**2. Historic review and classification for acute pancreatitis**

Atlanta classification was the first classification for acute pancreatitis and was originally stated in 1992; giving the opportunity to the universal surgical community to have a common aspect regarding its definition [3]. However, soon this terminology proved to be inadequate and confusing and became outdated. In addition, definition of the pancreatic fluid collections was not well-established and there was huge variety among the surgeons [4]. Better comprehension of the etiology and the pathophysiology of the acute pancreatitis has led to revision of the Atlanta Classification for the acute pancreatitis, two decades later, correcting the aforementioned deficiencies. This revised classification differentiates the acute pancreatitis into two phases: early and late onset as well as the severity as mild, moderate, and severe [5].

Regarding the pancreatic and the peripancreatic fluid collections, terms such as "acute pseudocyst" and "abscess" were misleading and therefore discouraged. Instead, there was a clear distinction between collections that are consisted of sole fluid and those with debris (solid components due to necrosis). Another important factor affecting the categorization of the fluid collections is the presence of infection and certainly the duration of existence (**Figure 1**) [5]. In the Atlanta classification, PPC was described as a well-defined extra-pancreatic fluid collection with minimal solids, which lasts more than 4 weeks as the pancreatitis recedes.
