Epithelial tumours

Epidermoid cysts of the distal phalanx are rare, usually secondary to trauma with implantation of epidermis into subcutaneous tissue or even into bone. An old trauma often goes unnoticed. The cyst may develop on a scar after surgery. The phalanx progressively expands and clubbing becomes obvious. The pain is of late onset, sometimes on the occasion of a pathological fracture. Histological investigation shows an epidermoid cyst filled with orthokeratin and lined with a thin layer of epidermis.

Radiographs depict a round, accurately rimmed erosion of the distal phalanx without septa or peripheral sclerosis. In early stages the bone erosion is absent or subtle and is occult on radiographs. Magnetic resonance imaging shows a regular mass with slight heterogeneous content and intermediate signal on ^-weighted and ^-weighted images; heterogeneous enhancement is noted after injection of gadolinium (Figure 11.20). A thin, regular rim with a high signal identical to that of normal epidermis is due to the peripheral epidermal layer. Bone erosions, even subtle, are highly visible on axial images. The area of an old penetrating injury may be marked by dark artefacts on gradient-echo images.

Keratoacanthoma is a rare, benign, but rapidly growing tumour located in the most distal part of the nail bed. The lesion may start as a small, painful keratotic nodule beneath the free edge of the nail plate. Magnetic resonance imaging shows a dome-shaped nodule with a homogeneous signal (intermediate on T1, hyperechoic on T2) and strong enhancement. A central area of low signal may indicate a central plug of horny material filling

Onychomatricoma Mri

Figure 11.20

Epidermoid cyst—axial 7^-weighted image. The cyst appears spontaneously with a slight high signal. Bone erosion is arrowed.

Figure 11.20

Epidermoid cyst—axial 7^-weighted image. The cyst appears spontaneously with a slight high signal. Bone erosion is arrowed.

Figure 11.21

Keratoacanthoma. Sagittal post-gadolinium ^-weighted image

Figure 11.21

Keratoacanthoma. Sagittal post-gadolinium ^-weighted image showing strong peripheral enhancement (arrow) and acro-osteolysis (arrowhead).

the crater, but this is inconstant. The limits may be ill defined owing to oedema in the surrounding tissues. Magnetic resonance images show a deep infiltrating lesion and detect more accurately than radiographs a frequent bone erosion (Figure 11.21).

Onychomatricomas must be suspected from clinical signs, with a filamentous tufted tumour in the matrix of a funnel-shaped nail. Histologically there is epithelial proliferation of the matrix or surrounding epidermis. The lobules are delimited by normal basal cells and are composed of keratinocytes identical to those of the matrix. After removal of these parakeratotic cells, an invagination remains resembling the infundibulum of a hair follicle. Sagittal MRI is essential to highlight the tumoral core in the matrical area and the invagination of the lesion into the funnel-shaped nail plate (see Figure 11.4). The centre shows a low signal on all sequences, with a peripheral rim with a signal identical to that of normal epidermis. The distal part with the filamentous extensions presents a higher signal on T2-weighted images due to a mucoid stroma (see Figure 11.5). Axial slices accurately show the holes in the substance of the nail plate, filled with the filamentous extensions.

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