Pilomatrixoma often presents as a hard, subcutaneous, slow-growing mass that may be tender, although usually not. The tumor is fixed to the skin but mobile over deeper underlying structures. They most often have a discoloration, with blue being the most common color [4], but this can be obscured by telangiectasia, hyperkeratosis, hemosiderin deposition, and erosion. The “tent sign” described by Graham and Merwin in 1978 [20] is the demonstration of the pilomatrixoma’s multifaceted, often calcified nature by stretching the skin tightly over the tumor .

Currently there is no laboratory test for pilomatrixoma, nor does this neoplasm cause any systemic physiologic changes that can be detected via laboratory testing. While fine-needle aspiration of these lesions is sometimes performed, the results can often be misleading [21–23]. Definitive diagnosis is most frequently made upon excisional biopsy.

In cases of patients who present with multiple pilomatrixomas, genetic screening may be warranted, as there have been cases where these lesions present as an early marker of myotonic dystrophy and adenomatous polyposis coli [24–26].

There is no specific imaging modality that provides a definitive diagnosis of pilomatrixoma; generally, imaging is not necessary. CT scan shows well-circumscribed masses in the subcutaneous tissue that may or may not have calcification. MRI reveals a nonenhancing, low to intermediate signal abnormality [7]. Ultrasound may be helpful in detecting the presence of calcifications and determining the position of the lesion with relation to deeper structures. This is particularly useful for tumors that may be large or in the parotid region [27]. Compared to CT and MRI, ultrasound is a less expensive noninvasive modality that does not require sedation or anesthesia, making it an attractive initial choice for evaluation in children.

Histology provides the definitive diagnosis in all cases. The tumor is in the deep subepidermal layer. Classic findings include a mass of cells in a circular configuration with enucleated “shadow cells” or “ghost cells” in the center and basophilic nucleated cells in the periphery. These cells are the immature basaloid cells’ attempt to manufacture hair. Hair shafts are absent in these lesions, as the basaloid cells fail to differentiate into follicles. These cells invoke a foreign body reaction and giant cell formation. Calcifications are typically present [7, 28, 29] and most commonly noted as stippled areas of cytoplasm [19]. The overlying epidermis is generally not involved, but is separated by a fibrous component (pseudocapsule) that gives the impression on physical exam of adherence to the skin (see Fig. 35.1 and Table 35.3).