Thecoma, fibroma, and fibrothecoma are all tumors belonging to the thecoma-fibroma group of stromal tumors (per the World Health Organization classification of ovarian sex cord–stromal tumors).

Thecoma-fibroma tumors are a closely related group of benign tumors that arise from ovarian stroma and are often difficult for the imager and even pathologist to distinguish. The fibroma contains spindle cells, and is the most common type of sex cord-stromal tumor, accounting for 6% of all ovarian tumors. They are usually asymptomatic although they can be large.

Fibromas/thecomas are typically solid, rounded, or oval ovarian lesions and may attain a large size (fibroma) and have low to moderate vascularity. Sonographically, fibromas tend to cast an intense acoustic shadow, making it difficult to see beyond the anterior surface of the tumor. They can also be solid but “stripy,” with linear streaks appearing vertically within the lesion. Vascularity is typically low by color Doppler, but occasional lesions may be more vascular, such as cellular fibromas. There can be small cystic areas within the solid mass; however, large cystic components are extremely rare.

These solid tumors cast intense acoustic shadows either as stripes or simply a global shadow from the entire lesion. A pedunculated fibroid is the most likely differential diagnosis to mimic this appearance. The finding of a normal ovary separate from the solid tumor can help to make that differentiation. Other solid ovarian tumors such as teratomas, dysgerminomas, and some solid malignancies are unlikely to cast a shadow and tend to be more vascular throughout the lesion than the fibroma/thecoma.