Case Report: Marked elevation of serum procalcitonin before radiographic confirmation of recurrence in stage IVB pulmonary large cell neuroendocrine carcinoma.

Serum procalcitonin (PCT) is conventionally used as a biomarker for bacterial infection, however, emerging evidence suggests that PCT may also be elevated in patients with lung cancer in the absence of infection. We report a patient with stage IVB pulmonary large cell neuroendocrine carcinoma (LCNEC) who experienced tumor recurrence after achieving partial response(PR) with multimodal therapy. During remission, serum PCT remained relatively stable at a baseline level of 1.62 ng/mL (normal range: 0-0.05 ng/mL). Subsequently, PCT increased markedly to 61.57 ng/mL despite the absence of definitive radiographic progression and rose further to 93.14 ng/mL when tumor recurrence was ultimately confirmed. Notably, marked elevation in PCT preceded radiographic confirmation of recurrence by more than one month, with no clinical or microbiological evidence of concurrent infection. This biochemical lead time suggests that serial PCT monitoring may have potential value as an adjunctive biomarker for the early detection of recurrence in advanced LCNEC, although further studies are required to establish its clinical utility.