Teaching Case

A 56-year-old man presented with a two-year history of a painless slowly growing mass in the perineum. On physical examination, there was a soft and well-demarcated oval-shaped mass in the anterior perineal area. Past medical history and laboratory studies were unremarkable.

Pelvic MR imaging showed a well-defined perineal mass displacing the spongiosum corpus at the radix penis level, surrounding the spongiosum corpus at the pelvic diaphragm and extending to the pelvis surrounding the right prostatic apex (Fig. 1). The tumour showed intermediate signal intensity on T1-weighted images (Fig. 2), heterogeneous hyperintense signal on T2-weighted images (Fig. 1) and pronounced enhancement after intravenous gadolinium administration (Fig. 2). A swirled or layered appearance was seen on T2-weighted and gadolinium-enhanced images.

The patient underwent surgical resection of the perineal tumour. The mass was densely adherent to the spongiosum corpus and prostatic apex, and complete surgical resection was not possible.
The excised perineal specimen appeared well-defined with elastic consistency and light brown colour with whitish areas (Fig. 3a). Histopathologic examination was consistent with a vascular myxolipoma (angiomyxolipoma) (Fig. 3b and 4).

Angiomyxolipoma (vascular myxolipoma) is a rare variant of lipoma, with very few cases reported in the literature in patients from 9 to 69 years [1, 2]. This tumour usually presents as a slowly growing, well-demarcated, painless subcutaneous mass on the scalp or extremities [2-5]. To our knowledge, our case is the first one presented as a large perineal mass extending from subcutaneous tissue to the pelvis through the pelvic diaphragm. This is a known route of tumoural extension between the pelvic cavity and the perineum [6], which is a challenge in management, given the proximity of the tumour to genitourinary and anorectal structures.
The histopathologic features of angiomyxolipoma include an admixture of a myxoid area with relatively few spindle-cells, mature adipose tissue without lipoblasts, and numerous thin- and thick-walled vessels [1-3]. Spindle cells in myxoid areas immunoreactivity for vimentin and CD34 [2, 3]. The mature adipocytes express S-100 protein, and the vessel walls expressed CD34 and SMA The tumour is negative for HMB45 [2, 3].
The MRI features of angiomyxolipoma have been previously described in only one case report, involving the suprapatellar area [4]. In this report, the tumour presents as a well-defined complex mass with a large peripheral fatty component and intense enhancement of the nonadipose central area. Our case appears to be quite different from previous reported cases, except for the well-defined margin and the intense enhacement after gadolinium administration. In our case, the mass showed intermediate signal intensity on T1-weighted images, although aggregates of microscopic fat within the tumour were seen on the pathological specimen. Microscopic fat would not be visible at MRI, unless chemical-shift imaging was performed. We also observed layered or swirled hypointense signal intensity within the hyperintense myxoid component of the tumour on T2-weighted MR images.
The swirled or layered appearance on MRI was attributed to the enhancement of the fibrovascular stroma. It usually develops in tumours that are stretched as they protrude through the pelvic diaphragm and has been reported as a characteristic finding in aggressive angiomyxoma and angiomyofibroblastoma [6, 7].
The tendency to infiltrate the surrounding soft tissue are key features in differentiating aggressive angiomyxoma from angiomyxolipoma [6]. Angiomyofibroblastoma was excluded in our case because it is miofibroblastic differentiation with HMB-45 inmunoreactivity, it is CD34 negative, the vessels are branching pattern and the tumour has epiteliod and plasmocitoid cells.