Case Report: Malignant Interstitial Cell Tumor with Pulmonary Metastases in an Aging Naïve Fischer 344 Rat.

Fischer 344 rats are a well-characterized laboratory animal. Of note, their popularity as a research model declined due to the high incidence of spontaneous tumor development in the strain. Among these tumors, testicular interstitial cell tumors (ICTs) are frequently observed. We describe here three aged male Fischer 344 rats that were submitted for diagnostic pathology due to an increase in morbidity (increased respiratory effort, general malaise, and weight loss) in the research cohort. In all animals, there was bilateral enlargement of the testes. On the cut surface, multiple well-demarcated masses were compressing and replacing the original testicular tissue. Histologically, these masses were identified as ICTs. The tumors were composed of 3 distinct cell types that correlate with varying degrees of interstitial cell maturation. In one rat, intravascular pulmonary metastases were observed, which resembled the ICT cells morphologically. Malignant ICTs are an unusual finding despite the very high incidence of ICTs in Fischer 344 rats. Our assessment suggests that there are no morphologic criteria that reliably predict malignant and metastatic behavior of ICTs in F344 rats, highlighting the need for clinical monitoring of animals for which castration is chosen as a treatment.