Fisher, Cyril

With the first series of PRRX1 -rearranged tumors published in 2019, the spectrum of these so-called fibroblastic tumors has been expanded. Since then, several smaller case series have been published; however, our understanding of them continues to be quite limited given their rarity. We herein studied 18 additional cases, the largest series to date. Eighteen tumors present in 9 male, 8 female, and 1 nonbinary patient with a median age of 35 years (range: 11 to 70 y) and involved the neck (5), the chest region (4), thigh (3), back (1), shoulder (1), forehead (1), lower leg (1), axilla (1), and the parapharyngeal region (1). Clinical follow-up (9/18 tumors; 50%; median: 10 mo; range: 4 to 40 mo) showed consistent indolent behavior without local recurrences or distant metastases. On morphology, these tumors were characterized by well-circumscription and distinctive peripheral crescent-shaped vessels. They were composed of uniform spindle and round cells growing in short fascicles within often densely hyalinized collagen lacking significant mitotic activity, necrosis, or cytologic atypia. Immunohistochemically, about half of the tested tumors expressed focal to rarely diffuse S100 with occasional co-expression of SOX10. Interestingly, almost half of the tested cases also showed complete loss of RB expression. All but 1 tumor harbored a PRRX1::NCOA1 fusion, while 1 case harbored a novel PRRX1::EP300 fusion. We herein provide additional data on these exceptionally uncommon tumors, expand their molecular spectrum, and compare them to their close morphologic mimics to aid in accurate diagnosis and avoid confusion with potentially more aggressive neoplasms.

Objectives. Desmoplastic small round cell tumor (DSRCT) is an aggressive round cell sarcoma that arises in the abdominal cavity/pelvis of young males. We sought to expand its clinicopathologic spectrum. Methods. Cases of DSRCT presenting in patients >30 years of age or tumors arising outside of the abdominal cavity/pelvis were retrieved. Results. Thirty-four cases were identified. Sixteen tumors arose at atypical sites (head/neck, intracranial, thigh, axilla/shoulder, inguinal/paratesticular, intraosseous, and uterine corpus). The remaining 18 patients were older than 30 years, and their tumors involved the abdomen or pelvis. The majority of cases showed areas with classic histology, while 6 cases exhibited solid growth and 5 showed macronodular architecture. Cytologic appearance included round cell, rhabdoid, epithelioid, and small cell. Conclusion. DSRCT may arise at nonabdominal locations in both pediatric and adult populations, as well as intra-abdominally in older adults, and these tumors exhibit high rates of metastasis and morbidity.