Background The use of RNA‐based next‐generation sequencing (NGS) assays to detect gene fusions for targeted therapy has rapidly become an essential component of comprehensive molecular profiling. For cytology specimens, the cell block (CB) is most commonly used for fusion testing; however, insufficient cellularity and/or suboptimal RNA quality are often limiting factors. In the current study, the authors evaluated the factors affecting RNA fusion testing in cytology and the added value of smears in cases with a suboptimal or inadequate CB. Methods A 12‐month retrospective review was performed to identify cytology cases that were evaluated by a targeted RNA‐based NGS assay. Samples were sequenced by targeted amplicon‐based NGS for 51 clinically relevant genes on a proprietary platform. Preanalytic factors and NGS quality parameters were correlated with the results of RNA fusion testing. Results The overall success rate of RNA fusion testing was 92%. Of the 146 cases successfully sequenced, 14% had a clinically relevant fusion detected. NGS testing success positively correlated with RNA yield (P = .03) but was independent of the tumor fraction, the tumor size, or the number of slides used for extraction. CB preparations were adequate for testing in 45% cases, but the inclusion of direct smears increased the adequacy rate to 92%. There was no significant difference in testing success rates between smears and CB preparations. Conclusions The success of RNA‐based NGS fusion testing depends on the quality and quantity of RNA extracted. The use of direct smears significantly improves the adequacy of cytologic samples for RNA fusion testing for predictive biomarkers.

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There have been major developments in targeted therapeutics with the clinical development of selective BRAF inhibitors (BRAFi) for patients with metastatic BRAF V600E mutant melanoma. Objective response rate of almost 50% has been witnessed in BRAFi clinical trials. Frequent side effects range from squamoproliferative lesions, including hyperplasia, keratoacanthomas, and squamous cell carcinomas to second primary melanomas. We describe a 50-year-old Hispanic woman with BRAF V600E mutant metastatic melanoma who was treated with surgery, radiation therapy, interleukin-2, and was enrolled on a BRAFi (dabrafenib) trial. Two months after initiation, she developed multiple erythematous, indurated, tender subcutaneous nodules bilaterally on the anterior thighs, posterior arms, and left dorsal forearm without overlying epidermal change. Punch biopsy revealed panniculitis with necrotizing granulomata. Infectious and other causes for panniculitis were excluded. We believe the histology likely represents a reaction to BRAFi therapy based on the temporal relationship of its onset to initiation of BRAFi therapy and previously reported cases of neutrophilic panniculitis associated with BRAFi therapy. Panniculitis has been emerging as an important unusual side effect of BRAFi therapy. Our case illustrates a unique presentation of BRAFi-associated panniculitis demonstrating necrotizing granulomata.