wide Local Excision (WLE), a surgery to remove the tumor site and a margin of healthy tissue. The width of this surgical margin is determined by the Breslow thickness to minimize the risk of local recurrence. For deep lesions, particularly those over 2.0 millimeters thick, a 2.0-centimeter margin of healthy tissue is typically removed. The excision must extend down to the underlying muscle fascia or deep subcutaneous tissue.
Following surgery, many patients with high-risk disease (Stage IIB, IIC, or Stage III) are candidates for adjuvant therapy to eliminate microscopic cancer cells. Systemic treatments include immunotherapy, which uses anti-PD-1 inhibitors (e.g., nivolumab or pembrolizumab) to harness the immune system against the cancer. These agents have demonstrated significant success in preventing recurrence.
For approximately 50% of melanomas that harbor the BRAF gene mutation, targeted therapy is an option. Targeted therapy uses drug combinations, such as dabrafenib and trametinib, that specifically block the activity of the mutated BRAF protein, stopping cancer growth. The choice between immunotherapy and targeted therapy is based on the tumor’s genetic profile and the patient’s overall health. Radiation therapy may also be used for local control when clear surgical margins are difficult to obtain or for deep tumors that have recurred locally.