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Cutaneous Metaplastic Synovial Cyst.

A 31-year-old male patient presented to the surgical outpatient department at the Armed Forces Hospital, Muscat, Oman, in September 2014 with a swelling on the left side of his forehead that had been on-going for 10 months. A clinical examination showed a subcutaneous, mobile and non-tender swelling. He had a past history of a similar swelling at the same site, which had been operated on four years previously at another hospital. No reports were available regarding the nature of this previous swelling. 
 
At presentation, the swelling was diagnosed clinically as a recurrent dermoid cyst and was excised and sent for histopathological examination. The pale grey firm nodular mass (1.2 × 1.0 × 0.8 cm) showed a solid cystic mass when sliced. An irregular cyst lined by hyperplastic synoviocytes forming broad villi and papillae with hyalinised cores infiltrated by lymphocytes and plasma cells was observed on microscopic examination [Figure 1]. Foci of chond-roid metaplasia, haemorrhage and pigment-laden macrophages surrounded by skeletal muscle with atrophic changes and giant cell formation were noted. A histopathological diagnosis of cutaneous metaplastic synovial cyst (CMSC) was made. Immunohistochemistry (IHC) showed vimentin-positive [Figure 2A] and cluster of differentiation (CD) 68-positive [Figure 2B] synoviocytes. However, IHC was negative for pan-cytokeratin and CD34. 
 
 
 
Figure 1: 
 
Haematoxylin and eosin stain at ×4 magnification showing the cyst wall with broad papillae and villi lined by synoviocytes (red arrow) with hyalinised cores (black arrow) in a 31-year-old patient with a cutaneous metaplastic synovial cyst. 
 
 
 
 
 
Figure 2A & B: 
 
Immunohistochemistry stains at ×20 magnification showing (A) vimentin-positive (arrows) and (B) cluster of differentiation 68-positive (arrows) synoviocytes in a 31-year-old patient with a cutaneous metaplastic synovial cyst.