Imaging Studies
Imaging studies are not usually indicated, but they may show a lobulated cystic mass, coarse calcification, or ringlike mineralization.
Because some subcutaneous tumors located in the midline of the body may have connections to the central nervous system (eg, scalp cavernous angioma, which may be part of the symptom complex known as sinus pericranii), imaging tumors in this location with CT or MRI prior to removal should be considered.
The best modality to determine bony invasion or erosion is CT scanning, and proliferating pilar tumors (PPTs) are frequently found as incidental subcutaneous nodules on brain CT scans. [19] They most frequently display isointensity on T1-weighted images and heterogeneous signal on T2-weighted images. [20] However, for deeper tissue invasion, MRI is best.
Additional imaging may be necessary on a case-by-case basis. A 62-year-old with a PTT on the shoulder had a positron emission tomography scan that displayed a high uptake of 18F-FDG (fluorodeoxyglucose). [21] In a different case, an endoscopic ultrasound with fine-needle aspiration was used to discover PPT metastasis to the pancreas in a 65-year-old man. [22]
Procedures
Performing an excisional biopsy is recommended. Send as much of the lesion as possible for pathologic evaluation. Ideally, the entire lesion should be excised and submitted at the time of the biopsy.
Histologic Findings
Proliferating pilar tumors (PPTs) are well circumscribed, with islands of squamous epithelium undergoing trichilemmal keratinization. Brightly eosinophilic keratinocytes are present in the outer portion of the tumor and cornify into compact cystic spaces filled with keratin. Horn pearls or squamous eddies may be present, as may foci of calcification and glycogen-rich clear cells.
The epithelial cells lining the cyst lack intercellular bridges. The peripheral layers palisade, while the deeper layer cells are swollen.
Anticytokeratin 5/6 may also stain strongly positive in this neoplasm. This is a monoclonal antibody that recognizes high molecular weight keratin intermediate filaments.
An increase in staining of nucleolar organizer regions, an indicator of proliferation, has also been proposed as an adjunct to differentiate benign and malignant proliferating trichilemmal tumors. [23]
Calretinin is a calcium-binding protein member of the EF-hand family and is reported to be focally positive. Calretinin may be helpful to identify the innermost cell layer of the outer root sheath in an anagen hair follicle, with the cutaneous adnexal proliferations showing differentiation toward this structure. Calretinin immunoreactivity supports eccrine differentiation in some sweat gland neoplasms, and it is also useful in identifying neoplasms with ductal sebaceous differentiation. [24]
Immunohistochemical staining with Ki-67, P53, and CD34 has been using to classify malignant PTT from PTT. [25]
A series of histological slides follows:
Proliferating trichilemmal cystic neoplasm. Well-circumscribed neoplasm with central cornified cells (2X). Courtesy of Steve A. McClain, MD.
Proliferating trichilemmal cystic neoplasm (20X). Courtesy of Steve A. McClain, MD.
Proliferating trichilemmal cystic neoplasm (400X). Note the pleomorphism of keratinocytes and mitotic figures. Courtesy of Steve A. McClain, MD.
Ye et al proposed a stratification of PPTs into the following three groups [9] :
-
Group 1 - Circumscribed silhouettes with "pushing" margins; modest nuclear atypia; and an absence of pathologic mitoses, necrosis, and invasion of nerves or vessels
-
Group 2 - Similar to group 1 but manifest as irregular, locally invasive silhouettes with involvement of the deep dermis and subcutis
-
Group 3 - Invasive growth patterns, marked nuclear atypia, pathologic mitotic forms, and geographic necrosis, with or without involvement of nerves or vascular structures.
Group 1 may be regarded as benign, group 2 as having the potential for locally aggressive growth, and group 3 as also having metastatic potential. The latter 2 categories might be equated with low and high grades of malignancy among PPTs of the skin. Occasionally, PPTs have been misdiagnosed at squamous cell carcinomas. [26]
Therefore, determining the malignant potential of a proliferating pilar tumor may be challenging and additional parameters are often needed. For instance, malignant proliferating pilar tumors are more likely to stain positive with p53 and Ki-67 relative to benign proliferating pilar tumors and trichilemmal cysts, and CD34 immunoreactivity may distinguish a malignant proliferating pilar tumor from a squamous cell carcinoma. [26]
The cyst cavity contains amorphous eosinophilic keratin. The content is commonly calcified.
The cyst may also have dedifferentiated parts, further emphasizing the need for careful analysis. [27]
-
Proliferating trichilemmal cystic neoplasm. Well-circumscribed neoplasm with central cornified cells (2X). Courtesy of Steve A. McClain, MD.
-
Proliferating trichilemmal cystic neoplasm (20X). Courtesy of Steve A. McClain, MD.
-
Proliferating trichilemmal cystic neoplasm (400X). Note the pleomorphism of keratinocytes and mitotic figures. Courtesy of Steve A. McClain, MD.
