Case Three – March 2017

  • Patient Information
    • 68 year old male presented with a 3 cm mass on the right scalp
    • No evidence of ulceration or epidermal change was seen
  • Clinical Diagnosis
    • Pilar cyst
    • Lipoma
  • Histology

    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


  • Final Diagnosis

    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
  • Summary
    • 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