Case Report: Contrast-enhanced ultrasonography for evaluating a newly detected contralateral pulmonary lesion during non-small cell lung cancer chemoimmunotherapy.

Background: New pulmonary lesions arising during chemoimmunotherapy for non-small cell lung cancer (NSCLC) pose a common diagnostic dilemma because they may reflect tumor progression, infection, treatment-related inflammation, or other benign processes. Computed tomography remains central to surveillance, but morphology alone is sometimes insufficient for confident characterization. Contrast-enhanced ultrasonography (CEUS) can provide real-time perfusion information in peripheral pulmonary lesions and may offer incremental value in selected subpleural lesions. Case presentation: A 78-year-old man with stage IIIB squamous NSCLC received four cycles of paclitaxel plus cisplatin combined with pembrolizumab. During treatment, he developed a newly detected contralateral lesion in the posterior basal segment of the left lower lobe. Transthoracic lung ultrasonography demonstrated a subpleural hypoechoic lesion measuring approximately 2.4 × 2.2 cm. CEUS showed rapid hyperenhancement in the pulmonary arterial phase, relatively homogeneous internal enhancement, and slight persistent hyperenhancement in the venous phase, favoring an inflammatory rather than a malignant lesion. This interpretation was supported by contemporaneous microbiologic evidence of respiratory infection and was subsequently confirmed clinically by marked lesion regression on short-interval follow-up computed tomography after anti-infective treatment. Conclusion: This case highlights the potential adjunctive role of CEUS in the evaluation of newly detected peripheral pulmonary lesions during chemoimmunotherapy. Current evidence indicates that CEUS is most useful when integrated with clinical, laboratory, microbiologic, and cross-sectional imaging data, rather than used as a stand-alone discriminator between benign and malignant disease.