Teaching Case

A 64-year-old female patient presented for evaluation of right gluteal mass that was found incidentally on CT of abdomen for abdominal pain. She reports pain over the mass if she sits on the area for a prolonged period of time. She denies any other symptoms.

A right gluteal mass was found incidentally on abdominal CT. MRI was obtained for further evaluation and showed an irregular lesion with spiculated margins that involves the superficial aspect of the right gluteus maximus muscle and extends into the subcutaneous fat. The mass is hypointense on T1, T2, and STIR MRI (Fig. 1-3). T1 post contrast MRI demonstrated enhancement of the lesion (Fig. 4). Non-enhanced CT images were obtained for biopsy planning (Fig. 5).

Background: Granular cell tumours (GCT) are rare neoplasms thought to be derived from Schwann cells. This is because GCTs have high rates of expression of the s-100 protein which is derived from neural crest cells. GCTs are usually benign, with less than a 2% malignancy rate [1]. GCT affects women more than men and occurs most commonly in ages between 20 and 40 [2].

Clinical Perspective: Clinically the tumours usually present as painless, firm, solitary nodules of less than 3 cm. GCT are most common in the oral cavity (40%) followed by skin and subcutaneous tissue (33%) with less common areas including the GI tract, larynx, and bronchus [3]. Definitive diagnosis is made by histopathological examination. Criteria suggestive of malignant GCT include necrosis, spindling, vesicular nuclei with large nucleoli, increased mitotic activity, high nuclear to cytoplasmic ratio, and pleomorphism [4]. In this case CT-guided biopsy showed granular cell tumour of benign variety.

Imaging Perspective: GCT are usually isointense or hypointense on T1 and T2 MRI, while malignant lesions are more likely to be hyperintense on T2 MRI. Characteristic imaging findings of benign GCT include size less than 4 cm, round shape, and superficial location without invasion into adjacent tissue. Features of malignant GCT include size larger than 4 cm, irregular shape, and deep location with infiltration into adjacent muscle [5]. Enhancement patterns have not yet been clearly defined. In this case the differential diagnosis following imaging included desmoid or fibroma. While this tumour had several features of malignancy such as irregular shape and infiltrating margins, it was confirmed to be benign GCT on biopsy.

Outcome: Prognosis primarily depends on whether the tumour is benign or malignant. Malignant tumours have a 40% mortality rate and high rates of recurrence and metastasis, whereas benign tumours rarely recur following complete resection. The treatment of choice for GCT is wide surgical excision [2]. Adjuvant chemotherapy and radiation therapy are options for malignant tumours, however, their effectiveness has not been clearly established. This patient underwent surgical resection with negative margins and experienced no complications during the procedure. The patient will undergo repeat MRI in 6 months for follow up.

Teaching Points: This case illustrates characteristic imaging findings of GCT. The definitive diagnosis of GCT is made by histopathological examination. Malignant GCT should be followed closely due to high rates of recurrence and poor prognosis.