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Background: Melanocytic neoplasms can be challenging to diagnose. One wellestablished diagnostic aid is the detection of copy number variation (CNV) in a few key genetic loci using conventional methods such as fluorescence in situ hybridization (FISH) and chromosomal microarray (CMA). Droplet digital polymerase chain reaction (ddPCR) is a novel, cost-effective, rapid, and automated method to detect CNV. Methods:We perform the first investigation of ddPCR to assay Ras-responsive element-binding protein-1 (RREB1), the most common CNV in melanoma using formalin-fixed, paraffin-embedded (FFPE) melanocytic lesion samples; CMA data are used as the gold standard. Archival samples from 2013 to 2021 were analyzed, including 153 data points from 39 FFPE samples representing 34 patients. Benign, borderline, malignant, and metastatic melanocytic neoplasms were examined.Results: ddPCR showed a sensitivity and specificity of 93.8% and 95.7% using one reference gene, and 87.5% and 100% using a different reference gene for RREB1 gain detection.Conclusions: Here we show that ddPCR can provide inexpensive, rapid, and robust data on the commonest copy number alteration in melanoma. Future development and validation could provide a useful ancillary tool in the diagnosis of challenging melanocytic lesions.

Mixed medullary and follicular cell‐derived thyroid carcinoma (MMFTC) is a rare primary thyroid carcinoma with morphologic and immunophenotypic evidence of admixed parafollicular and follicular cell‐derived tumor populations within the same tumor. We herein present the fine‐needle aspiration biopsy (FNAB) cytology of a case of MMFTC that was diagnosed histologically and discuss potential clues to the diagnosis for cytologists. We also provide a literature review of this uncommon primary thyroid tumor. The patient was a 47‐year‐old man with a history of hypothyroidism who presented with ear and neck pain. Imaging demonstrated thyroid nodules with regional lymphadenopathy. FNAB samples of two thyroid nodules and an involved lymph node were diagnosed as papillary thyroid carcinoma (PTC). The subsequent total thyroidectomy specimen demonstrated classic‐type PTC which transitioned to a morphologically and immunophenotypically distinct medullary thyroid carcinoma (MTC) component within the same lesion, indicative of MMFTC. The patient experienced recurrence of the medullary component 20 months later and received chemotherapy with subsequent external beam radiation. As in this case, the cytologic diagnosis of MMFTC is almost never made prospectively. Retrospective review of the preoperative FNAB samples showed subtle cytomorphologic features suggestive of MTC in two of three biopsies, an impression confirmed by calcitonin immunohistochemistry on cell block material. In the broader literature, most MMFTCs on FNAB have been diagnosed as MTC, which is usually the more aggressive component of the mixed neoplasm.

Liposarcomas are categorized into four distinct histopathological subtypes: atypical lipomatous tumors (ALT)/well-differentiated liposarcoma (WDL), dedifferentiated, myxoid, and pleomorphic. Dedifferentiated liposarcomas account for approximately 18% of all liposarcomas, characteristically arising in the deep soft tissue. They are reported to have lower rates of metastasis compared to other pleomorphic sarcomas. 1-3 The classic histopathologic appearance is ALT/WDL admixed or juxtaposed with a predominantly nonlipogenic sarcoma. Epithelioid features are rare, appearing in as few as 3% of tumors, and have not previously been reported in a superficial location. Herein, we present a 57-year-old male with intradermal and subcutaneous metastasis of his known deep dedifferentiated liposarcoma with epithelioid features.By H&E the tumor featured cords and sheets of crowded, plump, epithelioid cells with thick nuclear membranes and prominent nucleoli, which raised a broad differential including carcinoma and melanoma. By immunohistochemistry the tumor was diffusely positive for MDM2 and CDK4, on the other hand stains for Sox10, Melan A, MITF, CKAE1/3, desmin, and S100 protein were negative. This case serves as an opportunity to raise awareness of this rare morphological subtype, which can involve the skin and mimic epithelial and melanocytic malignancies. It can be a potential diagnostic pitfall, especially if metastases are the first presentation. K E Y W O R D S cutaneous, dedifferentiated, epithelioid, liposarcoma, MDM2, metastasis

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