Post-transplant lymphoproliferative disorder (PTLD) is a term used to describe a range of lymphoproliferative disorders that occur after solid organ transplant. Although the clinical presentation is variable, primary cutaneous PTLD typically presents as isolated nodules that appear as dermal-based proliferations. We present a case of a 70-year-old woman with a history of a kidney transplant who presented with a 2-month history of an asymptomatic, erythematous plaque on the right shin, clinically suspected to be squamous cell carcinoma in situ. Histomorphology demonstrated a dermal proliferation of atypical plasma cells with dense chromatin, variable nucleoli, and irregular nuclear borders. The atypical plasma cells were positive for Epstein-Barr virus by in situ hybridization and markedly kappa-restricted by RNAscope in situ hybridization. A diagnosis of cutaneous monomorphic PTLD, plasma cell neoplasm variant, was rendered, a rare diagnosis in the skin. Treatment for PTLD typically involves reduction of immunosuppression, although our patient progressed and developed new lesions despite this intervention. In this study, we present an atypical presentation of cutaneous PTLD, plasma cell neoplasm variant, presenting as squamous cell carcinoma in situ.

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Primary cutaneous B-cell lymphomas (PCBCLs) are diagnostically challenging entities due to significant overlap in clinical and morphologic features with reactive lymphoid proliferations. Traditional methods for evaluating clonality such as immunohistochemistry (IHC) and chromogenic in situ hybridization (CISH) are limited by low sensitivity, which leads to additional costly and time-consuming molecular clonality assays. More recent technology has introduced ultrasensitive bright-field RNA in situ hybridization (BRISH) to the field, which can detect single molecules of light-chain mRNA. The current study evaluated 274 cases of PCBCL in addition to atypical and reactive lymphoid infiltrates, with CISH or BRISH performed on 180 (65.7%). CISH was performed on 105 (58.3%), and BRISH was performed on 75 (41.7%). Significantly fewer immunoglobulin heavy-chain (IGH) rearrangement studies were performed on cases that were evaluated with BRISH as compared with CISH (P=0.02). Subgroup analysis demonstrated that cases with restriction by BRISH were significantly less likely to have subsequent IGH studies performed (P=0.01). The expected costs of cases using CISH versus BRISH were $1053.89 versus $810.32 to the patient and $245.63 versus $225.23 to the laboratory. The use of ultrasensitive BRISH to evaluate clonality in PCBCL reduced the use of IGH rearrangement studies when compared with CISH. In particular, cases with light-chain restriction by BRISH did not result in confirmatory molecular testing. Despite slightly higher costs to the laboratory to perform BRISH, routine use of this methodology can result in cost savings to both the patient and laboratory by decreasing the use of expensive molecular methods.

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