Clear Cell Stromal Tumor of Lung: A Clinicopathologic, Immunohistochemical, and Molecular Characterization of 8 Cases.

Clear cell stromal tumor (CCST) is a recently described mesenchymal neoplasm of the lung, characterized by spindle cells with variably clear-to-pale eosinophilic cytoplasm and prominent vascularity, as well as a recurrent YAP1::TFE3 gene fusion in most cases. Diagnosis can be challenging given its rarity and the lack of supportive immunohistochemical markers aside from TFE3. To date, less than 20 cases have been reported and data on clinical behavior are also limited. While most appear to be benign, aggressive behavior has been reported rarely. Here, we present the largest multi-institutional series of CCST to date, comprising a total of 8 cases and including 6 previously unpublished cases. We investigate its clinicopathologic and genomic features, while also assessing the diagnostic utility of immunohistochemistry (IHC) for YAP1 C-terminus (YAP-CT). Five patients were male and three were female. The median age was 59 years (range: 35 - 84). In all cases, a TFE3 rearrangement was demonstrated by either FISH or DNA / RNA sequencing. In 7 tumors, the YAP1::TFE3 fusion was identified by sequencing. We demonstrate that the combination of YAP1-CT loss and TFE3 overexpression by IHC reliably predicts an underlying YAP1::TFE3 fusion in these neoplasms and may be more sensitive than TFE3 FISH. Although the median follow-up time for our study was short (18 months, available in 7 cases), all cases pursued a benign clinical course, with no recurrences or metastases. Our study provides further characterization of this novel entity, supporting its wider recognition.

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