Original ContributionWhorling cellular perineurioma: A previously undescribed variant closely mimicking monophasic fibrous synovial sarcoma

- Whorling cellular perineurioma (WCP) morphologically strongly resembles monophasic fibrous synovial sarcoma.
- Morphology, IHC and electron microscopy corresponded to the perineurial cell derivation.
- The perineurial cells were packed and created a cellular whorls and/or sheets in a scarce stroma.
- 3 of 4 cases occurred on the hands or feet.
- WCP may represent an extremely cellular variant of sclerosing perineurioma.

The authors present a distinctive perineurioma (PN) variant which morphologically strongly resembles monophasic fibrous synovial sarcoma (MSS). The patients were 3 males and 1 female. The age ranged from 15 to 61 years (mean: 44 years). Locations included the sole, lower jaw, palm and foot. The tumor size ranged from 1.3 cm to 2.5 cm in the largest dimension (mean 1.8 cm). Morphologically, all tumors had an identical, monotonous appearance. The perineurial cells were closely packed and created a confluent cellular whorls and/or sheets in a scarce stroma, with only focally discernible long, slender cytoplasmic processes typical for perineurial differentiation. The nuclei were rounded or slightly elongated to tapered, without nuclear atypia. Mitoses were rare to completely absent. Atypical mitoses, hemorrhage, necrosis or calcifications were not present. The proliferative index (Ki-67) was 1-3%. All analyzed tumors were positive for EMA, Claudin-1, GLUT-1 and negative with S100 protein, CD34, OSCAR, CK7 and TLE-1. Two cases were tested by fluorescence in situ hybridization and neither showed alterations of the SYT gene. One case studied by electron microscopy showed characteristic features of perineurial differentiation. Follow-up was available for two patients both of which showed no evidence of disease at 8 years and 6 months, respectively. Based on their bland morphology, perineurial features and presumably benign clinical outcome we propose the term “whorling cellular perineurioma” for these tumors, which may represent an extremely cellular variant of sclerosing PN. Awareness of this PN subtype and its distinction from MSS is of utmost clinical significance.