PET/CT model helps distinguish primary lung cancer from colorectal metastasis

Patients with colorectal cancer who develop a solitary pulmonary nodule face a difficult diagnostic question: is the nodule a new primary lung cancer, or has the colorectal cancer spread to the lung? A new study published in EJNMMI Research reports that a model built on basic 18F-FDG PET/CT parameters, combined with clinical risk factors, can help differentiate between these two possibilities, with a combined model achieving an area under the ROC curve of 0.871.

The research, published on 1 October 2026, addressed a clinically challenging situation. Distant metastasis is the leading cause of death after curative resection for colorectal cancer, and accurately distinguishing a primary lung cancer from a solitary lung metastasis in patients with solitary pulmonary nodules remains difficult because of the limitations of biopsy and conventional imaging. The study aimed to explore the diagnostic value of a basic 18F-FDG PET/CT parameter-based model for this differential diagnosis.

The cohort included 101 patients, of whom 75 were male and 26 female, with a mean age of 62.31 plus or minus 9.52 years and a range of 32 to 80 years. Pathological verification revealed 53 solitary lung metastases and 48 primary lung cancers. This means every diagnosis in the study was confirmed by pathology, the reference standard for distinguishing these two entities.

The researchers compared three diagnostic approaches: a PET/CT parametric model based on basic imaging parameters, a clinical model built from clinical risk factors, and a combined model integrating both. The ROC-derived area under the curve was 0.757 (95% CI: 0.663–0.851) for the PET/CT parametric model, 0.768 (95% CI: 0.676–0.861) for the clinical model, and 0.871 (95% CI: 0.800–0.942) for the combined model.

Internal validation was performed using 1000 bootstrap resamples. After this validation, the corrected AUCs were 0.757 (95% CI: 0.666–0.851) for the PET/CT parametric model, 0.767 (95% CI: 0.687–0.867) for the clinical model, and 0.870 (95% CI: 0.809–0.949) for the combined model, indicating that the apparent performance of the models was stable within this cohort.

The combined model showed significantly superior diagnostic performance compared with the PET/CT parametric model alone (0.871 versus 0.757, P = 0.006) and compared with the clinical model alone (0.871 versus 0.768, P = 0.003). By contrast, no statistically significant difference was observed between the PET/CT parametric model and the clinical model (0.757 versus 0.768, P = 0.860), suggesting that the imaging parameters and the clinical factors carried largely complementary rather than redundant information.

The authors concluded that a basic 18F-FDG PET/CT parametric model enables effective discrimination between primary lung cancer and solitary lung metastasis presenting as solitary pulmonary nodules in post-operative colorectal cancer patients, and that the combined model integrating PET/CT parameters with independent clinical risk factors markedly improves diagnostic efficacy relative to single models, showing favourable clinical practicability.

Several caveats are worth noting. The reported AUCs come from a single cohort with internal bootstrap validation only, so the models have not yet been tested in an external patient population. The published version is an early-release, peer-reviewed accepted article that remains subject to further edits before the final Version of Record.

Even so, the study offers a practical signal for clinicians: when a patient treated for colorectal cancer presents with a solitary lung nodule, combining routinely available PET/CT parameters with clinical risk factors may improve the pre-biopsy assessment of whether the lesion is a new primary lung tumour or a metastasis, potentially informing decisions about further work-up and treatment planning.

Subject of Research: Using 18F-FDG PET/CT parameters and clinical factors to distinguish primary lung cancer from solitary lung metastasis in colorectal cancer patients

Article Title: The role of 18F-FDG PET/CT parameters in differentiating primary lung cancer from solitary lung metastasis in patients with colorectal cancer

Article References: Wang, N., Zhu, H., Liu, Y., Jing, F., Dai, M., Zhao, Y., Sun, Y., Wang, J., Zhang, J., Zhao, X., & Zhang, Z. (2026). The role of 18F-FDG PET/CT parameters in differentiating primary lung cancer from solitary lung metastasis in patients with colorectal cancer. EJNMMI Research. https://doi.org/10.1186/s13550-026-01519-4

Image Credits: AI Generated

DOI: 10.1186/s13550-026-01519-4

Keywords: colorectal cancer, 18F-FDG PET/CT, solitary pulmonary nodule, primary lung cancer, lung metastasis, diagnostic model, ROC analysis, AUC, bootstrap validation, nuclear medicine, EJNMMI Research, differential diagnosis