Atypical Behavior in a Typical Carcinoid: When Minimal Proliferation Meets Lympho-Vascular Invasion-A Case Report

Typical carcinoid tumors (TC) are a low-grade neuroendocrine tumor characterized by low proliferation activity and an overall favorable prognosis. Lympho-vascular invasion is uncommon in TC and is more frequently associated with atypical carcinoid. We reported a 46-year-old non-smoking woman who underwent right lower lobectomy for a right pulmonary hilar mass after multidisciplinary team (MDT) evaluation (cT1cN0M0, AJCC 9th edition, IA3). Postoperative histopathological examination confirmed typical carcinoid tumor with a low Ki-67 proliferation index (< 2%), and no tumor necrosis was identified, fulfilling the diagnostic criteria for typical carcinoid according to the current WHO classification of thoracic tumors. Unexpectedly, focal lympho-vascular invasion in the form of tumor emboli was identified. Additionally, the resected lung showed caseating granulomatous inflammation with acid-fast bacilli, suggestive of a non-tuberculous mycobacteria (NTM) infection; however, species-level identification was not performed. No lymph node metastasis was detected (pT1cN0M0, AJCC 9th edition, IA3). The patient had minimal pleural effusion on the surgical side postoperatively, and no signs of recurrence or metastasis have been detected during 20 months of follow-up to date. This report highlights a pathological discordance between the low proliferation index and the presence of lympho-vascular invasion in TC. The concomitant granulomatous inflammation may have contributed to diagnostic complexity, although a causal relationship cannot be established. This case underscores the importance of careful histopathological evaluation for invasive features in typical carcinoid tumors, even in the context of low proliferative activity. Surgical treatment alone might be safe in this specific scenario.