Usefulness of strain elastography in the differential diagnosis of ruptured epidermal cyst and superficial abscess.

Purpose: The aim of this study was to evaluate the usefulness of strain elastography (SE) in the differential diagnosis of ruptured epidermal cyst and superficial abscess.

Methods: This retrospective study included 34 patients with ruptured epidermal cysts and 17 patients with superficial abscesses who underwent ultrasonography (US) including SE. The SE characteristics were classified into four grades (1 to 4) according to elasticity. The largest length and height of the lesion and their ratio were evaluated on the US images. Involvement of more than half of the depth of the dermis and the presence of the submarine sign were assessed.

Results: The inter-reader agreement of US and SE findings showed excellent or almost perfect agreement. The height, length, ratio of height to length, and more-than-half-depth sign did not significantly differ between ruptured epidermal cysts and superficial abscesses for either reader (reader 1, P=0.071, P=0.129, P=0.806, and P=0.102, respectively; reader 2, P=0.173, P=0.053, P=0.669, and P=0.060, respectively). The submarine sign was significantly more frequent in ruptured epidermal cysts than in superficial abscesses (both readers, P<0.001). The difference in SE scores between ruptured epidermal cysts and superficial abscesses, which are harder than ruptured epidermal cysts, was statistically significant (reader 1, P=0.046; reader 2, P=0.028).

Conclusion: The SE score and submarine sign may be useful characteristics for distinguishing ruptured epidermal cyst from superficial abscess.