Would you have symptoms of small cell lung cancer for 18 months prior to diagnosis, or does small cell grow too rapidly for this to happen?

Depends on the grade: After a biopsy, when small cell carcinoma is diagnosed, it is often given a "grade" by the pathologist (e.g., low-grade or high-grade) based on the appearance under a microscope. There are small cell cancers that can grow slowly, & only be discovered on screening CT scans (not causing any symptoms). Others cause symptoms & spread quickly (high-grade). Not all small cell cancers behave alike.