PDL1 Expression and Outcomes in Histiocytic Neoplasms.

Introduction: Histiocytic sarcoma (HS) and interdigitating dendritic cell sarcoma (IDCS) are both extremely rare and aggressive histiocytic neoplasms that are most often treated with lymphoma-based chemotherapy; however, there is growing use of immune checkpoint inhibitors (ICI) in the second- or further-line settings as prognosis is typically poor in these neoplasms. Case Presentations: We describe two cases: first, HS in a 60-year-old man with primary tumor in the right anterior lower limb, and second, IDCS in an 82-year-old man in the right para-parotid region after initial wide local excision of a right postauricular mass suggestive of a pleomorphic undifferentiated sarcoma. Systemic imaging showed retroperitoneal, inguinal, and pelvic adenopathy in the first case and no distant metastases in the second case. Both were diagnosed via biopsy with extensive immunohistochemistry and were also found to be strongly positive on PDL1 testing. Treatment was initiated with ICIs in both patients, which was paired with palliative radiotherapy for the HS patient. Both cases exhibited a durable response to treatment upon repeated systemic imaging. Conclusion: To our knowledge, these are the first-reported cases of the use of ICIs in the first-line setting in these histiocytic neoplasms and highlight their efficacy. The significant elevation in PDL1 expression seen in both cases may serve as a potential biomarker of excellent response in these neoplasms.