Clinical utility of pathologist-directed comprehensive comparative molecular profiling for the classification of separate primary lung cancers vs. intrapulmonary metastasis.

8039 Background: Lung cancers can present as multiple pulmonary tumors, representing either separate primary lung carcinomas (SPLCs) or intrapulmonary metastases (IPMs) arising from a single advanced cancer. Distinguishing SPLCs from IPMs is important for effective staging, prognosis, and treatment. Comparative molecular profiling (CMP) of paired tumors can elucidate their clonal relationship. We report real-world data on the use of CMP to distinguish SPLCs versus IPMs. Methods: Paired lung tumors from the same patients, submitted for FoundationOne CDx (F1CDx) comprehensive genomic profiling within a 12-month period from one another between 2014 and 2024, were centrally reviewed by a board-certified pathologist. After excluding clonal hematopoiesis and germline alterations, 1) tumors with different driver alterations were classified as SPLCs, 2) tumors with shared drivers and other alterations as IPMs, and 3) tumors with 1 common driver but different other alterations as SPLCs. The molecular landscapes of SPLCs and IMPs as defined by CMP were subsequently compared. Results: In all, 359 paired lung cancers were identified and analyzed using pathologist-directed CMP. Of these, 32.6% (117/359) were classified as SPLCs, 63.8% (229/359) as IPMs, and 1.1% (4/359) as misdiagnoses (i.e., not lung cancer). Among SPLC pairs, 89.7% (105/117) harbored actionable genomic alterations in one tumor but not the other. Classification of 2.5% (9/359) of pairs was inconclusive due to suboptimal sequencing quality control metrics. Misdiagnoses, based on the molecular results, included 2 metastatic HPV-associated carcinomas (supported by high-risk HPV reads), and 2 metastatic cutaneous squamous or basal cell carcinomas (supported by ultraviolet mutational signatures). Of 31 paired lung tumors with different histologies, 67.7% (21/31) were SPLCs, while 32.3% (10/31) were IPMs. Molecular landscape analysis showed that IPMs had significantly higher frequencies of CDKN2B (20.1% vs. 10.3%, P=0.03, OR=2.2) and ERBB2 (6.3% vs. 0.9%, P=0.03, OR=7.8) alterations, while SPLCs were more likely to have elevated tumor mutational burden (i.e., TMB ≥10 Muts/Mb) (39.8% vs. 21.3%, P<0.0001). Conclusions: Pathologist-directed CMP using F1CDx facilitates the classification of multiple lung cancers as SPLCs or IMPs and may identify diagnostic errors associated with conventional histopathological examination. The higher prevalence of ERBB2 alterations in IPMs suggests a role in their development and opportunities for targeted therapy, while the increased rate of elevated TMB in SPLCs indicates a field cancerization effect, driven by smoking or other exposures, and possible responsiveness to immunotherapy. Accurately classifying SPLCs and IPMs is important for proper staging and therapeutic planning in patients with lung cancer.