Benign cysts may have slightly thickened walls or contain septa or calcifications that can blur their distinction from cystic neoplasms. Some cystic lesions will fulfill the criteria for simple cysts with the exception that their contents will have higher attenuation than expected. Due to the relative frequency of these complicating characteristics, the radiologist should provide an opinion on the clinical significance of these findings and try to sort out those that can be safely ignored or followed up from those that require surgical intervention. The referring physician is unlikely to be satisfied with a "malignancy cannot be excluded'' disclaimer and, not unexpectedly, will seek guidance from the radiologist regarding the likelihood of malignancy to establish a management plan.
As an aid to distinguishing imaging characteristics of those cystic masses most likely to be benign from those that require surgery, Morton A. Bosniak developed a descriptive categorization system in 1986 (7). He ascribed increasingly complex features to four categories of cysts from I to IV, ranging from simple cysts to cystic neoplasm. Cysts in Categories I and II were considered benign, while the more complex lesions in Categories III and IV mandated surgery.
It was a reasonable expectation that assigning simple cysts into Category I and grossly obvious cystic tumors into Category IV would not be difficult. Not surprisingly, the greatest challenge in using the system was separating the mildly complex Category II lesions, which could be ignored, from the Category III lesions, many of which were malignancies. In 1993, Bosniak modified the classification system to include a group of lesions that were not of sufficient complexity to warrant surgery but which deserved imaging follow-up to assure stability (8,9). These he deemed Category IIF lesions, the "F" standing for the follow-up imaging that he recommended in these instances.
Although based primarily on CT features, the Bosniak cyst classification system is aided by and can be extended to MR with some caution. Experience derived from more recent series of Bosniak's followed group of patients has further refined the system, and recommendations that stem from its use are described below (10-13).
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