Can mucus or phlegm show up as densities on a chest xray, and what else can?

Yes it is possible. My son had pneumonia when he was young, and obviously, an x-ray was taken to confirm the doctor's pre x-ray diagnosis. It showed as a shadow over a portion of the lung...and the doctor explained to us that the shadow was the phlegm clogging the lung that time. ps: if detected early pneumonia can be cured, and my son was cured with daily antibiotic injections and stuff. pps: if yours showed up as a nipple shadow, the first thing that came to my mind is... .maybe (stress on maybe) there's a tumour (it can be benign or malignant). Evidently, you are seeing a doctor..since you indicate you are having x-rays. I suppose your doctor will know what to do from the x -ray (he should!). If in doubt, go for a second opinion. But as in all cases, early detection is key to cure...I wish you all the best and hope it is nothing serious.

many thing ..would show.....a high density,....on X ray....which has ..a calcium content..... since most carninoges..are also.....composed of calcium...that is a possibility..... but than so is bone... but ..many hard soild are not visible on xray... the best way of going around is to get a ct scan....to confirm it....