What are the factors that make a lung nodule benign vs malignant?

Changes or biopsy: Lung nodules that are small and not worrisome looking(as in spiculated edges, growth, large size, newly developed, etc...) can be observed over 3-6 months with another ct scan for comparison. If no change then likely benign. If something is growing or larger than 2cm or ugly looking by a radiologist then it could b. Malignant. Biopsy is the way to know. If it is too small biopsy may not possible.

Difficult to explain: Tumors, primary or metastatic tend to grow more rapidly, may be associated with weight loss, positive positron scan, malignant cells in sputum, fine needle aspirate or on biopsy. Often the distinction is difficult.