Your doctor either has a weird sense of humour or is an idiot. Spud is right...if he sees Xrays every day, he knows
all about what causes this. Most probably he read somewhere that capsules have been found intact in the sewer system...this is very true, but mostly from dumping. Gelatin
capsules will dissolve in a PH of 5 in a few hours. Even with virtually no stomach acid they would be gone.
The cause is air bubbles on the Xray sheets in the processing stage. Hopefully the doc is less sloppy than his tech staff who failed in darkroom 101.