Case 106

A 78-year-old female with a posterior neck skin indurated lesion.

1. What's the possible process of lesion here?

   A: Neoplastic, benign

   B: Neoplastic, malignant

   C: Inflammatory conditions

   D: Hematolymphoid disorders

   E: Neoplastic, sarcoma

   
   

IHC

• Pan-keratin
• CD3
• CD20
• SMA

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2. The section is negative for pan-keratin, Melan-A, HMB-45, SMA, Desmin, but positive for S100 and SOX10. What's the diagnosis?

   
   

Answer

This is a case of desmoplastic melanoma.

Provided outside immunohistochemistry is reviewed:

S100: Positive

SOX10: Positive

Melan A: Negative

HMB45: Negative

PRAME: Negative (<50%)

AE1/AE3: Negative

EMA: Negative

CD68: Negative

CD34: Negative

SMA: Positive (subset)

Desmin: Negative

Ki-67: 10%

Histologic Features:

• Tumor characteristics:

  • Dermal-based, paucicellular tumors with spindle-shaped melanocytes resembling fibroblasts.

  • Typically non-pigmented and dispersed among abundant sclerotic or mature collagen bundles.

  • Stroma often appears fibromyxoid.

• Cellular distribution:

  • Usually haphazard, but may form parallel bundles or storiform patterns.

  • Spindle cells often extend into subcutis, deep fascia, and interlobular septa.

• Epidermis and dermis:

  • Overlying epidermis is thin and atrophic.

  • Adjacent superficial dermis typically shows severe solar elastosis.

• Inflammatory infiltrate:

  • Aggregates of lymphocytes and occasional plasma cells often present within or at tumor margins.

• Cytology:

  • Spindle cell atypia ranges from minimal to severe.

  • Often includes scattered cells with large, hyperchromatic nuclei.

  • Cytoplasm is typically poorly defined.

• Diagnostic clues:

  • In subtle cases, lymphoid aggregates may be key for diagnosis, especially with solar elastosis.

  • Easily missed on punch or shave biopsies.

  • More cellular forms may mimic scar or benign neural lesion.

Epidermal Involvement:

• May show lentigo maligna or atypical melanocytic proliferation.

• However, junctional component is minimal or absent in ~50% of cases.

Neurotropism:

• Seen in ~30% of cases.

• Patterns include:

  • Perineural invasion (tumor surrounding nerves).

  • Intraneural invasion (tumor invading endoneurium).

  • Neural transformation (tumor mimicking nerve structures).

• Can also occur in melanomas without desmoplasia, though less commonly.

Other Features:

• Vascular invasion is rare.

• Associated acquired nevus may be present.

Classification (Busam et al., 2004):

• Pure desmoplastic melanoma:

  • 90% of invasive tumor is desmoplastic.

  • Associated with more favorable outcomes.

• Combined desmoplastic melanoma:

  • Contains mixed areas of desmoplastic and non-desmoplastic melanoma.

  • Desmoplastic component <90%.

Reference: https://tumourclassification.iarc.who.int/chaptercontent/64/60

Case credit: UCSD Pathology

Author: Wangpan Jackson Shi, MD