Background <p>Lymphatic dissemination is widely recognized as an important pathway in the progression of non-small cell lung cancer (NSCLC), and nodal involvement often accompanies more advanced thoracic spread (Rawal N. Eur J Anaesthesiol. 2016;33(3):160–71). Pleural involvement represents a distinct form of intrathoracic dissemination and is generally classified as M1a disease; when a primary tumor extends to or invades the pleura, tumor cells may gain access to the pleural cavity (Gehling M, Tryba M. Anaesthesia. 2009;64(6):643–51). Nevertheless, pleural lesions may occasionally be discovered unexpectedly during surgery in patients who were considered to have potentially resectable early-stage disease on preoperative evaluation. Such cases create an intraoperative management dilemma and may reflect biological heterogeneity. Here, we report a case of central ALK-rearranged lung adenocarcinoma with unexpected localized pleural dissemination identified intraoperatively, in which pathological examination of dissected lymph nodes showed no evidence of metastasis.</p> Case presentation <p>A 74-year-old man with a 50-pack-year smoking history was referred to our institution because of an incidentally detected right upper lobe pulmonary nodule. Preoperative computed tomography showed a 1.5 × 1.0 × 1.1 cm centrally located solid lesion without direct pleural contact, pleural indentation, or obvious bronchial invasion, and the lesion was clinically staged as cT1bN0M0 (stage IA2). During video-assisted thoracoscopic surgery, approximately 4–5 nodular lesions were unexpectedly observed on the superior mediastinal pleura. Frozen-section examination of one pleural nodule demonstrated invasive carcinoma. The planned pulmonary resection was therefore abandoned, and lymph node dissection of stations 2, 4, 10, and 12 was performed for staging. Final pathological examination, together with immunohistochemistry, supported poorly differentiated pulmonary adenocarcinoma. No metastatic carcinoma was identified in the dissected lymph nodes. Molecular testing revealed an ALK rearrangement. The patient subsequently received alectinib hydrochloride at 600 mg twice daily. Follow-up computed tomography performed 6 months after treatment initiation showed marked tumor shrinkage, and the patient remained asymptomatic with ongoing radiological disease control.</p> Conclusions <p>This case documents an unusual presentation of localized pleural dissemination without pathologically confirmed nodal involvement in a central ALK-rearranged lung adenocarcinoma. It highlights the importance of intraoperative pathological confirmation, accurate staging, and timely molecular testing. However, mechanistic and therapeutic implications should be interpreted cautiously in light of the single-case nature of this report.</p>