Abstract
INTRODUCTION AND IMPORTANCE: Histiocytic sarcoma (HS) is a rare malignant neoplasm showing morphologic and immunohistochemical features of histiocytes. It is characterized typically by extranodal presentation and a poor clinical course, particularly in cases with disseminated disease. CASE PRESENTATION: This report documents a case of bifocal and aggressive HS in small bowel and lung revealed by acute peritonitis in a 63-year-old man. CLINICAL DISCUSSION: Despite its rarity, we believe that the correct diagnosis of HS is crucial for clinical treatment and prognostic prediction. CONCLUSION: The collection of additional cases of HS are important to obtain further progress in prognosis and guide treatment decisions.