Case Three – March 2017

• 68 year old male presented with a 3 cm mass on the right scalp
• No evidence of ulceration or epidermal change was seen

• Pilar cyst
• Lipoma

There is a complex, solid and cystic squamous proliferation present entirely within the dermis without an epidermal connection.

The undulating squamous epithelium shows evidence of trichilemmal differentiation with absence of a granular cell layer and presence of dense keratin.

The squamous cells are mitotically active and exhibit cytologic atypia with nuclear pleomorphism and hyperchromasia.

Differential Diagnosis:

• Trichilemmal carcinoma
• Squamous cell carcinoma with trichilemmal differentiation
• Pilar cyst
• Malignant proliferating pilar tumor

Malignant proliferating pilar tumor

• Rare malignant squamous neoplasm that represents malignant transformation from a pre-existing pilar/trichilemmal cyst
• Most often presents as a nodule on the scalp of women during the fourth to eighth decades of life
• These tumors exhibit increased complexity compared to a pilar cyst with solid and cystic areas, nuclear atypia, and an infiltrative growth pattern
• Due to the presence of cytologic atypia and solid areas with an infiltrative growth pattern, malignant proliferating pilar tumors mimic cutaneous squamous cell carcinoma histopathologically
  • Primary squamous cell carcinoma should demonstrate connection to the overlying epidermis and often also show an overlying in situ component
• Malignant proliferating pilar tumors are positive by immunohistochemistry for CD34, which is a marker for trichilemmal differentiation
  • Cutaneous squamous cell carcinoma is typically negative for CD34
• The histologic differential diagnosis also includes trichilemmal carcinoma, which a malignant neoplasm arising from a trichilemmoma
  • Often presents on the face and ears
  • Positive for CD34 immunohistochemical stain
• Malignant proliferating pilar tumors may potentially recur locally or regionally
  • Regional metastasis also has been reported
• Wide local excision is the treatment of choice for malignant proliferating pilar tumors to ensure complete removal and to prevent recurrence

• Malignant proliferating pilar tumors are malignant squamous neoplasms with trichilemmal differentiation that arise from a pre-existing pilar/trichilemmal cyst
• Malignant proliferating pilar tumors mimic cutaneous squamous cell carcinoma and trichilemmal carcinoma histopathologically
• Although arising from a pre-existing pilar/trichilemmal cyst, malignant proliferating pilar tumors can locally recur and rarely metastasize to lymph nodes
• Wide local excision is the treatment of choice to prevent local recurrence