ALL is defined as a supernumerary liver lobe composed of a normal liver parenchyma, including bile ducts and blood vessels [6]. Previous studies reported that ALL was caused by embryological abnormalities [11] or acquired trauma [12]. There are three types of ALL. Riedel’s lobe is a tongue-like elongation of the right liver, an ectopic lobe is completely independent of the liver parenchyma, which may exist anywhere in the body, and a pedunculated lobe is ALL that is continuous with the liver [6, 13]. The prevalence of Riedel’s lobe ranges between 3.3 and14.5% [14, 15] and is more frequent in women (4.5–19.4%) than in men (2.1–6.1%) [6]. Most cases of ALL are asymptomatic; however, torsion [16], bleeding [17], or extrinsic compression of the stomach [18] have been reported. A previous study showed that the incidence of ALL (pedunculated lobe) was 0.7% [10]. We herein presented two cases of ALL attached to the gallbladder during LC. ALL was classified as pedunculated lobes in both of these cases.

The diagnosis of ALL during preoperative imaging examinations may facilitate its treatment in surgery. Ultrasonography, enhanced CT, and MRI may be useful for its diagnosis before surgery [19]. However, in the two present cases, ALL was not detected in preoperative examinations. The size of ALL attached to the gallbladder was approximately 10 mm and it was compressed by the liver in a supine position; therefore, it was difficult to diagnose before surgery. Since ALL may be incidentally encountered, it is necessary to always be prepared for its treatment.

Pathological findings previously demonstrated that ALL contains three major structures: a portal vein, hepatic artery, and bile ducts [8, 9]. Vessels and bile ducts in ALL (particularly pedunculated lobes) may be connected to the main liver. Therefore, postoperative bleeding or bile leakage may occur if ALL is not completely resected. It is desirable to clip and resect bile ducts that enter the gallbladder bed (for example, a subvesical bile duct) in order to avoid postoperative bile leakage [20]. We resected the cord between ALL and the main liver using laparoscopic coagulation shears after clipping, and both cases were discharged without bile leakage after surgery. Due to surgical heat degeneration of the cord, communication between bile ducts in ALL and the main liver was unclear. Instead, bile ducts in ALL might be opened into the gallbladder in the second case; therefore, the preservation of ALL may also be a cause of bile leakage. It was very rare anatomy if the bile ducts in ALL opening directly into the gallbladder. However, there were bile ducts that communicate with the gallbladder other than cystic duct, such as Luschka's ducts or subvesical bile ducts [20]. It might be possible that the bile duct in ALL opening into the gallbladder due to an embryological abnormality. For our suggestion, ALL attached to the gallbladder needs to be completely resected along with the gallbladder in order to avoid bile leakage.

Since this was a retrospective observational study at a single institution with a small number of patients, further research on a larger number of patients is necessary.