Abstract: Estimated to comprise approximately 10% of lung cancer cases, multiple pulmonary lesions pose a diagnostic and therapeutic challenge in thoracic oncology. Distinction between multiple primary lung cancer (MPLC) and intrapulmonary metastasis (IPM) directly affects tumor staging and clinical management. In equivocal cases in which the lesions are histopathologically indistinguishable, targeted sequencing can provide key additional evidence for differential diagnosis. Herein, we describe an unusual patient who presented with seven lung lesions that consisted of primary tumors and metastatic lesions, each showing distinct clonality status based on histomolecular findings. Specifically, the 45-year-old female never-smoker underwent a surgery that removed one invasive lepidic predominant adenocarcinoma and five microinvasive adenocarcinomas. Next-generation sequencing revealed three of the lesions to carry a clonal driver mutation EGFR  p.L858R, supporting an IMP diagnosis. EGFR  p.L858R was not detected in two other surgical specimens, which instead harbored respective oncogenic BRAF  p.G469A and an uncommon EGFR  p.G779F. These results led to diagnosis of the two lesions as primary tumors of lineages different from that of the metastases. The patient had achieved a recurrence-free survival of 21 months as of the latest follow-up. In this rare case that presented with evidence of both MPLC and IPM, targeted sequencing proved valuable in facilitating the diagnostic workup.
Keywords: multiple primary lung cancer, intrapulmonary metastasis, targeted sequencing, mutational profiling, clonality