Description of the MRI and ultrasound imaging features of giant epidermal cysts.

Abstract

Guidelines suggest that lesions over 5 cm in dimension should be referred to a specialist sarcoma centre due to the possibility of malignancy. Few epidermal cysts (ECs) reach or exceed this size and are termed giant ECs (GECs). The purpose of this study is to report on a large series of GECs. Retrospective review of histologically proven GECs over an 8-year period. Patient demographics with MRI and ultrasound (US) appearances were evaluated. A total of 14 cases were included with eight males and six females. Mean age was 51 years. 11 lesions were oval and three bi-lobed in shape, while 12 demonstrated dermal apposition. All were hyperintense on water-sensitive sequences and isointense to slightly hypointense on T1W imaging. Internal clefts were seen in 13 cases and 11 demonstrated chemical shift artefact (CSA) on MRI. On US, 12 showed well-defined linear hypoechoic clefts, with 66.6% having dis-organised compared with 33.3% peripherally located clefts. One 'pseudo testis' pattern and one showing irregular striped echogenicity termed novel 'pseudo muscle' appearance. No cases demonstrated internal vascularity on Doppler US. MRI signal findings of GECs are often characteristic with hyperintensity on water-sensitive sequences, dermal apposition, CSA and internal clefts while US features of disorganised or clumped hypoechoic clefts and absence of neovascularity were commonly seen. Recognition of combinations of both US and MRI features of GECs should reduce the requirement for pre-excisional needle biopsy to confirm the diagnosis. 1. Identification of common imaging features of GECs should avoid unnecessary pre-excisional biopsy despite their large size in the appropriate MDT setting.2. A novel 'pseudo-muscle' appearance is described on MRI and US.