{"paper_id":"cc2d2937-6334-4628-9757-0ac930f46e72","body_text":"Nodular histiocytic/mesothelial hyperplasia (NHMH) is a benign reactive process characterized by nodular histiocytic proliferation and scattered mesothelial cells [ 1 ]. The lesion was described by Rosai and Dehner in 1975 in a hernia sac, without the use of electron microscopy or immunohistochemistry [ 2 ]. NHMH has commonly been identified on surfaces lined by mesothelium, including the omentum, pleura, and pericardium, as well as in non-mesothelial lined organs such as the lung, inguinal region, urinary bladder, and pelvic cavity. Microscopically, NHMH consists of sheets of mesothelial cells and histiocytes within a fibrin meshwork, with or without an intervening vascular network. These lesions are noteworthy because they may be mistaken for malignancies both radiologically and pathologically [ 3 - 4 ]. We report a case of NHMH of the greater omentum in a 56-year-old HIV-positive male who presented with umbilical swelling and multiple omental nodules, which mimicked peritoneal carcinomatosis on imaging.\n\nA 56-year-old HIV-positive male patient presented with umbilical swelling; on examination, blood pressure was 120/70 mmHg, heart rate was 82 beats per minute, respiratory rate was 15 breaths per minute, and temperature was 37°C. There was no pallor, jaundice, or lymphadenopathy. On chest examination, abdominal CNS examination was unremarkable. The patient underwent surgical herniorraphy of umbilical hernia and excisional biopsy from the greater omentum.\nUpon surgical admission, laboratory test results for the CBC are found in Table  1 , and the microbiology workup is presented in Table  2 . Flow cytometry analysis shows a markedly inverted CD4/CD8 ratio due to a moderately increased CD8 absolute count with a relatively decreased CD4 count. NK and B cells are preserved. Increased expression of HLA-DR on CD8 T cells denotes the progression of HIV. CT of the abdomen reveals diffusely infiltrated greater omentum by soft tissue nodules with fat stranding and increased attenuation (misty appearance), as shown in Figure  1  (A, B, C). This finding is associated with several processes, including infiltration by tumors, inflammatory cells, edema, and fibrosis. The radiological differential diagnosis includes peritoneal carcinomatosis, peritoneal lymphomatosis, and tuberculous peritonitis. Additionally, there is a fat-containing umbilical hernia with fat stranding, as seen in Figure  1  (D).\nWBC: white blood cells, RBC: red blood cells, HGB: hemoglobin, HCT: hematocrit, MCV: mean corpuscular volume, MCH: mean corpuscular hemoglobin, MCHC: mean corpuscular hemoglobin concentration, RDW-SD: red cell distribution width standard deviation, RDW-CV: red cell distribution width coefficient of variation\nTB: tuberculosis, DNA: deoxyribonucleic acid, AFB: acid-fast bacilli, MGIT: mycobacteria growth indicator tube, L-J: Löwenstein–Jensen, IgG: immunoglobulin G, IgM: immunoglobulin M, anti-HCV: anti-hepatitis C virus, HIV: human immunodeficiency virus\nCT: computed tomography, IV: intravenous\nThe macroscopic examination of the surgically removed specimen described an irregular rectangular piece of fatty tissue with congested blood vessels, measuring 13.5 × 7.5 × 3 cm. Histological examination (Figures  2 - 3 ) reveals vascularized fibroadipose tissue showing sheets and aggregates of large histiocytic cells with round-ovoid nuclei, some with distinct nucleoli, and abundant eosinophilic cytoplasm intermixed with inflammatory cells dominated by plasma cells, along with occasionally interspersed eosinophils and a few multinucleated giant cells. No granulomas, necrosis, or malignancy were observed.\nH&E: hematoxylin and eosin\nH&E: hematoxylin and eosin\nImmunohistochemical staining on paraffin-embedded sections with adequate internal control was performed and revealed histiocytic cells that were immunoreactive for CD163, exhibiting a strong and diffuse pattern of staining (Figure  4 ). Calretinin staining highlighted the mesothelial lining of the omentum (Figure  5 ), and CK7-immunoreactive mesothelial cells were also observed (Figure  6 ).\n\nThis case involves a 56-year-old male HIV-positive patient with NHMH, which represents a rare occurrence in HIV-positive individuals. NHMH is a reactive process characterized by the non-neoplastic proliferation of histiocytes and mesothelial cells [ 5 - 6 ]. There are no characteristic clinical manifestations, and it is usually an incidental finding. The age of presentation ranges from four to 85 years, with a mean age of presentation between 40 and 50 years. However, the disease has also been reported in children and young adults. Reported cases in the literature show that there is no sex predilection for the lesion. A study by Chung et al. indicated that the lesion is slightly more prevalent in females [ 3 ]; however, a study by Michal et al. reported that the disease is more common in females, with a female-to-male ratio of 2:1 [ 4 ].\nNHMH is most commonly found on mesothelial-lined surfaces, namely the pleura, peritoneum, and pericardium. The most common anatomical site for NHMH is the peritoneum, followed by the skin, thymus, pleura, pericardium, and mediastinum (Table  3 ) [ 4 , 5 , 7 ]\nThe pathophysiology underlying NHMH is unknown. However, it occurs as a consequence of an unspecific reaction to an injury (e.g., trauma, inflammation, infiltrating malignancy, and surgical procedure) [ 4 ]. It has been suggested that the nodular aggregation of these histiocytes and mesothelial cells is mediated by adhesion molecules [ 5 ]. However, this lesion was found in organs that are devoid of a mesothelium, such as the endocardium, and inside a dissecting aneurysm. A possible explanation for that is the accidental insertion of mesothelial cells by a variety of procedures. These mesothelial cells, once inside the vascular space, lead to mechanical irritation, resulting in histiocytic proliferation. Mesothelial cells express CD34 and other adhesion molecules such as ICAM-1 and VCAM-1, which mediate the adhesion of mesothelial cells to histiocyte/monocyte elements by the L-selectin [ 12 , 14 ].\nNHMH is reported to be found in association with a variety of diseases, including ovarian cancer, chronic lymphocytic leukemia, and antiphospholipid syndrome. This lesion is very rare, and, to the best of our knowledge, this is the first reported case that is associated with HIV infection [ 4 ].\nMichal et al. published a study of 50 cases that described Langerhans cell histiocytosis (LCH) as a differential diagnosis of NHMH. LCH comprises histiocytes with unique oval-shaped grooved nuclei associated with eosinophilic infiltration, along with the expression of CD1a by histiocytes. Additionally, the proliferating histiocytes in NHMH may exhibit cytological atypia that could be misinterpreted as malignancies [ 4 ]. Furthermore, Chan et al. reported two cases of NHMH in transbronchial biopsies that were initially considered to be neuroendocrine neoplasms or meningiomas [ 7 ]. Chung et al. reported a case of pelvic nodular histiocytic aggregates/mesothelial hyperplasia in a patient with endometriosis and leiomyoma [ 3 ].\n\nNHMH is a benign lesion that should be considered in the differential diagnosis of all diffuse nodular masses in mesothelial-lined tissue. The diagnosis of NHMH is primarily based on histopathological examination, supported by immunohistochemical staining. In this study, we document the occurrence of NHMH in an HIV-positive patient. In our case report, NHMH presented with soft tissue infiltration of the greater omentum, exhibiting an appearance that mimicked peritoneal carcinomatosis on imaging. Awareness of this pathological entity is crucial to avoid erroneous diagnoses and aggressive, unnecessary management, which can negatively affect overall patient outcomes and prognosis. Accurate diagnosis is achieved through adequate histopathological assessment and clinicopathological correlation.","source_license":"CC-BY-4.0","license_restricted":false}