Given the characteristic clinical presentation, the most likely diagnosis is pilomatrixoma.
Pilomatrixomas are benign adnexal tumors that arise from immature matrix cells of the hair follicles located on dermal or subcutaneous tissue.
The cause of pilomatrixoma remains unclear. Recent studies have suggested that the development of pilomatrixoma are related to mutations in the Wnt signaling pathway, where beta-catenin gene (CTNNB1) mutation is the most frequently reported.1-4
Pilomatrixomas are more common in children and often present before 10 years of age.1,2,5 They commonly appear in head and neck, as well as upper extremities, trunk, and lower extremities.2,6
The clinical manifestations of pilomatrixomas are diverse and according to their appearance five classic clinical types are described: mass, pigmented, mixed, ulcerated, and keloid-like.2,3 The mass type is the predominant form, where it generally presents as a hard and freely mobile nodule covered by skin that may present a firm calcified protruding nodule. Other less common types include: lymphangiectasic, anetodermic, perforating, and bullous.2,6,7
University of California, San Diego
Dr. Lawrence F. Eichenfield
Pilomatrixomas are mostly solitary, whereas multiple forms are reported to be associated with familial inheritance or syndromic conditions, such as myotonic dystrophy, Gardner syndrome, Turner’s syndrome, and Rubinstein-Taybi syndrome.2-4 However, children and adolescents occasionally present with multiple pilomatricomas with no associated syndrome.
On physical exam a helpful features for the diagnosis is the “teeter-totter sign,” which can be illustrated by pressing on one edge of the lesion that will cause the opposite edge to protrude from the skin. Another helpful tool is to use a light to transilluminate and the calcification produces a bluish opaque hue,8 as light cannot transmit through the calcification, often differentiating it from epidermal inclusion cysts or other noncalcified lesions.
What is the differential diagnosis?
Because of the diverse clinical presentations, pilomatrixomas are frequently misdiagnosed. The percentage of correct preoperative diagnosis reported is low, varying from 16% to 43% in different series.1,9-11 They most frequently are misdiagnosed as other types of cysts such as epidermal, dermoid, or sebaceous.2,3,5,12,13 Rapidly growing pilomatrixomas can be also be misdiagnosed as malignant soft-tissue tumors, cutaneous lymphoma, or sarcomas.5,13
Dr. Alreem Al-Nabti
When presenting with a classic history and physical features, diagnosis is clinical, and no further studies are recommended.14 To improve diagnostic accuracy when encountering unusual subtypes, imaging is recommended, including ultrasound. Ultrasound adds a high positive predictive value (95.56%).2 Generally, on ultrasound a pilomatrixoma is described as an oval, well-defined, heterogeneous, hyperechoic subcutaneous mass with or without posterior shadowing.2 The definitive diagnosis is, however, made by histopathologic examination.
Pilomatrixomas do not spontaneously regress, therefore complete surgical resection is the standard treatment. During the follow-up period, very low recurrence rates have been reported, varying from 1.5% to 2% which generally occurs because of incomplete resection.2,3
Dr. Sofía Guelfand
Plexiform neurofibromas are usually congenital tumors of peripheral nerve sheath associated with neurofibromatosis type 1, often with a “bag of worms” feel on palpation. Epidermoid cysts generally present as dermal nodules often with a visible puncture, mobile on soft and mobile on palpation. Dermatofibromas present as firm, usually hyperpigmented papule or nodules that are fixed to subcutaneous tissue, thus often “dimpling” when pitched. Dermatofibrosarcoma protuberans is a rare soft-tissue sarcoma which presents as a firm, slow growing indurated plaques growing over months to years.