The floating balls sign refers to the appearance of multiple mobile globules/spherules of solid, usually fatty, tissue within an adnexal cyst. It is pathognomonic for ovarian mature cystic teratoma 1,2.
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Terminology
Alternative names include the meatballs 3, truffle sign 4, and boba sign 13.
When a single large ball is floating at a fat-fluid interface, the name Pokemon (Poké) ball sign has been used 1.
The term sack of marbles sign has also been applied to this appearance but it is more commonly used to describe dermoid cysts of the head and neck 11,12.
Epidemiology
Though thought to be uncommon, the sensitivity is up to 25% in some case series 1.
Pathology
The balls represent spherules of sebaceous or keratinoid debris 1-3. They are different from Rokitansky nodules, which are nonfloating mural protuberances 9. The balls float due to their lower specific gravity compared to cyst fluid 10.
Radiographic features
The balls are discrete spherical structures located within a cystic mass. They range in size, even within the same cyst, from 5 mm to 4 cm diameter 2. They also range in number, sometimes in excess of 100 within one cyst 2.
Two-thirds contain fat demonstrable by CT or MRI 1.
Ultrasound
The balls are echogenic on a background of anechoic/hypoechoic fluid 1-7. No color Doppler signal is present within.
CT
The balls vary in density, from fat density to soft tissue density (-70 to +35 HU) 1. Sometimes the balls have a fluid or soft tissue density core surrounded by a fat density rim 3.
MRI
The signal characteristics of the balls usually reflect the sebaceous composition 5,6.
- T1: hyperintense rim compared to surrounding fluid, relatively hypointense center
- T2: hypointense rim compared to the surrounding fluid, relatively hyperintense center compared to the outer portion
Fat-saturated sequences or an out-of-phase T1-weighted gradient-echo sequence may demonstrate signal drop, but this is not always present 8.