Peritoneal Metastasis Mimicking Chemotherapy-Induced Complications in Lung Adenocarcinoma: A Diagnostic Challenge of a Case Report.

We report a case of a 64-year-old man with advanced non-small cell lung cancer (NSCLC) who developed peritoneal metastasis during systemic treatment. Initially diagnosed with lung adenocarcinoma with pleural dissemination and bone metastases, he received carboplatin, pemetrexed, and pembrolizumab, followed by docetaxel due to clinical progression. While primary lung lesions responded to docetaxel, the patient developed new-onset abdominal pain and ascites. Radiologic findings suggested peritoneal thickening, raising suspicion for either docetaxel-induced toxicity or disease progression. Given the rarity of peritoneal metastasis in NSCLC and concurrent treatment response elsewhere, drug-induced complications were initially considered. However, worsening symptoms and further imaging prompted cytological evaluation of ascitic fluid, which confirmed metastatic adenocarcinoma consistent with lung origin. This case highlights the diagnostic challenge of distinguishing treatment-related adverse events from disease progression, especially in patients presenting with nonspecific abdominal symptoms during therapy. Clinicians should maintain a high index of suspicion for uncommon metastatic sites when new symptoms arise, even in the setting of apparent response at the primary site.