Teaching Case

A 52-year-old woman with no relevant medical history was referred to the orthopaedic oncology clinic for assessment of a slowly growing, non-painful, non-tender lump in the subcutaneous tissue of the posteromedial thigh. This had been present for 4 years, measuring approximately 2cm in maximal diameter on clinical examination.

Ultrasound demonstrated a well-delineated heterogeneously hypoechoic lesion within the subcutaneous fat of the posterior thigh containing a small number of vessels on colour doppler (fig. 1a, 1b).

MRI including contrast sequences demonstrated a subcutaneous lesion extending to the skin measuring 2.1cm x 1.9cm x 1.3cm. This was largely intermediate signal on T1WI, with an incomplete peripheral low signal margin (fig. 2). Heterogenous high signal was observed on T2WI (fig. 3) and T1 TIRM sequences (fig. 4). Post-contrast, there was avid enhancement with some discrete internal, relatively hypoenhancing septations (fig. 5a, 5b). No calcifications were evident on MRI.

The lesion had no invasive features with no deeper underlying abnormality. Following percutaneous biopsy and subsequent marginal excision by the sarcoma surgeons, histology demonstrated a collagenous and myxoid background containing ductal structures and epithelial strands with some squamous differentiation, keratocyst formation, and focal calcification consistent with benign chondroid syringoma (CS).

CS is a rare tumour of sweat gland origin (<0.01% of all primary skin tumours) usually occurring in the head and neck but also reported in locations including extremities, scrotum, eyelids and brain [1,2,3,4].

Presentation may be with a slowly enlarging lump with symptoms dependent on location [2,5]. Most cases are benign but rare malignant cases can occur which are typically invasive with a high rate of recurrence. Features concerning malignancy include extremity and torso origin, size >3cm and female gender [1,4,6].

Few reports focus on the imaging features of CS and most are MRI based. Benign lesions have been described as heterogeneous, isointense to muscle & low signal on T1WI, and heterogeneous and high signal on T2WI [3,5,6,7,8]. Kakitsubata et al described a lesion with a solid component containing chondroid material and fat, surrounded by fluid and an incomplete peripheral rim of low signal on T1, T2 and fat-saturated T2 sequences [7]. Kerimoglu et al described a lesion with focal high signal on T1WI, homogenous high signal on STIR and cystic components [6]. Fibrovascular septae have also been described [5]. In two cases post-contrast findings were not described [3,7]. Otherwise, enhancement is a consistent finding [5,6,8].

Ryu et al describe benign CS on ultrasound as a well capsulated hyperechoic mass with a central hypoechoic scar-like portion from which radiated hypoechoic hypervascular septa-like structures[5].

We found 2 cases describing malignant CS on imaging. Park et al reported a lesion of low signal on T1WI “higher than that of surrounding muscles” and heterogeneous signal on T2WI. However, review of their images demonstrates a heterogenous lesion on T1WI. They also describe peripheral low signal and heterogenous enhancement [2]. Nicolau et al reported heterogeneous predominantly intermediate signal on PDWI, heterogeneous internal high signal with peripheral intermediate signal on T2WI and heterogeneous high signal on STIR [9].

With limited reports, imaging characteristics of CS are nonspecific and there is overlap in the appearances of benign and malignant cases. Features such as heterogeneous signal on T1 & T2WI, enhancement and a peripheral rim of low signal are described in both. Imaging is however useful in narrowing the differential diagnosis and in our case diffuse enhancement, lack of macroscopic calcifications and no exophytic components made epidermoid inclusion cyst, pilomatrixoma and dermatofibrosarcoma protuberans less likely respectively. However, other malignancies including pleomorphic dermal sarcoma could look similar, ultimately necessitating biopsy for confirmation of diagnosis.

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