I had a chest x-ray that showed a 3.5cm mass on my lung. I had a CT today and the radiologist said that masses can't be diagnosed by x-rays?

CT scan is better. We can find some masses on chest X-ray, but a CT scan would be more accurate for smaller lesions, would be more detailed for lesions in general, and would provide information about the three-dimensionality of a lesion that you simply can't get on an X-ray.