This Literature - Spontaneous case, initially received on 13-Sep-2016, was reported by a health professional and concerns a 71-year-old male patient. Weight: not provided; Height: not provided.  Administration of company suspect drug(s): On an unknown date, the patient received Influenza Virus Vaccine (INN) for indication not reported. Dose regimen: Not reported. Batch number: Not reported. Site: Left upper arm. No additional suspect drugs. Adverse reactions/events and outcomes: On an unknown date, the patient presented with a 5.0-cm nontender, erythematous, and scaling plaque with an indurated border on the left upper arm. The lesion appeared at the site of an influenza vaccination that he received 7 days prior. The patient denied systemic symptoms and any such reaction with previous vaccinations. The initial clinical suspicion was that of an injection site hypersensitivity reaction. A punch biopsy was performed, and a topical high-potency steroid, diflorasone diacetate 0.05%, was prescribed. After histologic and immunoprofiling of CD41, CD561, TCL1, and CD1231, a diagnosis of blastic plasmacytoid dendritic cell neoplasm (BPDN) was rendered. Differential diagnosis included extramedullary myeloid sarcoma and an acute leukemia with ambiguous lineage. Absence of myeloperoxidase excluded myeloid sarcoma, and absence of CD34, myeloperoxidase, CD117, and muramidase ruled out acute leukemia. This diagnosis was confirmed by a senior hematopathologist and at major cancer center.  At the 2-week follow up, the patient reported that the entire lesion had resolved with topical steroid treatment. Despite the lack of clinical evidence of disease, radiation therapy was delivered at the site. At the time of this report, bone marrow and blood involvement were not found and leukemia was excluded, but watchful follow-up is necessary. Relevant test results: The biopsy found an atypical dense lymphoid infiltrate in the papillary and reticular dermis. A grenz zone was present. The infiltrate shows prominent perivascular distribution. The individual lymphocytes had hyperchromatic nuclei with irregular nuclear contour and nucleoli. Crush artefact was present at the periphery of the biopsy section. Immunohistochemical profiling found the lesion to be diffusely CD41 and CD561. Most of the lesional cells were also CD1231 and TdT1. Repeat CD123 staining found more intense and diffuse positivity. Admixed were rare CD791 B cells. Tests for myeloperoxidase, CD34, MUM1, PAX5, CD3, CD10, CD21, kappa, and lambda were negative. Epstein Barr virus in situ hybridization results were also negative. In addition to these markers, an outside hospital found that our lesion was TCL11 and CD117 and muramidase negative. The event blastic plasmacytoid dendritic cell neoplasm and associated symptoms is serious (medically significant) and the outcome is reported as recovered / resolved. Reporter's Comments: Reporter's assessment: The diagnosis of this lesion is dependent on histologic and immunologic evaluation. To date, there is no specific molecular test to confirm a diagnosis of BPDN. Therefore, based on the current standards, a diagnosis of BPDN was made on our biopsy. However, the atypical presentation and behavior of our case raises the question if our lesion was indeed a distinct presentation of BPDN that carries a good prognosis or was a BPDN-like reaction to the vaccination.; Sender's Comments: Company comments: The event of BPDN is unlisted in the product CCSI and PI.  The event of BPDN is considered related to the vaccination secondary to temporal relationship only. There remains to be no evidence in literature or cases of association of vaccination with BPDN. BPDN is most commonly observed in elderly individuals; the patient for this case is 71 years old and disease occurrence secondary to other risk factors is also plausible.The possibility of a BPDN-like reaction on the injection site is also considered.