STAT6-positive intraorbital papillary tumor: a rare variant of solitary fibrous tumor?

We experienced a peculiar case of orbital mesenchymal tumor in a 22-year-old Japanese woman. The tumor showed a papillary proliferating pattern, but no typical hemangiopericytomatous staghorn vessels. The tumor was composed of round to oval shaped cells with oval nuclei and mild nuclear atypia. Abundant vascular cores were present in the central portion of papillary proliferations of tumor cells. Immunohistochemistry revealed that the tumor cells were positive for CD34 and bcl2. Moreover, they showed positive nuclear signals of STAT6, which have recently been shown to be specific for solitary fibrous tumors. In the literature, only one case of solitary fibrous tumor with papillary and retiform growth pattern has been reported, but the case partially showed the typical staghorn vessel pattern. Although the definite diagnosis is difficult in the settings of the unusual histology and the deficiency of NAB2-STAT6 fusion, it is possible that this STAT6-positive intraorbital papillary tumor is a very rare variant of solitary fibrous tumor.