Basically, the contrast dye used in iodine based one. Sometimes, there may be adverse effect from the radiocontrasts as these substances improve the visibility of internal organs allowing healthcare practitioners see the abnormal growths.
Also, the contrast dye can cause symptoms that can range from mild to life threatening. Sometimes anaphylaxis-like reaction and kidney damage will occur.
Basically, the allergic reactions that involve the immune system is called anaphylaxis reactions. Immune system is not involved if anaphylaxis reactions occur in response to contrast dye. So it is not considered true anaphylaxis. Also, the kidney damage may occur as the kidneys must break down the contrast dye.