Proliferating Pilar (Tricholemmal) Cyst/Tumor

 Multicystic squamous neoplasm composed of mature keratinocytes lining keratin-filled spaces






Etiology/Pathogenesis




• Postulated that most cases arise in preexisting pilar (tricholemmal) cyst; may be related to chronic inflammation or trauma


Clinical Issues




• Typically occur on scalp (90% of cases) in older adults
• Much more common in female than male patients

• Most cases of PPT behave in benign fashion, but malignant PPTs are aggressive tumors that have high rate of metastasis

image Complete surgical excision is recommended in order to prevent recurrence and malignant transformation


Macroscopic




• Large tumors, 6 cm or greater in most cases

• Often multicystic dermal-based tumors that may involve subcutis


Microscopic




• Cystic spaces are irregularly formed and anastomosing

• Cysts show keratinization without granular layer

• Large, multicystic, dermal-based tumor with squamous lining and spaces containing dense keratin

• Peripheral palisading of basilar layer is typically present, and there may be thickened basement membrane

• Occasional mitotic figures are present, but no high-grade atypia or increased mitotic activity should be present


Top Differential Diagnoses




• Pilar/tricholemmal cyst
image Unicystic structure lined by mature squamous cells

• Proliferating epidermoid cyst
image Shows overlapping features with PPT but has granular layer and laminated keratin

• Malignant PPT (squamous cell carcinoma arising in PPT)
image Larger than benign PPTs, clinically often present as rapidly enlarging nodular mass lesion

image Show greater cytologic atypia and mitotic activity, infiltrative features

image
Low Magnification of PPT
Scanning magnification of proliferating pilar tumor (PPT) shows a smooth-bordered image, circumscribed tumor composed of thickened cords of squamous cells surrounding a large cystic cavity image containing abundant keratin material.


image
High Magnification of PPT
High-magnification examination of a benign PPT shows bland squamous keratinocytes with abundant eosinophilic cytoplasm surrounding keratin-filled spaces. Note the lack of an intervening granular layer image.

image
PPT With Bland Cytologic Features
High-magnification examination shows the bland cytologic features of the squamous cells, which show uniform nuclei, small nucleoli, and abundant amounts of dense eosinophilic cytoplasm. Scattered dyskeratotic keratinocytes are present image.

image
PPT With Keratinization
High-magnification examination shows several small foci of dense keratin material surrounded by bland-appearing squamous cells.


TERMINOLOGY


Abbreviations




• Proliferating pilar tumor (PPT)

• Proliferating pilar cyst (PPC)


Synonyms




• Proliferating trichilemmal cyst/tumor


Definitions




• Multicystic squamous neoplasm composed of mature keratinocytes lining keratin-filled spaces


ETIOLOGY/PATHOGENESIS


Unknown




• Postulated that most cases arise in preexisting pilar (tricholemmal) cyst; may be related to chronic inflammation or trauma


CLINICAL ISSUES


Epidemiology



Apr 24, 2017 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Proliferating Pilar (Tricholemmal) Cyst/Tumor
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