Lipomas are the most common benign extremity soft tissue tumors. They can occur in the subcutaneous tissues or in deep somatic tissues either within muscle or more commonly between muscles. Lipomas in general have an unpredictable growth rate. In particular, those that are deep can become very large and efface neurovascular structures and adjacent bone. Unlike malignant tumors, lipomas generally do not invade or erode bone and, despite their prodigious size, cause minimal pain ^[1,2,3,4].  We describe the clinical and radiographic findings in three patients with large lipomas that caused erosions in bone. It is important to consider lipoma on the differential when a patient presents with the findings and radiographic findings described below, as treatment will differ from a malignant tumor.

Case 1

A 57-year-old female presented with a 1-month history of a progressively enlarging mass and pain in her left buttock. The pain was worse with activity but also present at rest. She required analgesic medications for relief of her symptoms. There was no history of antecedent trauma and she denied paresthesias or weakness in the affected lower extremity. She had no bowel or bladder dysfunction and reported no constitutional symptoms or weight loss. On physical examination, there was a large, slightly firm, and non-tender mass occupying nearly the entire left buttock. There were no changes in the skin overlying the mass and examination of the spine revealed no deformity or focal tenderness to deep palpation. She had a full painless, active, and passive range of motion in her lumbosacral spine and hips. She had no neurologic deficit in either lower extremity and her vascular examination was normal. Anterior-posterior (AP) pelvis radiographs showed no apparent osseous abnormality. A technetium-99m pyrophosphate bone scintigram demonstrated no abnormal focal uptake in the involved area. Magnetic resonance imaging (MRI) showed a large lobulated mass in the left buttock abutting the ilium with a homogenous and hyperintense signal on both T1 and T2 axial and coronal images. There was a focal area of hyperintense signal penetrating the outer table of the ilium and replacing the marrow; the inner table was intact (Fig. 1a). Tumor extension into the left sciatic notch was also seen with mild displacement of the pelvic viscera (Fig. 1b). Computed tomography (CT) of the pelvis better demonstrated osseous scalloping of the outer table of the ilium with a well-defined border between the bone and the soft tissue mass (Fig. 1c).

The patient’s history, physical examination, and imaging studies were consistent with a deep intramuscular fatty tumor; however, the finding of osseous involvement was unusual and unexpected. Intraoperatively, the tumor was situated deep to the gluteal muscles. The mass itself was smooth, soft, and yellowish in appearance with a thin well-defined capsule. The tumor was completely resected with an intact capsule. A small segment of the lesion was found to penetrate the outer table of the ilium, replacing the cancellous bone but leaving the inner table of the ilium intact. The sciatic nerve was displaced but minimally compressed by a protruding lobule of tumor in the sciatic notch. The mass measured 24 × 22 × 10 cm. Histopathologic evaluation revealed mature appearing lipocytes with small flattened nuclei. No lipoblasts were identified; there was focal organized fat necrosis.

Case 2

A 68 year-old man presented with a recurrent, non-painful mass in the right buttock. He had two prior resections at outside facilities. The first resection was of an intrapelvic mass performed 27 years before presentation through an anterior approach. The patient reported that this mass was a lipoma but no documentation of the histopathologic evaluation was available for review. The second resection was of a right buttock mass done 1 year before presentation through a posterior approach. Histopathologic interpretation of this mass at the outside hospital was that of a lipoma. Since the second resection, he noted a slow return of the mass along with pain in his right buttock that was associated with prolonged periods of sitting. On physical examination, there was a fullness of the right buttock with an apparent soft tissue mass that was soft and non-mobile. MRI demonstrated a mass that was hyperintense on T1 and T2 pulse sequences consistent with a lipomatous mass. The mass was predominantly in the buttock but extended through the sciatic notch into the retroperitoneal space (Fig. 2a). The majority of the mass appeared to be benign fat with some smaller areas with increased stranding. The mass abutted the posterior aspect of the ilium and appeared to have eroded through the posterior cortex in several places (Fig. 2b and c). There were no changes in the marrow adjacent to the erosions into the ilium. The changes in the ilium were better demonstrated on the CT scan with scalloping of the posterior ilium adjacent to the mass (Fig. 3a and b).

Intraoperatively, the mass was adherent to the posterior aspect of the ilium. It was separated bluntly from the bone and removed intact utilizing a posterior approach. The mass measured 14 × 14 × 5 cm. Histologically, the mass was consistent with a lipoma with fat necrosis and fibrosis. There were no atypical or overtly malignant features observed microscopically.

Case 3

A 79-year-old woman presented with a complaint of a non-painful soft tissue mass in the left axilla of 2-month duration. An incomplete resection of the mass was performed at another institution 6 months before presentation. The histopathologic interpretation of this mass at the outside hospital was that of a low-grade well-differentiated liposarcoma, likely referred to what is now named an atypical lipomatous tumor. On physical examination, there was a large, deep mobile, and non-tender mass adjacent to the chest wall and lateral to the scapula. There was another large, non-tender, bulging mass overlying the posterior surface of the left scapula that did not appear on examination to be contiguous with the axillary mass. The patient was unaware of the posteriorly based mass. An AP radiograph showed focal, well-defined radiolucent defects in the neck and spine of the scapula (Fig. 4). CT and MRI showed a large, multilobulated soft tissue mass around the posterior aspect of the scapula and deep to the supraspinatus and infraspinatus muscles. The mass perforated the scapula from posterior to anterior and extended laterally into the axilla; it was contiguous with the axillary mass (Fig. 5a and b). The portion of the mass over the posterior aspect of the scapula was homogenous and hyperintense on T1 and T2 pulse-weighted sequences, whereas the axillary mass was more heterogeneous and somewhat more hypointense on T1 pulse-weighted sequences.

Intraoperatively, the posterior fatty mass in the supra- and infraspinatus regions penetrated anteriorly through the base of the scapular neck as well as the body of the scapula. The perforations had well-defined, rounded margins. The posterior and axillary components of the tumor were completely excised. Histopathologic analysis of the resected tumor showed mature lipocytes with interspersed fat necrosis and reactive changes but no changes that could be ascribed to an atypical lipomatous tumor or a malignancy. The axillary portion of the mass demonstrated infarcted adipose tissue with osseous metaplasia and organizing fat necrosis, accounting for the heterogeneous signal changes on MRI. These changes may have been caused by previous surgical manipulation.

Lipomas, the most common benign soft tissue tumors of the extremities, can often be accurately diagnosed with advanced imaging studies ^[1,2,3,4]. Lipomas are typically homogeneously hyperintense on both T1 and T2-weighted pulse sequences, suppress on STIR images, and lack prominent septations. Rarely, lipomas erode adjacent bone as demonstrated in the cases described herein. Parosteal, intraosseous, and lipoma arborescens lipomas are among the tumor types that should be considered on the differential. Lipomas that are primarily osseous in origin are described as either intraosseous lipomas or parosteal lipomas ^[5]. Parosteal lipomas arise on the surface of the bone, originate in the outer fibrous layer of the periosteum, and may exhibit varying degrees of mineralization in the soft tissues ^[6,7,8]. However, in the cases presented herein, the lipomas arose adjacent to the bone rather than within the bone or periosteum. Histologically, lipomas demonstrate mature lipocytes with interspersed fat necrosis and reactive changes, without findings that could be ascribed to an atypical lipomatous tumor or a malignancy (Fig. 6).

Large juxtacortical malignant soft tissue masses can penetrate bone, although the periosteum is usually an effective barrier to tumor growth. Benign soft tissue tumors, except for neurofibromas and fibromatosis, rarely erode long bones. The large flat bones however are relatively devoid of periosteum and the cortices are thin, thus rendering these bones vulnerable to pressure erosion by expanding soft tissue masses. Presumably, pressure produced by these large soft tissue masses at the bone soft tissue interface induces focal osteoclastic resorption of bone, resulting in the defects observed radiographically (specifically, well-defined lucencies). Grossly, these changes in the bone appear as large scallops. To our knowledge, there have been very few reported cases of lipomas causing osseous erosions, and no reports of these erosions occurring in the pelvis. Lloyd reported a case of a lipoma causing osseous erosions in the scapula ^[9]. Leffert reported a case of erosion of an adjacent metacarpal by a lipoma arising in the palm and Kitagawa reported a case of erosion of a distal phalanx by a lipoma in a finger ^[10,11]. Other reports of cases of erosion of bone include those by Shawker (a rib erosion from an intrathoracic lipoma in an infant), Braunschweig (erosion of the bones of the foot by a spindle cell lipoma) and by Preul (angiolipomas of the spine) ^[12,13,14]. Chae reported a case of lipoma arborescens of the glenohumeral joint eroding into the humeral head and Ryu described marginal erosions in subchondral bone in patients with lipoma arborescens of the knee ^[15,16]. In the three cases reported here, the erosion of the ilium and the scapula is postulated to have occurred due to direct pressure on the bone by the large expanding lipomas growing deep to the gluteal muscles in the first two cases and deep to the posterior scapular muscles in the third case. The underlying cortex of the ilium and scapula is quite thin and thus more vulnerable to pressure erosion by an expanding adjacent soft tissue mass.

In summary, large lipomatous tumors arising around the flat bones may cause erosions in bone that can be seen on MRI as lobular extensions of fat into the outer cortex. The erosions are best observed on CT imaging. These tumors may also have areas of mineralization and fat necrosis, resulting in heterogeneity that may be interpreted as atypical or overtly malignant. It is important to recognize that both of these findings may be seen in patients with benign tumors of fat, where the appropriate treatment is marginal tumor excision alone.

When a patient presents with a soft-tissue mass that erodes adjacent bone, a benign lipoma should be considered in the differential diagnosis. Although many referring attendings will assume that the findings are associated with a malignant process, further histopathological analysis can differentiate the mass as a benign lipoma eroding adjacent bone, which is a much more benign condition than a malignant tumor.