This video, the first in a series on melanocytic lesions in dermatopathology, provides an introductory primer on diagnosing benign nevi. The speaker, Jared Gardner, emphasizes the critical importance of distinguishing benign nevi from malignant melanomas for patient safety and to avoid misdiagnosis-related harm or malpractice. He highlights the subjective nature of cytologic atypia and acknowledges ongoing controversies within the field, encouraging expert consultation for difficult cases. The core content focuses on macroscopic and microscopic architectural and cytologic features characteristic of benign nevi, including symmetry, maturation (zonation), specific patterns of melanocyte growth within the epidermis and dermis (Types A, B, C), and the significance of mitotic activity. The video also showcases various examples of benign nevi, including dysplastic and congenital types, illustrating their wide range of appearances and common "unusual" features that are, in fact, benign.
Understanding Benign Nevi in Dermatopathology: A Primer
0:00 Introduction to Melanocytic Lesions: This video is the first in a series focused on diagnosing melanocytic lesions (nevi and melanoma) in dermatopathology, acknowledging the topic's complexity and diverse opinions.
1:04 Critical Diagnostic Skill: Distinguishing between a benign nevus (mole) and malignant melanoma is one of the most important skills for a dermatopathologist due to potential patient harm and medical malpractice implications.
1:38 Balancing Diagnosis: Pathologists must avoid missing melanomas, but also not over-diagnose benign nevi as atypical or malignant, which can lead to unnecessary patient fear and treatment.
2:21 Importance of Consultation: The speaker stresses that this video is for education only and encourages seeking expert consultation for difficult melanocytic lesions.
2:40 Controversies Acknowledged: The field of melanocytic pathology has strong, sometimes dogmatic, schools of thought regarding lesions like dysplastic nevus and Spitz nevus, which will be discussed in later videos.
3:31 Starting with Basic Principles: The video's primary focus is on the basic features of benign nevi as a foundational understanding.
3:34 Low Power First Approach: Always start by evaluating the lesion at low power to assess overall architectural patterns before examining cytologic details, as high power can lead to over-interpreting individual atypical cells.
6:42 Key Feature: Symmetry: Benign nevi typically appear symmetrical, meaning the left and right sides of the lesion have a similar architectural pattern.
9:15 Understanding Nevus Types: Melanocytes are classified by location: junctional (epidermis only), intradermal (dermis only), or compound (both). Precise classification is less critical than recognizing benignity.
12:13 Junctional Component of Nevi: Melanocytes are arranged in nests or as single cells primarily at the basal layer and tips of rete ridges. They should avoid high-level spread (Pagetoid spread) or forming a solid, confluent line along the basal layer.
15:03 Dermal Component & Maturation (Zonation): In nevi with a dermal component, melanocytes change their appearance as they go deeper. Superficial cells are larger and epithelioid (Type A), becoming smaller and lymphocyte-like (Type B) deeper down, and sometimes spindled and neural-looking (Type C) at the deepest parts, trickling out into the dermis. This "maturation" or "zonation" is a key benign feature.
26:05 Cytologic Atypia is Subjective: The evaluation of cytologic atypia is highly subjective. Architectural features should be assessed first.
26:45 Nevoid Melanomas: Some melanomas can mimic nevi at low power but show severe nuclear atypia and dermal mitotic activity at high power. Therefore, high-power examination is important after low-power assessment.
30:13 Mitotic Activity in Nevi: Benign nevi can have mitoses, particularly in the junctional component. A single mitosis in an otherwise benign-appearing nevus should not automatically lead to a melanoma diagnosis, but multiple mitoses or deep dermal mitoses warrant caution.
36:09 Pigment Distribution: Dark-brown pigment in the epidermis is typically in keratinocytes, not melanocytes. In the dermis, dark pigment usually belongs to melanophages. Melanocytes themselves often have pink or gray cytoplasm. Pigment amount is not a reliable indicator of malignancy.
39:15 Macro-melanosomes: Large, perfectly round melanin dots seen in keratinocytes are a benign feature, often in pigmented nevi.
47:23 Congenital Nevus Features: Large/thick nevi, especially congenital ones, can present with unique benign features, including seborrheic keratosis-like epidermal changes, tangential sectioning artifacts, evenly spaced large single melanocytes in the epidermis, and tracking along adnexal structures (nerves, vessels, eccrine ducts, hair follicles, arrector pili muscles).
58:00 Multinucleated Dermal Melanocytes: Large, hyperchromatic, multinucleated cells can be seen in the dermis of benign nevi (especially congenital types) and should not be over-interpreted as severe atypia.
1:01:06 Neurotized Nevi and Neurofibromas: Some nevi (neurotized nevi, often congenital pattern) can have extensive spindled melanocytes (Type C) and features resembling neurofibromas, including Wagner-Meissner-like bodies. Distinguishing between them often relies on finding obvious nested nevus cells or melanin in other areas of the lesion, rather than relying solely on immunostains.
1:02:40 Fatty Metaplasia & Pseudo-Vascular Change: Benign fat can be present within nevi, and some nevi can exhibit pseudo-vascular spaces where melanocytes mimic endothelial cells. These are benign findings.
1:14:36 Core Features of Benign Nevi: Symmetry, maturation, basal layer orientation with lack of Pagetoid/confluent spread, bland cytology (lack of severe atypia), and low/no dermal mitotic activity are key indicators of a benign nevus.