Pancreatic ACC is defined as a tumor that exhibits differentiation toward pancreatic acinar cells [2]. It shows an expansile growth pattern and is primarily found in the pancreatic head [3]. On CT and magnetic resonance imaging, well-defined borders with an outward protrusion are observed. Smaller lesions often exhibit a relatively uniform enhancement pattern, although less intense than that of normal pancreatic parenchyma [4]. Large ACCs can exhibit hemorrhage and cystic changes. Histologically, ACC is characterized by cells with round nuclei, distinct nucleoli, and granular cytoplasm that exhibit acinar-like structures [5].

Differential diagnoses are neuroendocrine carcinoma and solid pseudopapillary neoplasm, and immunohistochemistry is essential for their distinction. In addition to traditional markers such as Trypsin and Lipase, BCL10 has recently been reported to be a useful marker for the diagnosis of ACC [6]. BCL10 is exclusively expressed in normal acini and shows positive expression in 82% of resected ACC cases and 50% of adenosquamous carcinoma cases, while exhibiting negative expression in other subtypes of pancreas neoplasms [6]. In the present case, BCL10 positivity in immunohistochemistry played a key role in the diagnosis of ACC.

The accessory papilla, situated approximately 2 cm proximal to Vater’s papilla on the anterior wall of the descending duodenum, is present in most people [7]. It is often described as a potential secondary drainage route for pancreatic juice and a safety valve to prevent acute pancreatitis, although many aspects of its function remain unclear [7]. The accessory papilla is considered to be formed by the accessory pancreatic duct after penetrating the duodenal muscle layer and the surrounding fibrous connective tissue. However, the accessory papilla is not always surrounded by the sphincter like Vater’s papilla, which is surrounded by the sphincter of Oddi, and thus the border of the accessory papilla is not well-defined [7,8,9]. Therefore, we refer to this area as “the accessory papilla region” in this report. Pancreatic tissue is known to exist within the accessory papilla region, with a reported frequency of 59.0–76.4% in autopsy cases; this incidence is higher than the 9.2% for Vater’s papilla [10, 11]. Neoplasms in the accessory papilla region rarely occur, except for duodenal adenomas. In our search in PubMed between 1985 and 2023, we found a few reports of neuroendocrine tumor and adenocarcinoma but no reports of ACC.

An intriguing aspect of our case is the tumor’s origin. The most likely origin of ACC occurring in this region is infiltration from pancreatic ACC into the accessory papilla or progression of pancreatic ACC through the pancreatic duct to the accessory papilla. Indeed, ACC reportedly can grow and spread in the pancreatic duct, mimicking pancreatic intraductal neoplasm [2]. However, in the present case, pathological analysis revealed the tumor and the pancreatic parenchyma to be separated by the duodenal muscle layer with no continuity between them. In addition, no tumor cells were found within the surrounding pancreatic duct. Therefore, the scenario of pancreatic ACC extending to the accessory pancreatic duct was denied. Moreover, based on the observation that pancreatic acinar tissue was identified in the accessory papilla region and that it was quite close to the tumor, we think that the tumor may have arisen from this pancreatic tissue found in the accessory papilla region.

There remains a question as to whether the pancreatic tissue in the accessory papilla region could be considered an ectopic pancreas. The ectopic pancreas is defined as pancreatic tissue that lacks continuity with the pancreatic parenchyma and differs in vascular supply [12]. It is commonly found in the gastrointestinal tract, such as the duodenum, stomach, and jejunum [13]. Although there may be controversy as to whether the vascular supply of the accessory papilla and the pancreas is the same or different, pancreatic tissue in the accessory papilla region, as observed in the present case, lacks continuity with the pancreatic parenchyma. When considering the development of the pancreas in the fetus, the accessory papilla arises from the dorsal pancreatic bud of the duodenum, and during its formation acinar cell differentiation may occur in the region where it eventually becomes an accessory papilla in adulthood. Indeed, pancreatic tissue was observed in the accessory papilla in 59.0–76.4% of autopsy cases [10, 11]. Therefore, it may be possible to consider the pancreatic tissue in the accessory papilla to be an ectopic pancreas, although its formation is likely linked to the development of the pancreas. We conducted a literature review of ACC originating from ectopic pancreatic tissue. Although the occurrence of this condition is extremely rare, our searches in PubMed between 1999 and 2023 identified 15 case reports, including 8 in the stomach, 2 in the jejunum, 1 in the liver, and 4 in the duodenum; 2 of the duodenal cases involved the duodenal wall and the remaining 2 involved Vater’s papilla [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28]. This case presents the first report of ACC of the accessory papilla.

The primary treatment for resectable pancreatic ACC is surgery, and the prognosis is typically better compared to pancreatic ductal adenocarcinoma [29]. While there is no standard regimen for both adjuvant chemotherapy and chemotherapy for recurrent or advanced cases of ACC [30], it is often conducted following a regimen similar to that used for pancreatic ductal adenocarcinoma [30]. The most common forms of recurrence in pancreatic ACC are liver metastasis followed by peritoneal metastasis [29]. Our case was diagnosed and treated at a relatively early stage, and a favorable prognosis, similar to that with pancreatic ACC, could be expected. However, given that a few reports exist on cases of ACC originating from ectopic pancreatic tissue, and as its prognosis still remains unclear, close follow-up is essential.