A Rare Case of Large Cell Neuroendocrine Carcinoma of the Thymus Presenting as a Lower Neck Mass in a Young Male Patient.

Large cell neuroendocrine carcinoma (LCNEC) of the thymus is a strikingly unusual and high-grade thymic tumor with a poor prognosis. Studies with large series of patients on the clinical, epidemiological, and biological behavior of the disease are not available due to its low prevalence and short survival. We report here such a rare case of LCNEC in a 33-year-old male patient who primarily presented with a mass in the suprasternal and left supraclavicular regions with multiple bilateral cervical lymph nodes. Fine needle aspiration cytology from the neck mass demonstrated hypercellular smears with numerous variably sized, loosely cohesive clusters, including large hypercellular ones, of medium-sized atypical cells with scant cytoplasm, hyperchromatic nuclei, and granular chromatin, with a few having small, inconspicuous nucleoli. Peripheral nuclear palisading or rosette-like structures were noted in many places. An anterior mediastinal invasive mass lesion, possibly of thymic origin, extending to the neck and encasing the major vessels, including branches of the aortic arch, without any evidence of lung parenchymal extension, with multiple bilateral cervical lymph nodes was identified on contrast-enhanced computed tomography. Incisional biopsy revealed large atypical cells mostly arranged in nests, solid sheets, and trabeculae, with hyperchromatic nuclei, coarse chromatin, a moderate amount of eosinophilic cytoplasm, frequent mitoses, and apoptotic bodies. The cells were immunopositive for pancytokeratin, EMA, synaptophysin, chromogranin A, CD56, and TTF-1, with a high Ki-67 labeling index, while negative for all other relevant markers. Misdiagnosis or delay in the correct diagnosis of this rare but aggressive thymic tumor may lead to disease spread and progression to systemic involvement. Many such cases of thymic LCNEC need to be studied to design and optimize standard management.