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Melanocytic Dermpath Basics: Benign Nevus
Jerad Gardner, MD
Overview
This video serves as an introductory guide to diagnosing benign melanocytic nevi, focusing on key histological features that differentiate them from melanoma. The presenter emphasizes the importance of starting with low-power examination to assess architectural patterns like symmetry and maturation. Key microscopic features discussed include the arrangement of melanocytes in nests or as single cells, their location within the epidermis or dermis, and the concept of maturation where cells change in size and shape as they descend into the dermis. Cytological atypia and mitotic activity are also covered, with a caution against over-interpreting these features in benign lesions. The video highlights that pigment presence alone is not indicative of malignancy and discusses variations seen in congenital nevi, such as tracking around dermal structures.
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Chapters
- Diagnosing melanocytic lesions is crucial for dermatopathologists, balancing the risk of missing melanoma with over-diagnosing benign nevi.
- There are significant controversies and differing opinions in melanocytic pathology, particularly regarding dysplastic nevi and Spitz nevi.
- This video focuses on the basic features of benign nevi as a starting point, with future videos to cover melanoma and specific nevus subtypes.
- Expert consultation is essential for difficult cases; this video is for educational purposes only.
Understanding the critical importance and inherent challenges in diagnosing melanocytic lesions sets the stage for the detailed examination of benign nevi, highlighting the need for careful and nuanced interpretation.
The presenter shares personal disclaimers about the complexity and subjective nature of melanocytic pathology, encouraging viewers to seek expert consultation when needed.
- Begin assessment at low power to evaluate overall architectural features before examining cytology.
- Symmetry is a key feature: the lesion should appear relatively similar on both its left and right sides.
- Maturation is observed in nevi with a dermal component, where melanocytes change in appearance as they descend into the dermis.
- Nevi are classified as junctional (epidermis only), intradermal (dermis only), or compound (both epidermis and dermis).
Low-power examination provides a global view, allowing for the identification of fundamental architectural patterns that are strong indicators of a benign process, guiding subsequent higher-power analysis.
The presenter demonstrates symmetry by showing a lesion where the left and right halves appear mirror images of each other in terms of silhouette and cell distribution.
- In the junctional component, melanocytes are typically arranged in nests at the tips of rete ridges or as single cells spaced apart.
- Avoid features like pagetoid spread (melanocytes high in the epidermis) or confluent growth (melanocytes replacing the basal layer) in benign nevi.
- Maturation involves a transition from larger, epithelioid (Type A) melanocytes in the superficial dermis to smaller, more spindled (Type B and C) melanocytes deeper in the dermis.
- Type C melanocytes are spindled and can resemble Schwann cells, a pattern seen in 'neurotized' nevi.
Understanding how melanocytes are arranged and how they mature provides crucial clues to distinguish benign proliferation from malignant changes, as these patterns differ significantly between nevi and melanoma.
The presenter illustrates maturation by showing a nevus where larger, rounder cells (Type A) are present superficially, transitioning to smaller, darker cells (Type B) deeper down, and finally to single cells trickling between collagen fibers.
- Cytologic atypia (abnormal cell appearance) is subjective and best evaluated after architectural assessment.
- Benign nevus cells typically have bland, oval to round nuclei with fine chromatin and small nucleoli.
- While melanomas often have significant atypia and dermal mitotic activity, benign nevi can occasionally show a single mitosis.
- Mitotic figures in the epidermis can be difficult to definitively identify as melanocytic versus keratinocytic.
Evaluating cellular details and mitotic activity helps confirm or refute suspicions raised by architectural patterns, but it's essential to apply appropriate thresholds and avoid over-interpretation in the context of a benign lesion.
The presenter points out a single mitotic figure within a nest of melanocytes, explaining that while mitoses are concerning, their presence in a small number in an otherwise benign nevus does not automatically indicate melanoma.
- Melanin pigment is primarily stored in keratinocytes, not typically in melanocytes, which often have pale or gray cytoplasm.
- Dark pigment in the dermis usually represents melanophages (macrophages containing pigment), not melanocytes.
- The presence of pigment alone does not distinguish nevi from melanoma; many benign nevi are heavily pigmented.
- Some nevi, particularly lentiginous nevi, can appear darker and more atypical at low power due to increased pigment and elongated rete ridges.
Understanding the distribution and appearance of pigment, as well as variations like those seen in lentiginous nevi, helps prevent misinterpretation and over-diagnosis of malignancy based solely on pigmentation.
The presenter shows a heavily pigmented lesion with numerous dark brown cells in the epidermis, explaining that these are likely keratinocytes containing melanin, while the underlying melanocytes have a paler cytoplasm.
- Congenital pattern nevi often exhibit features that might appear unusual but are benign.
- These can include epidermal changes mimicking seborrheic keratosis and melanocytes tracking along dermal structures like nerves, vessels, and hair follicles.
- Melanocytes in congenital nevi can be large and evenly spaced in the epidermis, and show maturation with single cells trickling into the dermis.
- Features like tracking around nerves are more common in benign nevi than melanoma and should not be mistaken for perineural invasion.
Recognizing the specific benign features of congenital pattern nevi is crucial to avoid misinterpreting them as malignant, especially given their potential for unusual architectural arrangements.
The presenter demonstrates a large nevus where melanocytes are seen extensively wrapping around a nerve bundle and eccrine ducts, explaining this is a normal finding in congenital nevi.
Key takeaways
- Always start melanocytic lesion assessment at low power to evaluate architecture before focusing on cellular details.
- Symmetry and maturation are key benign features; look for orderly changes in melanocytes as they descend into the dermis.
- Benign nevi typically have melanocytes arranged in nests at the tips of rete ridges, with minimal pagetoid spread or confluent growth.
- Cytologic atypia and mitotic activity can be present in benign nevi, but severe or widespread findings should prompt further investigation.
- Pigment alone is not a reliable indicator of malignancy; many benign nevi are heavily pigmented.
- Congenital pattern nevi can display unique features like tracking around dermal structures, which are benign variations.
- The distinction between benign nevi and melanoma requires a holistic assessment of multiple architectural and cytologic features, not just isolated findings.
Key terms
Melanocytic nevusMelanomaDermatopathologyJunctional nevusIntradermal nevusCompound nevusMaturationZonationPagetoid spreadConfluent growthCytologic atypiaMitotic activityMelanophagesCongenital nevusLentiginous nevusNeurotized nevus
Test your understanding
- What are the primary architectural features to assess at low power when evaluating a melanocytic lesion for benignancy?
- How does the concept of maturation help differentiate benign nevi from melanoma microscopically?
- What specific patterns of melanocyte arrangement in the epidermis are considered reassuring for a benign nevus?
- Why is it important to consider the context and overall pattern when evaluating cytologic atypia and mitotic activity in melanocytic lesions?
- What are some distinguishing features of congenital pattern nevi, and why are they important to recognize?