Lipoma and Atypical Lipomatous Tumor/Liposarcoma: Differential Ultrasound Diagnosis
Ultrasound Features of Typical Lipoma
Lipoma is visualized as a well-demarcated hypoechoic lesion with prominent linear echogenic septa (partitions separating aggregates of adipose tissue). Characteristic findings include acoustic enhancement (sound transmission through fat is greater than through muscle) and minimal or weak blood flow. The long axis of the lesion is typically parallel to the skin surface. Power Doppler does not demonstrate significant intralesional blood flow. Typical ultrasound findings are generally diagnostic, and if all margins are clearly defined, additional imaging is not required.
Lipoma Variants
Lipoma variants include angiolipoma (vascular variant, appearing on ultrasound as a small hyperechoic lesion in the subcutaneous tissue), spindle cell lipoma, hibernoma, chondroid lipoma, osteolipoma, and infarcted lipoma. Calcification, small non-lipid foci, myxoid tissue, and fat necrosis may be seen in lipoma variants but are not, in themselves, diagnostic features of liposarcoma.
Features Raising Concern for Liposarcoma
The best indicators of liposarcoma are considered to be large size, increase in size, deep localization, and heterogeneity. However, not all heterogeneity within a tumor necessarily indicates liposarcoma. Liposarcoma should be suspected when nodularity, heterogeneity, stranding, calcification, necrosis, and vascularity are present.
| Feature | Lipoma | Liposarcoma (Concerning) |
|---|---|---|
| Borders/Structure | Well-defined, linear echogenic septa | Nodularity, stranding |
| Echotexture | Homogeneous, predominantly hypoechoic | Heterogeneity, non-lipid hypoechoic foci, necrosis |
| Blood Flow | Minimal/weak | Moderate to marked vascularity |
| Size/Localization | — | Large size, growth, deep localization |
Special Forms of Liposarcoma
Well-differentiated liposarcoma (WDL/ALT, low-grade) may have a homogeneous appearance and is indistinguishable from lipoma on ultrasound. High-grade or poorly differentiated liposarcoma is heterogeneous but predominantly hypoechoic, similar to other sarcomas, and may macroscopically contain very little fatty component or none at all. In myxoid liposarcoma, a large intramuscular heterogeneous lipomatous lesion is visualized with moderate internal vascularity and foci of non-lipid hypoechoic tissue.
Management and Role of MRI
Ultrasound usually allows diagnosis of lipoma; MRI is performed to exclude features of liposarcoma and better differentiates lipoma from liposarcoma, serving as a good baseline study. CT or MR may be required to assess the full anatomic extent of large lesions. FNA/biopsy should not be used as a method for confirming the diagnosis of lipoma. Differentiation of well-differentiated liposarcoma from lipoma requires correlation with clinical history and surgical excision.
Frequently asked questions
Can well-differentiated liposarcoma (WDL/ALT) be distinguished from lipoma on ultrasound?
No. WDL/ALT may appear homogeneous and is indistinguishable from lipoma on imaging. Differentiation requires MRI, correlation with clinical history, and surgical excision.
Which ultrasound features best indicate liposarcoma?
Large size, increase in size, deep localization, and heterogeneity. Also concerning are nodularity, stranding, calcification, necrosis, and vascularity.
Does heterogeneity always indicate liposarcoma?
No. Not all heterogeneity within a tumor necessarily indicates liposarcoma; calcification, small non-lipid foci, myxoid tissue, and fat necrosis may occur in lipoma variants as well.
Is MRI necessary for a typical lipoma on ultrasound?
If all margins of the lesion are clearly defined on ultrasound, additional imaging is not required. MRI is added to exclude features of liposarcoma; CT/MR is used to assess the extent of large lesions.
What is a hyperechoic lesion in subcutaneous tissue when lipoma is suspected?
One variant is angiolipoma, a vascular variant of lipoma appearing as a small hyperechoic subcutaneous lesion on ultrasound.