Epidermoid cyst appearing as a malignancy-mimicking subcutaneous lesion on ultrasonography.

Purpose: It is challenging to diagnose epidermoid cysts on ultrasonography; except in typical, benign-appearing cases. The purpose of this study was to include epidermoid cysts in the differential diagnosis of diverse subcutaneous lesions, especially malignancy-mimicking lesions, as seen on ultrasonography.

Methods: We reviewed 19 cases of pathologically confirmed epidermoid cysts in 19 patients (male, 8; female, 11). Three radiologists, who were blinded to the pathology data, classified (by consensus) these epidermoid cysts as benign or malignancy-mimicking lesions, according to generally accepted ultrasonographic criteria, including the margin, shape, echotexture, and transitional zone with surrounding tissue, and also including the growth pattern and adjacent tissue change. The ultrasonographic data were then correlated with the pathology results regarding the ruptured or unruptured status of the cysts.

Results: Epidermoid cysts have been noted as showing a wide-spectrum of findings on ultrasonography. Twelve of our cases showed benign ultrasonographic features: six cases had typical, benign ultrasonographic features with unruptured status; two cases with ruptured status did not have clear ultrasonographic features, although we decided by consensus that there were benign ultrasonographic features; and four cases with unruptured status had peculiar internal echogenicities, described as "internal rod-like contents", that could be considered to be a variation of the typical ultrasonographic finding of epidermoid cysts. Seven cases showed malignancy-mimicking ultrasonographic features; all seven of these had ruptured status.

Conclusion: The diagnosis of ruptured epidermoid cysts should be included in the differential diagnosis of malignancy-mimicking subcutaneous lesions. The internal rod-like contents can be regarded as another typical ultrasonographic finding of epidermoid cysts.