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Subgaleal Lipoma: Imaging Findings

Subgaleal lipoma is a benign tumor of adipose tissue. It should be suspected when a semi-spherical avascular mass with well-defined margins, iso- or hyperechoic in most cases, with thin internal echogenic lines parallel to the long axis of the tumor, is observed between the galea aponeurosis and periosteum of the cranial bone. We report a series of cases of three patients who underwent surgical lesion excision and whose histopathological examination findings confirmed the diagnosis of lipoma. Main Teaching Point: The presence of long continuous echogenic lines within a lens-shaped soft tissue mass located beneath the galea aponeurosis may suggest the diagnosis of subgaleal lipoma.

Ultrasound is usually the first imaging technique used to investigate soft tissue masses of the scalp due to its accessibility and affordability. Lipoma of the scalp can be easily diagnosed on ultrasound because of its semi-spherical shape and the presence of thin internal echogenic lines parallel to the long axis of the tumor [1]. Subgaleal lipoma is recognized by the location between the galea aponeurosis and the cranial bone, most of them being reported in the forehead [2,3].
We report the imaging findings of subgaleal lipomas in three patients, reviewing the imaging characteristics of these lesions.

RESULTS
Our three patients were aged between 39 and 64 years at the time of diagnosis. The localization, size, and echogenicity of the lesions are presented in Table 1. All patients underwent surgical lesion excision, and histopathological examination confirmed the diagnosis of lipoma. The echogenicity of the lesions, compared to that of the adjacent fat on ultrasound, varied from one patient to the other. However, all lesions showed thin internal echogenic lines, and none showed calcifications or internal Doppler flow.
The ultrasonographic appearance of these subgaleal lipomas is shown in Figure 1. The computed tomography (CT) appearance of patient 1 is presented in Figure 2. The magnetic resonance imaging (MRI) of patient 2 is shown in Figure 3.

DISCUSSION
Subgaleal lipoma is a heterotopic tumor of adipose tissue typically occurring between the periosteum and the galea aponeurosis (epicranial aponeurosis) of the scalp [4]. Most of the subgaleal lipomas described in the literature are located in the forehead, but they can also develop in other parts of the scalp as in the case of two patients in our case series [1,4].
Ultrasound is usually the first imaging technique used for soft tissue masses owing to its accessibility and affordability. On ultrasound, the subgaleal lipoma appears as a solid mass, semi-spherical or lensshaped, with a flat or slightly concave base towards the periosteum of the cranial bone, and a convex surface towards the subcutaneous fat, parallel to the skin surface  [1]. Most of these lesions appear iso-or hyperechoic compared to the adjacent fat [1]. They contain multiple thin internal echogenic lines parallel to the long axis of the tumor [1,5]. Besides subgaleal lipoma, the differential diagnosis of scalp lumps also includes epidermoid cysts, dermoid cysts, trichilemmal cysts, intraosseous hemangiomas and other vascular malformations, and malignant lesions such as lymphoma, carcinoma, and metastasis [2].
CT scan is typically used to examine the bone alterations [2]. On unenhanced CT, the lesion appears oval or semi-spherical in shape, well-circumscribed, and  homogeneous, presenting a fat attenuation/density of -50 to -100 Hounsfield Units without calcifications [2,5].
MRI can be used to evaluate the extent of the lesion [2]. Subgaleal lipoma shows a high signal on T1-and T2weighted sequences, with a suppressed signal on fatsaturated sequences.

CONCLUSION
On ultrasound, the diagnosis of subgaleal lipoma should be suspected in the presence of a lens-shaped or semi-spherical soft tissue mass located between the galea aponeurosis and periosteum of the cranial bone that is iso-or hyperechoic, with thin continuous echogenic intralesional lines parallel to the long axis of the lesion.