Small breast cysts, which can be resolved by fine-needle aspiration, are often presented as hypoechoic masses on ultrasound examinations. They can be misinterpreted as benign tumors and cause unnecessary follow-ups or biopsies. The aim of this study was to find out the proportion of atypical breast cysts, which can be resolved by needle aspiration, among hypoechoic masses less than 1 cm in size and to investigate the relationship between the characteristics of a mass and its probability of being cystic.

Between March 2003 and May 2003, we found and aspirated 119 hypoechoic masses less than 1 cm in size from 104 consecutive patients.

Mass size ranged from 2.5 mm to 9.8 mm, with a median size of 5.5 mm. Among the 119 hypoechoic masses, 40 masses (33.6%) were cystic and 79 masses (66.4%) were solid. Among the 40 cystic masses, 37 were completely resolved by needle aspiration, one mass was reduced to half, and the 2 remaining masses were reduced to a third of their sizes. In patients in their 20's, 11.1% of the hypoechoic masses were cystic masses, the ratios for patients in their 30's, 40's, 50's, and 60's were 11.4%, 32.0%, 76.2%, and 75%, respectively (P<0.001). With decreasing internal echo of mass, the likelihood of being a cyst increased (P=0.026). However predicting the probability of being a cyst by the margin, the degree of posterior acoustic enhancement, and the location of the mass was difficult (P>0.05).

Needle aspiration must be considered in small hypoechoic masses not speculated as definite solid masses.