Onion Skin Sign (Testicular Epidermoid Cyst) | Radiology Signs

Answer:

The onion-skin (onion-ring) sign is the concentric pattern of alternating hypoechoic and hyperechoic rings seen within a well-defined, avascular intratesticular mass on grey-scale ultrasound, corresponding to alternating low and high signal intensity on MRI. It is the classic, though not universal, appearance of a testicular epidermoid cyst, a rare benign monodermal teratoma composed of desquamated keratin. Concentric rings are reported in roughly two-thirds of cases; the remainder show a target (or bull’s-eye) pattern with an echogenic keratin core surrounded by a hypoechoic rim, or a densely calcified rim. The lesion is sharply marginated, usually 2-3 cm or smaller, and shows no internal vascularity on colour Doppler or contrast enhancement on MRI, a feature that helps separate it from malignant germ cell tumours, which tend to be more vascular, less sharply marginated, and may coexist with raised serum tumour markers (AFP, β-hCG, LDH) or testicular microlithiasis. Recognising the onion-skin pattern preoperatively, together with normal tumour markers, supports testis-sparing enucleation rather than radical orchiectomy, provided intraoperative frozen section confirms benign histology and biopsy of the adjacent parenchyma excludes germ cell neoplasia in situ.

Why is it called so?

The concentric, alternating rings of laminated keratin resemble the layered cross-section of a cut onion, which gives the sign its name.

Pathophysiology

A testicular epidermoid cyst is a benign, purely ectodermal (monodermal) teratoma: its wall is lined by stratified squamous epithelium without skin appendages, and the cavity progressively fills with desquamated keratin debris. Where this keratin accumulates in successive, concentrically arranged laminations, it produces alternating bands of differing echogenicity, and differing MR signal, that appear as the onion-skin rings on imaging; where keratin instead compacts centrally, the same process produces the target/bull’s-eye pattern with a dense echogenic core. Because the cyst has no true internal blood supply, it remains avascular throughout, and its capsule can become fibrotic and calcify over time.

Alternative names: Onion-ring sign; onion-peel appearance

Other associated named signs: Target (bull’s-eye) sign of testicular epidermoid cyst, an echogenic keratin core surrounded by a hypoechoic rim, seen when the cyst does not form the full concentric onion-skin lamination

References

1. Langer JE, Ramchandani P, Siegelman ES, Banner MP. Epidermoid cysts of the testicle: sonographic and MR imaging features. AJR Am J Roentgenol. 1999;173(5):1295-9. PMID: 10541108.