A 'complex' breast cystic mass, or complex cyst, refers to cysts on the upper end of the continuum of abnormalities that are sometimes associated with breast cysts. Simple breast cysts are fluid filled, and with a uniformly thin and smooth wall to their oval shape. A complex breast cyst, however, contains a good percentage of solid elements suspended within the fluid, and may also feature segmentation (septation) and some regions of the cyst wall that are 'thicker' than others. Complex breast cysts account for approximately 5% of breast cysts that are screened, and are ussually followed up more closely in terms of time interval, and are often biopsied. There is a very very small chance that a complex breast cyst could be associated with malignant breast cancer, so they merit a higher degree of scrutiny.
There are no standard definitions or signature features of a complex breast cyst. They are a heterogeneous group of lesions with different presentations. On ultrasound a complex breast cyst will typically be evaluated in terms of internal echoes, presence or absence of posterior enhancement (evidence of an intracystic mass), thin septations, and a thickened and/or irregular wall (lobulations). But the results of the ultrasound can influence the strategy for followup evaluation or treatment.
Findings of septations (thin walls dividing the cysts into segments) is really of little concern. What the radiologist is looking for is hard evidence of an intracystic mass, which would be indicative of neoplastic cell growth, and that would probably lead to a histological evaluation. Even so, the chances of the neoplasm being breast cancer is very low.
Findings of internal echoes without a distinctly visible solid mass or, alternatively, an 'anechoic' lesion with no posterior-wall enhancement would be a bit of a judgement call. It likely means that various particles are floating in the cystic fluid and, and the cysts is extremely likely to be completely benign. ( It could be floating cholesteral crystals, blood, pus, or milk of calcium crystals.) The decision to biopsy, or apsirate, or simply follow up with observation would be somewhat subjective in this instance.
The presentation of complex cysts and actual malignancy development, when it rarely occurs, can be a little bit irrational however. The presence of an intracystic mass (probable neoplasm) does not statistically correlate with higher risk of malignancy. But a thick cystic wall, lobulation (irregular lobule shapes in the wall), and hyperechogenecity (many internal echoes), particularly when occurring in combination, may be associated with higher risk of underlying malignancy.
The vast majority of complex breast cysts turn out to be benign. As for those that eventually show as developing due to malignant breast cancer, there is quite a range of opinion as to the 'malignancy rate'. This is probably due to the fact that there is no consistent definition of a complex cyst. Some studies place the rate as high as 63%, others as low as 0.3 %. So, the best answer is, 'what does the histological evaluation say". The ultrasound confirmation of a complex breast cyst cannot be generalized as any having any particular or consistent rate of association with breast cancer. Of interest, personal or family history of breast cancer, and menopausal status appear to have no bearing on the development of a malignant neoplasm.
One of the trends in cancer research is to look for different biochemical profiles, or 'markers' which tend to be associated with particular types of breast cancers or benign lesions. Different cells may release different proteins and other chemicals, and that can give clues as to various cell growths and patterns that may be developing. With breast cysts, there does appear to be a tendency towards two basic 'lining patterns' : those lined with apocrine epithelium, and those lined with flat epithelium. (Apocrine epithelial cells are basically secretion cells, while 'flat epithelium' refers to a flat or columnar 'stacking pattern' or layering of normal epithelial cells.) The different cyst-lining cell patterns can also be measured biochemically using [Na+]/[K+], ( the 'sodium to potassium ratio' ) of the cystic fluid. There is some suggestion that cysts with 'low electrolyte' readings ( higher potassium and less sodium) may indicate a higher risk for breast cancer involvement, but this is purely speculative at this time.
By way of translation, this 'biochemical-analysis' approach for evaulating breast cysts suggests that where the epithelial wall is 'getting thicker' by adding certain types of cells, the risk of malignant association may be higher.