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Background: Follicular dendritic cell sarcoma (FDCS) is an extremely rare malignant tumor caused by proliferation of antigen-presenting follicular dendritic cells. The tumor most commonly arises in lymph nodes, with fewer than 30% of cases originating in extranodal sites. Case Presentation: We report the case of a 39-year-old previously healthy male patient who presented with paraneoplastic pemphigus in the setting of a large intraperitoneal mass with no associated lymphadenopathy. The biopsy results showed a cellular proliferation of mildly to moderately atypical oval-to-spindle-shaped cells admixed with a variably dense lymphoplasmacytic infiltrate suggestive of FDCS versus inflammatory myofibroblastic tumor. A pathology review following total resection of the abdominal mass confirmed the diagnosis of FDCS. The tumor cells showed positivity for CD35, CD21, CD45, and CD68, negative ALK staining, and limited smooth muscle actin immunoreactivity. The surgery was complicated by severe pneumonia and acidosis, resulting in a prolonged ICU stay and death. Conclusion: It is essential to maintain a high index of suspicion for FDCS and include it in the differential diagnosis of a spindle cell neoplasm, especially in the setting of paraneoplastic pemphigus. The histological and immunohistochemical features should be sufficient to confirm the diagnosis of FDCS.

Background: Pembrolizumab is a humanized monoclonal antibody which serves to enhance the antitumor immune response by targeting programmed cell death 1 receptor. The use of pembrolizumab plus carboplatin/pemetrexed combination therapy results in improvement in overall survival and progression-free survival rates for non-small cell lung cancer (NSCLC) patients as compared to chemotherapy alone. However, numerous immune-mediated toxicities of pembrolizumab have been reported. Case Presentation: We report the case of a 74-year-old male patient diagnosed with stage IIIA programmed death-ligand 1-positive non-small cell lung adenocarcinoma treated with 4 cycles of carboplatin/pemetrexed plus pembrolizumab combination therapy followed by 2 cycles of pembrolizumab treatment. Follow-up PET-CT scanning showed a very good response at the level of the tumor but new-onset activity in bilateral hilar and mediastinal lymph nodes. Biopsy of these lymph nodes revealed a benign pathology with noncaseating granulomas consistent with immune-mediated sarcoidosis. Conclusion: The pathogenesis of immunotherapy-induced sarcoidosis is not yet known but has been reported in different cancers and using different checkpoint inhibitors. To our knowledge, this case is the first in the literature displaying pulmonary sarcoidosis in a patient with NSCLC 4 months after having initiated chemotherapy plus pembrolizumab combination therapy.