Paraspinal spindle cell neck lipoma: A (rare) mimicker of malignant entities

A 69-year old male patient with history of diabetes and hypertension was referred to our tertiary Hospital for investigation of a painless neck mass. Following ENT evaluation and fine-needle aspiration cytology, a neck CT and a PET scan were performed. The patient subsequently underwent surgical resection of the lesion.

Preoperative contrast-enhanced neck CT depicts a hypodense right paraspinal lesion, with well-defined margins, measuring 49 x 45 x 29 mm. The lesion was centred to the posterior cervical triangle and contacted the anterior scalene muscle. Beam hardening artefact from the shoulder girdle compromised evaluation of the inferior pole of the lesion, where subtle small solid components were retrospectively apparent. Mean lesion density was -1.9 HU. A smaller ROI, centred to the solid component showed a mean density of approximately 44 HU.  The lesion was well demarcated from adjacent structures and showed low uptake of 18F-fluorodeoxyglucose. PET-CT further identified cervical lymph nodes that were considered to have unspecific metabolism. Lesion characterization by fine-needle aspiration was compatible with a malignant diagnosis. The patient was unable to undergo preoperative MR imaging, due to anxiety/claustrophobia issues. Images from an unenhanced CT obtained 3 years after surgical resection showed no evidence of lesion recurrence.

Spindle cell lipoma (SCL) is a rare variant of lipoma with a predilection for men in their fifth to seventh decades of life, that is more frequently encountered in the subcutaneous tissue of the lower neck, shoulder and back [1]. Indeed, when compared to liposarcoma, SCL tends to have a more superficial location [2]. However, as our case illustrates, these lesions are not always restricted to a subcutaneous location. The radiological features of SCLs are variable, reflecting the variable contribution of adipose and non-adipose components. Unlike typical lipomas, SCLs may have minimal fat content [3]. In turn, the non-adipose component of these lesions frequently shows contrast enhancement [1,4], further hindering their differential diagnosis from higher-grade lesions, such as liposarcoma. In addition to unspecific radiologic features, cytologic characterization of SCL may also be misleading [5], since overlap exists with that of other spindle cell tumours such as neurofibromas, schwannomas and myxoid lesions, including myxoid liposarcoma, myxoma, myxoid fibrosarcoma, among others. This was indeed the case with our patient, in whom initial fine-needle aspiration of the lesion was compatible with myxoid liposarcoma. The treatment of SCL is complete surgical resection, which also frequently provides the final diagnosis. Local recurrences are rare [6]. Histologic analysis of the surgical specimen depicts bland spindle cells interspersed with mature adipocytes and ropelike collagen bundles [5]. Lesions may also contain mast cells, lymphocytes, myxoid tissue, and vascular elements [6].

In summary, the radiological features of spindle cell lipoma are variable and unspecific. It is most important to be aware that these features may overlap with those of liposarcoma. The hypothesis of spindle cell lipoma should also be considered when investigating a subcutaneous lesion in the posterior neck of a middle-aged man. Cytologic/histologic characterization may aid in narrowing the differential diagnosis. In our case, initial lesion characterization was performed by fine-needle aspiration (FNA), which has the advantages of convenience, decreased cost, minimal morbidity, and a theoretically lower risk of local contamination. However, FNA obtains a limited tissue sample and, when compared to core biopsy (which provides the pathologist with a block of tissue preserving tumour architecture and cellular interrelation), it has a lower accuracy in the diagnosis of soft tissue masses [7]. With regard to lipomatous soft tissue masses, limited tissue sampling may even lead to misdiagnosis of malignancy, with low-grade liposarcoma being diagnosed as benign lipoma [8]. As our case illustrates, the opposite scenario is also a possibility: a benign lipomatous lesion being misdiagnosed as malignancy.

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