Microcystic stromal tumors (MCST), first described in 2009 by Irving et al., are rare ovarian neoplasms. The entity was introduced into the 2014 WHO classification of tumors of female reproductive organs in the group of sex cord-stromal tumors, which is rather heterogeneous. We studied three cases of ovarian tumor with the characteristic morphological features and immunohistochemical marker profiles of MCST. The three tumors showed micro, and macrocystic patterns with solid areas, and were composed of small round to spindle-shaped cells, without atypia. The tumors diffusely and strongly expressed CD10, FOXL2, and nuclear β-catenin, but without immmunoreactivity for hormone receptors, calretinin, or inhibin. Genome analyses showed no somatic mutation of exon 1 of the FOXL2 gene and of exons 24 and 25 of DICER1 gene, the latter not having been reported previously. The patients are well, without evidence of tumor progression 1 to 10 years after diagnosis.The absence of FOXL2 and DICER1 gene mutation, along with strong FOXL2 immunoreactivity provides additional evidence to place MCST within pure gonadal stromal rather than sex cord ovarian tumors.

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