20 Questions
What is the focus of the 2018 WHO classification of skin tumors in relation to melanoma?
Capturing the typical evolution of different subtypes of melanoma based on cell of origin, mutagenic factors, and oncogenic drivers
What is emphasized as essential for making the best diagnoses of melanocytic tumors?
Integration of clinical features and understanding of the pathogenesis and evolution
What does the text suggest about melanoma subtypes?
They are caused by different oncogenic drivers and mutagenic factors
What is the role of immunohistochemistry and molecular techniques in the diagnosis of melanocytic tumors according to the text?
Practical application in the context of pathway-based classification
Which genetic mutation is associated with the low-CSD melanoma pathway?
BRAF V600E
What distinguishes melanocytomas from conventional melanocytic nevi?
Distinct histopathologic and genetic features
What is the characteristic behavior of a well-characterized melanocytoma?
Low-grade biologic behavior
What type of melanoma can arise without an observable precursor nevus?
De novo melanoma
Which genetic mutation is present in 80-90% of conventional melanomas but very infrequent in Spitz nevi?
BRAF V600E
What can be used to guide classification when the lineage of a melanocytic proliferation is uncertain?
Genetic testing, such as gene panel testing or immunohistochemistry
Which kinase fusion can be identified by immunohistochemistry in some cases?
ALK and ROS1
Where are Spitz nevi more common?
In children
Which immunohistochemical feature is characteristic of BAP1-inactivated melanocytomas in the low-CSD pathway?
Loss of nuclear expression of BAP1 in melanocytes
What is the role of beta-catenin in DPMCs and melanomas with beta-catenin activation?
It is a 'master regulator' of Wnt signaling
Which feature distinguishes DPMCs from blue nevi and melanoma?
Abundant pigmented and vacuolated cytoplasm
What can immunohistochemistry for beta-catenin or LEF1 assess?
Beta-catenin activation characterized by strong uniform expression in tumor cells
Which genetic mutation is characteristic of PRKAR1A-inactivated melanocytoma?
BRAF V600E mutation
How are melanocytes in PRKAR1A-inactivated melanocytoma distributed?
As single cells rather than nests
Which tumor suppressor is often lost in uveal melanoma and melanoma arising within blue nevi?
CDKN2A
How can genetic inactivation of CDKN2A occur?
Homozygous deletion followed by loss of heterozygosity
Study Notes
- PRKAR1A-inactivated melanocytoma is a subset of pigmented epithelioid melanocytoma, also known as epithelioid blue nevus of Carney complex. It is most commonly found in the low-CSD pathway and harbors a BRAF V600E mutation.
- Genetically distinct from the blue nevus pathway, characterized by a mutation that activates the Gαq pathway instead of a BRAF V600E mutation.
- Melanocytes in PRKAR1A-inactivated melanocytoma have larger vesicular nuclei with large faintly eosinophilic nucleoli and are distributed as single cells rather than nests.
- PRKAR1A-inactivated melanocytomas are less common than deep penetrating melanocytomas and can be concerning for melanoma due to their need for two hits to a tumor suppressor.
- Loss of PRKAR1A expression can be identified through immunohistochemistry.
- PRKAR1A inactivation increases the risk of various characteristic tumors such as myxomas and psammomatous melanotic schwannomas in patients with Carney complex.
- Clear cell sarcomas demonstrate melanocytic differentiation but are typically amelanotic without epidermal involvement and can be confused with Spitz tumors, cellular blue nevi, and other sarcomas.
- These tumors are characterized by EWSR1 fusions, typically involving ATF1 and less commonly CREB1.
- Assessment of tumor suppressors including BAP1, PRKAR1A, CDKN2A, NF1, and SPRED1.
- BAP1 is often lost in uveal melanoma and melanoma arising within blue nevi, making it a critical tumor suppressor in melanoma diagnosis.
- CDKN2A inactivation is common in melanomas across all pathways and is defined as a Spitz tumor with isolated inactivation of CDKN2A without additional secondary driver alterations.
- CDKN2A-inactivated Spitz melanocytomas have benign biologic behavior and may require TERT promoter alterations or other telomere maintenance mechanisms for malignant transformation.
- p16 can be assessed through immunohistochemistry, but its interpretation can be challenging as it is not expressed at significant levels in normal melanocytes and is induced by oncogenic signaling.
- Genetic inactivation of CDKN2A can occur through homozygous deletion or a truncating mutation followed by loss of heterozygosity.
- p16 expression is often cytoplasmic or nuclear and occurs in mosaic fashion, with some melanocytes expressing p16 and others lacking expression.
- Other tumor suppressors such as NF1 and SPRED1 can also be assessed through immunohistochemistry, but their expression across different types of melanocytic nevi, melanocytomas, and melanomas remains to be further characterized.
Test your knowledge on differentiating between Spitz and conventional pathway acral melanoma, mucosal melanoma, TERT amplification, and more. These tumors can be challenging to differentiate, especially in childhood cases.
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