Intrathoracic myolipoma from parietal pleura with oestrogen and progesterone receptor

Abstract Myolipoma is a rare benign lipomatous soft tissue neoplasm, occurring most frequently in adults in the abdomen, pelvis or retroperitoneum. We presented a case of a 39‐year‐old female with an epipleural lesion at the left paraspinal region identified using computed tomography. Magnetic resonance imaging revealed a fat‐containing lesion attached to the pleura over the left paraspinal region. Surgical resection was performed, and histopathological analysis revealed a tumour composed of interlacing bundles of neoplastic smooth muscle cells with low cellularity and rare mitotic figures intermixing with mature adipocytes. The presence of oestrogen and progesterone receptors in smooth muscle cells was noted. The mass was reported as myolipoma. To the best of our knowledge, this case is the first description of an intrathoracic myolipoma.


INTRODUCTION
Myolipoma is an extremely rare benign soft tissue tumour composed of a mixed proliferation of adipose and smooth muscle tissues. It presents as a large, painless and palpable mass in most patients. The tumour is benign with no reported cases of recurrence nor metastasis after surgical resection. Most cases of this tumour have been reported in the retroperitoneum, pelvis and abdomen. There are fewer reports of the tumour in the round ligament, spinal cord, eyelid, subcutaneous tissue and pericardium. We present the first case of an intrathoracic myolipoma described in literature.

CASE REPORT
We present a case of a 39-year-old female who reported left upper back soreness over the past 6 years (since her first pregnancy). A chest x-ray revealed a faint bulging mass over the posterior mediastinum, left paraspinal region, which was not found in an x-ray taken 4 years previously. Computed tomography (CT) disclosed an epipleural, heterogeneous, contrast-enhancing,, fat-containing lesion at the left paraspinal region. Magnetic resonance imaging (MRI) showed an oval, fat-containing, contrast-enhancing mass in the left paraspinal region attached to the pleura ( Figure 1). Thoracoscopic tumour removal was performed in September 2020. An oval, fat-containing, pedunculated tumour over the posterior fourth intercostal space was noted. Surgical resection by way of disconnection of the tumour peduncle and the parietal pleura was performed. The mass measured approximately 8 Â 5.5 Â 3.5 cm in size and was made up of soft yellow tissue with firm white tissue. Histopathological analysis revealed the tumour was composed of smooth muscle cells with low cellularity and rare mitotic figures intermixed with mature adipocytes (Figure 2). Immunohistochemical analysis was positive for smooth muscle actin (SMA), HHF-35, desmin and hcaldesmon, and negative for melanoma markers (HMB45 and Melan-A), MDM2, CDK4, CD34 and Sat6, which suggested myolipoma. The presence of oestrogen and progesterone receptors was noted, strongly in smooth muscle cells and with low positivity in mature adipocytes. Followup occurred 9 months later (July 2021) and showed no significant evidence of recurrence or metastasis. To the best of our knowledge, this is the first description of an intrathoracic myolipoma.

DISCUSSION
Myolipoma, a rare benign tumour that was first described by Meis and Enzinger in 1991, was mostly reported in the retroperitoneum, pelvis and abdomen. 1 There are fewer reports of the tumour in the round ligament, spinal cord, eyelid, tongue base, mesentery and pericardium. Myolipoma occurs frequently in adults, with a slight predilection for women, and presents clinically as an enlarged painless mass or as a variety of symptoms according to anatomic site.  3 have similar radiological features that should be considered as part of a differential diagnosis; hence, histopathology is essential for the diagnosis of myolipoma. Myolipoma has been described as a completely or partially encapsulated mass. The presence of a stalk supporting the tumour was noted in the current case, but has been rarely mentioned in other published cases. Histologically, it consisted of mature adipocytes and well-differentiated smooth muscle. Differential diagnosis includes lipoleiomyosarcoma, well-differentiated liposarcoma, dedifferentiated liposarcoma, spindle cell lipoma, mammary-type myofibroblastoma and fat-forming solitary fibrous tumour. In the present case, the immunohistochemical stains were positive for SMA, HHF-35, desmin and h-caldesmon, and negative for melanoma markers (HMB45 and Melan-A), MDM2, CDK4, CD34 and Sat6. Differential diagnosis of myolipoma using immunohistochemical staining to distinguish other neoplasms is summarized in Table 1. HMGA2 immunohistochemical staining was not available; however, it is worth noting that a positive stain for HMGA2 is not a required marker for the diagnosis of myolipoma. However, it has been reported that approximately 60% of myolipoma express HMGA2, which indicates that approximately 40% of myolipomas are negative. HMGA2 is widely expressed in other lipomatous tumours, such as lipoma (86%), well-differentiated liposarcoma (86%) and dedifferentiated liposarcoma (67%). 4 Due to the low sensitivity and specificity of HMGA2, it may not be useful in the differential diagnosis of myolipoma.
Consistent with the current case, the presence of both oestrogen and progesterone receptors was reported in myolipomas of the broad ligament and pericardium. Symptoms were first disclosed following the patient's first pregnancy. Rapid tumour growth occurred in the past 4 years, in which a second pregnancy occurred. It has been suggested that the growth of the myolipoma could be affected by pregnancy-related hormones 5 ; however, further studies are required to confirm and expand on this suggestion.
Myolipoma presents with a benign clinical course, with surgical excision as a curative treatment. To date, there have been no reports of recurrence, metastasis or malignancy. In summary, myolipoma should be considered as a differential diagnosis of fat-containing intrathoracic masses in order to make an accurate diagnosis. Specific pathological and immunohistochemical characteristics of the tumour, including a positive stain for SMA, desmin and/or HMGA2, as well as a negative stain for HMB-45 and MDM2, are helpful in the diagnosis of myolipoma.

CONFLICT OF INTEREST
None declared.

ETHICS STATEMENT
The authors declare that appropriate written informed consent was obtained for publication of this case report and accompanying images.