Oil cysts of the breast are relatively common benign masses. Oil cysts may present as a nontender, palpable breast lump or may present as an incidental finding on mammogram, breast ultrasound, or MRI. Oil cysts are benign lesions that usually require no intervention, unless the cyst is symptomatic or causes anxiety for the patient. The etiology of oil cysts is felt to be most often an end-form of liquefaction fat necrosis of the breast resulting from trauma or surgery. However, some oil cysts arise independently, without a prior history of trauma or surgery. Mammographically, oil cysts typically appear as well defined, round or ovoid lucent (same density as breast fat) masses, often with a thin calcified rim ("eggshell calcification") 
Sonographically oil cysts may present in a variety of ways. Harvey et al  performed a sonographic study of 26 oil cysts in 15 patients. Most cysts were hypoechoic or anechoic with a variable degree of posterior enhancement or shadowing. Sometimes oil cysts mimicked intracystic neoplasms and ultrasound could not always characterize the masses as benign. The classic sonographic characteristics of an oil cyst is an ovoid anechoic mass with a thin capsule and slight posterior acoustic enhancement. If the mammographic findings suggest a lucent fat containing mass with a thin calcified rim and ultrasound demonstrates an anechoic cystic mass, those findings are essentially confirmatory of an oil cyst. An intramammary lipoma may have a similar mammographic appearance, but typically lacks the eggshell calcification and is typically hypoechoic, not anechoic, on ultrasound . Differentiating a lipoma from an oil cyst, is usually not clinically important, as both are benign "don't touch" lesions.
If the mammographic or ultrasound findings are equivocal, MRI can provide further useful information and determine if the mass is fat containing and non-enhancing. The MRI findings of a lipoma or an oil cyst will be similar. Both will be well- circumscribed, nonenhancing masses with characteristic fat signal contents(bright on T1, iso to fat on T2, with signal dropout on T1 fat-sat sequences.)
Reference(s): 1. Cardenosa G. Breast Imaging Companion. Baltimore, MD: Lippincott,Williams, and Wilkins; 1997: 271-275
2. Harvey JA, Sonographic features of mammary oil cysts.
J Ultrasound Med 1997;16(11):719-24