Context.— Intravascular large B-cell lymphoma (IVLBCL) is a rare hematopathologic entity, posing both a clinical and histologic challenge for diagnosis. Numerous pitfalls can hinder making the diagnosis. Objective.— To summarize recent developments in literature pertaining to IVLBCL and point out key pitfalls pathologists should be prepared to encounter. Data Sources.— Literature review via PubMed search and hospital (Darnall Medical Library) resources. Conclusions.— The 3 primary pitfalls of IVLBCL include masking of IVLBCL, mimicry by IVLBCL, and mimicry of IVLBCL. These scenarios illustrate the importance of histologic pattern recognition and subsequent usage of immunohistochemistry, especially in context of a clinical history that may be noncharacteristic.

Casestudy: The utilization of checkpoint inhibitors such as programmed cell death protein 1 (PD-1/PD-L1) inhibitors (nivolumab) and cytotoxic T-lymphocyte antigen 4 inhibitors (ipilimumab) for treatment of certain malignancies has steadily gained popularity. Medication related colitis is uncommon, with a reported incidence of 1–9% depending on the checkpoint inhibitor used, and the histologic features have been characterized in recent literature. Because of the immunomodulating effect of these drugs, infectious colitis is in the differential diagnosis of enteritis. Multi-drug therapy in many of these patients further complicates identification of the culprit drug. We present the case of a 63-year-old male with metastatic renal cell carcinoma being treated with both nivolumab and ipilimumab who presented with acute on chronic non-bloody diarrhea. His clinical course was complicated by hypotension, acidosis and coagulopathy. The clinical differential for his colitis was cytomegalovirus infection versus a checkpoint inhibitor colitis. Colonoscopy revealed continuous circumferential loss of vascularity and diffuse erythema throughout the colon. Histology showed acute colitis with prominent apoptosis, cryptitis, crypt abscesses, and rare ringed mitotic figures, but without architectural distortion. Occasional smooth purple crystals consistent with pill material were present in the mucosa, but without significant tissue reaction. No pathogenic organisms were identified, and a cytomegalovirus immunostain was negative. These histologic findings in concert with the clinical history are consistent with checkpoint inhibitor colitis and multi-drug effect. A review of the patient’s chart showed cholestyramine was added to the patient’s regimen during hospitalization, which was consistent with the morphologic appearance of the crystals. Given the acute complications of checkpoint inhibitor induced enterocolitis and potential for increased morbidity (rare cases of bowel perforation and subsequent resection), accurate diagnosis is imperative. Management of checkpoint inhibitor associated colitis ranges from initiation of immunosuppression to checkpoint inhibitor cessation. When these findings are masked by multi-drug effect, accurate diagnosis can be difficult.

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