Acorn cyst | Radiology Signs

Answer:

The acorn cyst sign describes a benign complicated breast cyst on ultrasound composed of two distinct components: a deep, rounded anechoic fluid portion (the “acorn”) and a more superficial echogenic layer of proteinaceous or hemorrhagic debris (the “acorn cap”) capping part of the cyst. Together the two components resemble an acorn sitting within its cap. Recognizing this pattern helps radiologists categorize the lesion as a benign complicated cyst rather than a more suspicious complex cystic and solid mass; when the distinction is not confident, aspiration or biopsy can be performed to exclude malignancy.

Why is it called so?:

Named for its resemblance to an acorn: the rounded anechoic fluid component forms the “nut,” while the superficial layered echogenic debris forms the “acorn cap” sitting atop it, mirroring the two-toned, capped appearance of an acorn.

Pathophysiology:

Acorn cysts are complicated breast cysts containing a mixture of simple fluid and proteinaceous or hemorrhagic debris. On ultrasound this produces two sonographically distinct zones within the same cystic structure: a predominantly anechoic fluid-filled portion (the “acorn”) and a separate, more superficial layer of echogenic debris (the “acorn cap”). Because the debris component is avascular and non-solid, it typically shows no internal flow on Doppler interrogation and may shift or reposition with a change in patient position or transducer pressure, features that support a benign, complicated cyst rather than a fixed solid nodule. When these reassuring features are not clearly demonstrated, the lesion can mimic a complex cystic and solid mass, and short-interval follow-up or tissue sampling may be warranted to exclude malignancy.

Alternative names: None commonly cited in the literature.

Other associated named signs: None commonly cited in the literature for this specific finding.

References

1. Zhang Z, Maimone S. I saw the sign: Go nuts, it’s the “acorn cyst” sign. Clin Imaging. 2023.