Extreme procalcitonin elevation without infection in a patient with lung adenocarcinoma undergoing transformation to small-cell lung carcinoma: a case report

Abstract Objectives Procalcitonin (PCT) is widely used as a biomarker of bacterial infection and sepsis, but its specificity decreases substantially in patients with advanced malignancy. Case presentation We report a 55-year-old woman with surgically resected lung adenocarcinoma who subsequently underwent multiple systemic treatments, Gamma Knife radiosurgery for central nervous system metastases, immunotherapy, and KRAS G12C-targeted therapy. After disease progression, biopsy of a liver metastasis demonstrated histologic transformation to dominant small-cell lung carcinoma (SCLC) with neuroendocrine phenotype, including diffuse CD56 and synaptophysin positivity, thyroid transcription factor-1 (TTF-1) positivity, Napsin A loss, Ki-67 approximately 80 %, and focal retinoblastoma (RB) loss. The patient was admitted with markedly elevated inflammatory markers and persistently extreme PCT values above 20 ng/mL, while serum calcitonin was profoundly elevated (2,409 pg/mL). Extensive infectious evaluation showed three negative sets of blood cultures obtained before antibiotic therapy, a sterile urine culture, negative Aspergillus and Candida serology, negative mannan, galactomannan, and beta-D-glucan assays, and no radiological evidence of pneumonia, abscess, or intra-abdominal infection on contrast-enhanced computed tomography of the chest, abdomen, and pelvis. She remained afebrile and clinically stable, with no improvement attributable to broad-spectrum antimicrobial therapy; subjective improvement followed a packed red blood cell transfusion. Conclusion This case is clinically distinctive because persistent extreme PCT elevation occurred in parallel with histologically confirmed adenocarcinoma-to-SCLC transformation, marked hypercalcitoninemia, extensive tumor necrosis, and a consistently negative infectious workup, closely mimicking severe bacterial sepsis. In this setting, extreme PCT elevation was more consistent with a tumor-related inflammatory phenomenon than with bacterial infection.