Acinar Cystic Transformation of the Pancreas: Histomorphology and Molecular Analysis to Unravel its Heterogeneous Nature

Diagnostic Pearls and Pitfalls in the Evaluation of Biopsies of the Pancreas

CONTEXT.—

The examination of small pancreatic biopsies is a difficult task for pathologists. This is due to the scant and fragmented material often obtained from diagnostic procedures as well as the significant overlap between different neoplastic and nonneoplastic entities. In the upcoming neoadjuvant era, biopsies could become even more important, representing the only possibility to look at the real histomorphology of tumors before chemotherapy-induced modifications.

OBJECTIVES.—

To summarize and discuss the state-of-the-art diagnostic workflow for small pancreatic biopsies, including the most important morphologic and immunohistochemical features and molecular alterations. The main diagnostic pearls and pitfalls of this challenging scenario are also discussed. The most important topics of this review are represented by: (1) pancreatic ductal adenocarcinoma, along with its main differential diagnoses, including autoimmune pancreatitis; (2) solid hypercellular neoplasms, including neuroendocrine neoplasms, acinar cell carcinoma, pancreatoblastoma, and solid pseudopapillary neoplasms; and (3) cystic lesions. Real-world considerations will also be presented and discussed.

DATA SOURCES.—

Sources included a literature review of published studies and the author's own work.

CONCLUSIONS.—

The correct diagnosis of pancreatic lesions is a crucial step in the therapeutic journey of patients. It should be based on robust, standardized, and reliable hallmarks. As presented and discussed here, the integration of morphology with immunohistochemistry, and, in selected cases, with molecular analysis, represents a decisive step in this complex scenario.

Reappraising imaging features of pancreatic acinar cystic transformation: be aware of differential diagnoses

OBJECTIVES

Imaging features of pancreatic acinar cystic transformation (ACT) have been published. We aimed to describe the clinical and radiological characteristics of patients with a presumed pancreatic ACT diagnosis, reappraising the value of these published imaging criteria.

MATERIALS AND METHODS

Single-center retrospective study (2003-2021) of consecutive patients with a presumed diagnosis of ACT as suggested by the local expert multidisciplinary case review board. Patients without available imaging (CT or MRI) for review were excluded. Patients were classified into "certain" ACT (if ≥ 2 imaging criteria and no differential diagnosis) or "uncertain" ACT (if ≥ 1 imaging criteria and suggested differential diagnoses).

RESULTS

Sixty-four patients (35 males, [55%]) were included. ACT was considered "certain" for 34 patients (53%) and "uncertain" for 30 patients (47%). The number of ACT criteria did not differ between groups, with 91.2% of patients with ≥ 3 ACT imaging criteria in the "certain" group vs 93.3% in the "uncertain" group (p = 0.88). In the "uncertain" group, the main suggested differentials were branch-duct intraductal papillary mucinous neoplasm (18/30 patients, 60%), calcifying chronic pancreatitis (8/30 patients, 27%), both (three patients, 10%) and serous cystadenoma (one patient, 3%). Calcifications were significantly more frequent in the "uncertain" group (89% vs 63% in the "certain" group, p = 0.02).

CONCLUSION

Published ACT imaging criteria are frequently associated with features suggesting differential diagnoses. They appear insufficient to reach a final diagnosis in a subset of patients.

CLINICAL RELEVANCE STATEMENT

ACT displays a heterogeneous morphological imaging presentation challenging the non-invasive diagnostic work-up. Physicians' and radiologists' awareness of this entity is important to better understand its natural history and improve non-invasive diagnostic criteria.

KEY POINTS

The criteria to help diagnose ACT are frequently associated with features suggestive of differentials. The main alternatives suggested when ACT diagnosis was "uncertain" were branch-duct intraductal papillary mucinous neoplasm and calcifying chronic pancreatitis. Published ACT diagnostic imaging criteria can be insufficient for a definite non-invasive diagnosis.

Acinar Cystic Transformation of the Pancreas With Main Pancreatic Duct Dilation and Distal Pancreatic Atrophy

We report a rare case of a large, slowly progressive acinar cystic transformation (ACT) of the pancreas with main duct dilation and atrophy of the upstream pancreas. The diagnosis was made through endoscopic ultrasound-guided through-the-needle biopsy and histological identification of cuboidal acinar epithelium and characteristic immunohistochemistry findings. Distal pancreatectomy and splenectomy were performed because of elevated carcinoembryonic antigen levels, atypical cells on biopsy, and an increase in cyst size. Owing to the benign nature of this case, postresection surveillance was not necessary.

Acinar cystic transformation in the pancreatic tail

Pancreatic acinar cystic transformation (ACT) is a rare non-neoplastic cystic lesion that is predominantly located at the pancreatic head in females. Preoperative definitive diagnosis of ACT remains challenging despite advances in radiologic imaging methods. A 25-year-old male patient presented with abdominal discomfort and a 50-mm cystic lesion in the pancreatic tail. The patient underwent laparoscopic distal pancreatectomy, because branch duct intraductal papillary mucinous neoplasm cannot be ruled out and the presence of abdominal symptoms. The resected specimen revealed a collection of small and large cysts lined by a single cuboidal epithelium layer with scattered pancreatic tissue exhibiting fibrosis in the septal wall. The cystic lesion was epithelial, trypsin-positive, B cell lymphoma 10-positive, cytokeratin 19-positive, mucin 1-positive, and MUC6-negative with a differentiated lobular central conduit causing to an adeno-cystic cell, thereby supporting the ACT diagnosis. Distinguishing ACT from other pancreatic cystic tumors remains a diagnostic challenge despite improvements in radiologic imaging methods. Surgical resection may be justified when other cystic neoplasms cannot be excluded because of its heterogeneous nature, although the ACT is a non-neoplastic lesion, and cases of malignant transformation have never been reported to date.

Comprehensive characterisation of acinar cystic transformation of the pancreas: a systematic review

AIMS

Acinar cystic transformation (ACT) of the pancreas is a rare pancreatic cystic lesion. Owing to its rarity, comprehensive histomolecular characterisation of this entity is still lacking. We aim to perform a systematic review on this controversial entity.

METHODS

We searched PubMed, SCOPUS and Embase through May 2023 to identify all studies on ACTs. Clinicopathological, immunohistochemical (IHC) and molecular data have been extracted and analysed.

RESULTS

Overall, there were 121 cases of ACTs in the literature. ACT had a female predominance (65.3% of patients), and a mean size of 4.8 cm. ACT was more often unifocal (71.9%) and multiloculate (61.2%). Histologically, the cysts were lined by an acinar epithelium, sometimes harbouring ductal-like areas (18.2%). In five cases (4.1%), an intralesional pancreatic intraepithelial neoplasia (PanIN) was reported. Preoperative diagnosis is challenging. After surgical resection, all patients were alive and disease free during follow-up except one patient who developed a second ACT after resection. By IHC, all lesions were positive for acinar markers; cytokeratin 7 and 8/18/19 were usually positive, and Ki-67 was invariably ≤3%. At the molecular level, three cases demonstrated genetic alterations: one showed multiple chromosomal gains, and other two harboured somatic mutations of KRAS and SMO genes (one mutation per case).

CONCLUSIONS

Globally considered, our findings demonstrated that ACT is a benign entity, without the need of surgical resection with the exception of symptomatic lesions. The rare occurrence of intracystic PanINs and driver mutations suggest considering follow-up if a preoperative diagnosis of ACT can be made.

Pancreatic Acinar Cystic Transformation: Familial Forms Do Exist

ABSTRACT

Pancreatic acinar cystic transformation (ACT) is a recently described entity of the pancreatic cysts' spectrum. It is a nonneoplastic pancreatic lesion, mostly asymptomatic, and is considered a rare disease, with less than 100 reported cases. Its benign nature and the absence of reported cases of malignant transformation or invasive lesions were a plea for a conservative approach. As a consequence, little is known about the pathogenesis of this lesion. No familial history of ACT has been previously described. Here, we report a familial case of 2 siblings with typical ACT imaging lesions giving evidence for familial aggregation. No obvious environmental exposure was identified as a potential risk factor for ACT development. No physiological data exist to suggest a congenital nature of these lesions, but for the first patient, the calcifications seem to have appeared with time. Further research, with high throughput sequencing technologies, may elucidate genetic polymorphisms explaining potential ACT familial phenotype. In practice, careful family history collection must be performed to identify a possible familial form of ACT when this diagnosis is proposed.