Radiation Recall Pneumonitis with Pneumocystis jirovecii Superinfection and Treatment Induced Hyponatremia in a Patient with Non-Small-Cell Lung Cancer

Immune checkpoint inhibitors (ICIs) and thoracic radiotherapy are standard treatments for advanced non-small-cell lung cancer (NSCLC), especially in patients with high PD-L1 expression or symptoms such as superior vena cava syndrome (SVCS). Both therapies carry a risk of pulmonary toxicity, which may be exacerbated by opportunistic infections due to corticosteroid use. We report a unique case of a 65-year-old man with squamous-cell NSCLC and high PD-L1 expression (80%), who developed a rare complication: radiation recall pneumonitis (RRP), with superimposed <i>Pneumocystis jirovecii</i> pneumonia and severe symptomatic hyponatremia induced by trimethoprim/sulfamethoxazole (TMP-SMX). The coexistence of these three complications-radiotherapy- and immunotherapy-associated lung injury, opportunistic infection, and electrolyte imbalance-represents an exceptional clinical scenario not previously described in the literature. This report highlights the importance of differential diagnosis, early recognition of complications, and close monitoring of electrolytes in NSCLC patients undergoing complex treatment regimens.