What is melanoma in situ?

What is Melanoma In Situ?

Melanoma in situ (MIS) is an early, non-invasive form of melanoma where malignant melanocytes are confined to the epidermis and epithelial adnexa without invasion into the papillary dermis—essentially "stage 0 melanoma" with no metastatic potential. 1, 2

Key Defining Characteristics

MIS represents a collection of malignant melanocytes that remain entirely within the epidermis, meaning these cells have not yet breached the basement membrane to invade deeper skin layers. 2 This distinction is critical because:

• MIS has no potential for metastatic spread, unlike invasive melanoma 1
• The prognosis is excellent, with 5-year net survival rates exceeding 98-100% for all subtypes 2
• Treatment aims solely at complete excision with clear margins—no further staging or systemic therapy is required 1

Histologic Subtypes

MIS encompasses three distinct histologic patterns 2:

• Lentigo maligna (LM): The most common subtype, typically occurring on chronically sun-damaged skin of the head and neck in elderly patients (median age 66-72 years) 3, 4, 2
• Superficial spreading MIS: Associated with intense intermittent sun exposure, more common on trunk and extremities 2
• Acral lentiginous MIS: Found on non-hair-bearing skin (palms, soles), rare (0.6% of MIS) but proportionally higher in more pigmented skin types 2

Clinical Presentation

MIS typically begins as a tan-brown macule or patch that can progress to variegated pigmentation with dark black coloration or even amelanotic features. 4 The lesion appears flat without palpable elevation, distinguishing it from invasive melanoma. 3

Critical Management Consideration: Subclinical Spread

A major challenge with MIS, particularly lentigo maligna, is extensive subclinical spread—atypical melanocytes extend laterally beyond what is clinically visible, sometimes reaching several centimeters beyond visible borders. 5 This "field effect" of atypical junctional melanocytic hyperplasia makes clinical margin determination unreliable and accounts for the high local recurrence rates when inadequate margins are used. 1, 5

Treatment Approach

Surgical excision with histologically clear margins is the gold standard treatment for MIS. 1, 2 However, the approach differs by subtype:

Standard MIS (Non-LM Subtypes)

• A 0.5 cm clinical margin around the visible lesion is recommended 1
• On trunk and extremities, 1.0 cm margins are often used due to ease of closure and higher likelihood of achieving clear margins 1

Lentigo Maligna

• Standard 0.5 cm margins are frequently inadequate—margins >0.5 cm are often necessary to achieve histologically negative margins 1, 5
• Complete circumferential peripheral and deep margin assessment (CCPDMA) techniques are recommended, including Mohs micrographic surgery with permanent section analysis or staged excision with formalin-fixed sections 1, 5
• Approximately 50% of LM cases, especially on head and neck, require margins greater than 0.5 cm for clearance 1

Alternative Treatments (When Surgery Not Feasible)

Non-surgical options exist but have higher recurrence rates than surgery: 1

• Topical imiquimod (limited data, relatively low cure rates) 1, 6
• Radiotherapy 1, 6
• Cryotherapy 1, 2
• Observation only (very elderly patients with limited life expectancy) 1

Important Pitfall

A significant percentage of cases initially diagnosed as MIS by biopsy subsequently prove to have an invasive component upon complete excision. 6 This finding underscores why:

• Permanent section analysis of the entire excision specimen is mandatory 1
• Topical or destructive therapies should be used with extreme caution 6
• If Mohs surgery is performed for MIS, permanent section analysis of the central debulking specimen must be done to provide complete pathologic staging 1

Recurrence Patterns

Local recurrence of lentigo maligna is common and typically represents persistent disease (inadequate initial margins) rather than true metastatic recurrence. 1, 5 This is attributed to the "field effect" where atypical melanocytes extend laterally along the epidermis but are not clinically detectable. 1