Atypical melanocytic proliferation

by Jason Wasserman MD PhD FRCPC
November 18, 2024

Pathologists use the term atypical melanocytic proliferation when they see a skin lesion made up of melanocytes (the cells that produce pigment) that does not look completely normal under the microscope. These lesions show some unusual or atypical features but do not have all the features needed to confidently label them as benign (non-cancerous) or malignant (cancerous). Because they fall into this gray area, further evaluation or treatment is often needed to determine the best way to manage them.

Is an atypical melanocytic proliferation a type of cancer?

An atypical melanocytic proliferation is not the same as a diagnosis of cancer. Still, it also does not entirely rule out the possibility that the lesion could behave like a melanoma, which is a type of skin cancer. Instead, this term reflects uncertainty about whether the lesion is benign or malignant. Pathologists use this terminology when the features they see under the microscope make it challenging to make a definitive diagnosis. It is a way of saying that the lesion needs careful follow-up and sometimes additional treatment to ensure patient safety.

How is this diagnosis made?

The diagnosis of an atypical melanocytic proliferation is made after a skin lesion is biopsied or removed entirely and examined under the microscope by a pathologist. Sometimes, pathologists cannot confidently determine whether the lesion is benign or malignant based on the features seen. They may consult other pathologists or experts for a second opinion in these cases. The clinical context—such as the lesion’s location, the patient’s medical history, and how the lesion appeared before removal—can also play an important role in deciding how to classify the lesion and plan treatment.

What does an atypical melanocytic proliferation look like?

An atypical melanocytic proliferation may not look very different from other nevi (moles) or pigmented skin lesions when seen on the surface of the skin. It can appear as:

  • A spot or patch of unevenly pigmented skin with shades of brown, black, or even red.
  • A mole that has irregular borders or an unusual shape.
  • A lesion that is larger than normal moles, sometimes exceeding the size of a pencil eraser (about 6 millimeters).
  • A growth that has changed over time, becoming darker, larger, or more irregular.

Because these features can overlap with both benign and malignant lesions, it is important to have any unusual or changing lesions evaluated by a dermatologist. A biopsy is often performed to determine if the lesion has atypical features under the microscope.

What are the microscopic features of an atypical melanocytic proliferation?

Under the microscope, an atypical melanocytic proliferation shows unusual features but is not definitive for benign or malignant melanocytic lesions such as melanoma. Common features that may be seen include:

  • Atypical melanocytes: The melanocytes may appear larger, darker, or more irregular than normal.
  • Unusual growth patterns: The melanocytes may grow unexpectedly, such as spreading unevenly through the skin.
  • Involvement of deeper layers: The melanocytes may extend into deeper parts of the skin, which is sometimes seen in more concerning lesions.
  • Borderline characteristics: The lesion may show a mix of features typically seen in benign nevi (moles) and malignant melanoma, making it difficult to classify with certainty.

Because these features do not fit neatly into either a benign or malignant category, the lesion is labeled as an atypical melanocytic proliferation.

What is the recommended treatment for an atypical melanocytic proliferation?

The treatment for an atypical melanocytic proliferation depends on several factors, including the pathologist’s findings, the size and location of the lesion, and the patient’s medical history. Common recommendations include:

  • Complete surgical removal: The lesion is often completely removed with a margin of healthy skin around it to remove all atypical cells. This is called an excision.
  • Close monitoring: After removal, regular skin checks and follow-ups with a dermatologist are usually recommended to watch for any new or recurring lesions.
  • Expert consultation: In some cases, additional opinions from specialized pathologists or dermatologists may be sought to help guide treatment decisions.

Working closely with your healthcare provider is important to determine the best course of action. The goal of treatment is to minimize the risk of any potential problems while avoiding unnecessary procedures.

A+ A A-