The most important indication for galactography is spontaneous unilateral bloody nipple discharge from a single duct orifice. You’ll find that ductograms are also often ordered and performed for unilateral single duct serous discharge, but the etiology in those cases is almost always benign fibrocystic disease or ductal ectasia. Ductography is not indicated when the nipple discharge is bilateral, from multiple ducts (multiple openings on nipple surface), or when it’s green, brown, black, yellow, or milky in colour. Many specialists say that the exam is also not indicated when the discharge only presents upon manual expression of the fluid, but I would suggest that if the manually expressed discharge is bloody, a ductogram is still indicated. Note also that persistent bilateral milky discharge may be due to a pituitary prolactinoma, and therefore correlation with a serum prolactin level is reasonable in this situation (if elevated, a pituitary sella MRI would be the next step).
In 90% of women with unilateral bloody nipple discharge, a solitary discrete intraductal mass will be identified on galactography, and the majority of these will be benign intraductal papillomas. If multiple intraductal masses are discovered, the more likely diagnosis will be ductal carcinoma in situ (although intraductal debris can also give that appearance). The presence of a single mass or multiple intraductal masses requires surgical biopsy for diagnosis. Pre-operative localization is typically performed with repeat ductography using methylene blue dye to assist the surgeon in lesion localization intraoperatively. Excision is also therapeutic as it resolves the spontaneous nipple discharge.
References:1. Koskela A et al. AJR 2005;184:1795-8.