"a nodule that is not calcified in a benign pattern or that does not have other features strongly suggestive of a benign etiology, such as intranodular fat that is pathognomonic of hamartoma or a feeding artery and vein typical for arteriovenous malformation." - ACCP Guidelines. Chest 2013 PMC3749714
References
Clinical practice guidelines for the management of a pulmonary nodule
Gould MK et al. Evaluation of individuals with pulmonary nodules: when is it lung cancer? Diagnosis and management of lung cancer, 3rd ed: American College of Chest Physicians evidence-based clinical practice guidelines. Chest. 2013 May;143(5 Suppl):e93S-120S. doi: 10.1378/chest.12-2351. PMID: 23649456; PMC3749714 (open access).
(for subsolid nodules only) Naidich DP, Bankier AA, MacMahon H, Schaefer-Prokop CM, Pistolesi M, Goo JM, Macchiarini P, Crapo JD, Herold CJ, Austin JH, Travis WD. Recommendations for the management of subsolid pulmonary nodules detected at CT: a statement from the Fleischner Society. Radiology. 2013 Jan;266(1):304-17. doi: 10.1148/radiol.12120628. Epub 2012 Oct 15. PMID: 23070270
New, not yet incorporatedMacMahon H, Naidich DP, Goo JM, Lee KS, Leung AN, Mayo JR, et al. Guidelines for Management of Incidental Pulmonary Nodules Detected on CT Images: From the Fleischner Society 2017. Radiology. 2017 Feb 23:161659. PMID 28240562
Source studies for predicting the probability of malignancy in a pulmonary nodule
Swensen SJ, Silverstein MD, Ilstrup DM, Schleck CD, Edell ES: The probability of malignancy in solitary pulmonary nodules. Application to small radiologically indeterminate nodules. Arch Intern Med 1997;157(8):849-855. PMID: 9129544
Herder GJ, van Tinteren H, Golding RP, et al: Clinical prediction model to characterize pulmonary nodules: validation and added value of 18F-fluorodeoxyglucose positron emission tomography. Chest 2005;128(4):2490-2496. PMID: 16236914