RET gene rearrangements occur in a subset of pancreatic acinar cell carcinomas

A narrative review on rare types of pancreatic cancer: should they be treated as pancreatic ductal adenocarcinomas?

Pancreatic cancer is one of the deadliest malignancies in humans and it is expected to play a bigger part in cancer burden in the years to come. Pancreatic ductal adenocarcinoma (PDAC) represents 85% of all primary pancreatic malignancies. Recently, much attention has been given to PDAC, with significant advances in the understanding of the mechanisms underpinning disease initiation and progression, along with noticeable improvements in overall survival in both localized and metastatic settings. However, given their rarity, rare histological subtypes of pancreatic cancer have been underappreciated and are frequently treated as PDAC, even though they might present non-overlapping molecular alterations and clinical behavior. While some of these rare histological subtypes are true variants of PDAC that should be treated likewise, others represent separate clinicopathological entities, warranting a different therapeutic approach. In this review, we highlight clinical, pathological, and molecular aspects of rare histological types of pancreatic cancer, along with the currently available data to guide treatment decisions.

Paediatric pancreatic acinar cell carcinoma with a novel SEC31A-BRAF fusion gene

Paediatric pancreatic acinar cell carcinoma (PACC) presents a diagnostic challenge, often confused with pancreatoblastoma (PB) due to its rarity. It is crucial to differentiate between PB and PACC, given their distinct therapeutic strategies and prognoses. Histologically, the absence of squamoid nests and scarcity of tumor mesenchyme support PACC. Conversely, the identification of a BRAF alteration leans towards PACC. Here, we present the case of an 8-year-old girl with a well-defined mass in the pancreas. The tumor exhibited a SEC31A-BRAF fusion gene and amplification of 18p, showcasing unequivocal acinar differentiation and a minor degree of neuroendocrine differentiation. Additionally, the tumor displayed scant fibrous stroma, and an absence of squamoid nests, further supporting PACC. Notably, this is the first reported instance of a solid tumor featuring a SEC31A-BRAF gene fusion. The discovery of this novel fusion gene expands our understanding of BRAF fusion partner profiles, particularly in the context of paediatric PACC.

Fusion genes in pancreatic tumors

Gene fusions and rearrangements play a crucial role in tumor biology. They are rare events typically detected in KRAS wild-type (WT) pancreatic tumors. Their identification can inform clinical management by enabling precision oncology, as fusions involving BRAF, FGFR2, RET, NTRK, NRG1, and ALK represent actionable targets in KRAS-WT cancers, and serve diagnostic purposes since fusions involving PRKACA/B represent the diagnostic hallmark of intraductal oncocytic papillary neoplasms (IOPNs). Although they are rare, the therapeutic and diagnostic importance of these genomic events should not be underestimated, highlighting the need for quality-ensured molecular diagnostics in the management of cancer. Herein we review the existing literature on the role of fusion genes in pancreatic tumors and their clinical potential as effective biomarkers and therapeutic targets.

Post-therapy emergence of an NBN reversion mutation in a patient with pancreatic acinar cell carcinoma

Pancreatic acinar cell carcinoma (PACC) is a rare form of pancreatic cancer that commonly harbors targetable alterations, including activating fusions in the MAPK pathway and loss-of-function (LOF) alterations in DNA damage response/homologous recombination DNA repair-related genes. Here, we describe a patient with PACC harboring both somatic biallelic LOF of NBN and an activating NTRK1 fusion. Upon disease progression following 13 months of treatment with folinic acid, fluorouracil, irinotecan, and oxaliplatin (FOLFIRINOX), genomic analysis of a metastatic liver biopsy revealed the emergence of a novel reversion mutation restoring the reading frame of NBN. To our knowledge, genomic reversion of NBN has not been previously reported as a resistance mechanism in any tumor type. The patient was treated with, but did not respond to, targeted treatment with a selective NTRK inhibitor. This case highlights the complex but highly actionable genomic landscape of PACC and underlines the value of genomic profiling of rare tumor types such as PACC.

Cytomorphologic, immunophenotypical and molecular features of pancreatic acinar cell carcinoma

OBJECTIVES

As a rare tumor in pancreas, pancreatic acinar cell carcinoma (PACC) possesses a distinct molecular feature from pancreatic ductal carcinoma (PDAC). Though the diagnosis of PACC is often established based on cytology specimens, its cytologic diagnosis can be challenging. Furthermore, the correlation between PACC cytomorphology and its unique different molecular alterations have not been fully explored.

METHODS

Cytology features were analyzed in 8 histologically proven PACC and cytohistological correlation was performed. Immunocytochemistry for trypsin, chymotrypsin, BCL10, synaptophysin, chromogranin A, INSM1, β-catenin, and Ki-67 was assessed. Comprehensive molecular profiling and additional targetable treatment biomarker assessment were also performed.

RESULTS

The cohort included 4 mixed acinar-neuroendocrine carcinomas, 3 pure PACCs, and 1 mixed acinar-ductal carcinoma. Immunophenotypical features are consistent with diagnoses of PACC or PACC with neuroendocrine features. Identified genetic alterations included somatic mutations of CTNNB1, TP53, MAP2K1, PTEN, RAC1, germline mutations of NBN and BRAC2, and gene fusion of CCDC6-RET.

CONCLUSIONS

The current study is the first attempt to explore the correlation between the cytomorphology characteristics and molecular features of PACC and a few intriguing findings were observed. Further validation in larger cohorts is warranted.

Pan-tumor survey of RET fusions as detected by next-generation RNA sequencing identified RET fusion positive colorectal carcinoma as a unique molecular subset

BACKGROUND

RET fusions are driver alterations in cancer and are most commonly found in non-small cell lung cancer and well-differentiated thyroid cancer. However, RET fusion have been reported in other solid tumors.

MATERIAL AND METHODS

A retrospective analysis of RET+ solid malignancies identified by targeted RNA sequencing and whole transcriptome sequencing of clinical tumor samples performed at Caris Life Science (Phoenix, AZ).

RESULTS

As of March 22, 2022, a total of 378 RET+ solid malignancies were identified in 15 different tumor types and carcinoma of unknown primary (CUP) that underwent next-generation RNA sequencing. RET+ NSCLC and RET+ thyroid cancer constituted 66.9% and 11.1% of the RET+ solid malignancies, respectively. RET+ colorectal adenocarcinoma and RET+ breast adenocarcinoma constituted 10.1% and 2.6%, respectively. The estimated frequency of RET fusions within specific tumor types were NSCLC 0.7%, thyroid cancer 3.1%, colorectal cancer 0.2% and breast cancer 0.1%. KIF5B (46.8%) was the most common fusion partner followed by CCDC6 (28.3%) and NCOA4 (13.8%) in RET+ solid tumors. KIF5B-RET was the dominant fusion variant in RET+ NSCLC, NCOA4-RET was the dominant variant in RET+ colorectal carcinoma, and CCDC6-RET was the dominant variant in thyroid cancer. The most common single gene alterations in RET+ tumors were TP53 (34.8%), RASA1 (14.3%) and ARIAD1A (11.6%). RET+ CRC had a high median TMB of 20.0 and were commonly MSI-H.

CONCLUSIONS

RET fusions were identified in multiple tumor types. With a higher median TMB and commonly MSI-H, RET fusion positive CRC may be a unique molecular subset of CRC.

Clinical and Genomic Characterization of Pancreatic Ductal Adenocarcinoma with Signet-Ring/Poorly Cohesive Cells

Signet-ring cell (SRC)/poorly cohesive cell carcinoma is an aggressive variant of pancreatic ductal adenocarcinoma (PDAC). This study aimed to clarify its clinicopathologic and molecular profiles based on a multi-institutional cohort of 20 cases. The molecular profiles were investigated using DNA and RNA sequencing. The clinicopathologic parameters and molecular alterations were analyzed based on survival indices and using a validation/comparative cohort of 480 conventional PDAC patients. The primary findings were as follows: (1) clinicopathologic features: SRC carcinomas are highly aggressive neoplasms with poor prognosis, and the lungs are elective metastatic sites; (2) survival analysis: a higher SRC component was indicative of poorer prognosis. In particular, the most clinically significant threshold of SRC was 80%, showing statistically significant differences in both disease-specific and disease-free survival; (3) genomic profiles: SRC carcinomas are similar to conventional PDAC with the most common alterations affecting the classic PDAC drivers KRAS (70% of cases), TP53 (55%), SMAD4 (25%), and CDKN2A (20%). EGFR alterations, RET::CCDC6 fusion gene, and microsatellite instability (3 different cases, 1 alteration per case) represent novel targets for precision oncology. The occurrence of SMAD4 mutations was associated with poorer prognosis; (4) pancreatic SRC carcinomas are genetically different from gastric SRC carcinomas: CDH1, the classic driver gene of gastric SRC carcinoma, is not altered in pancreatic SRC carcinoma; (5) transcriptome analysis: the cases clustered into 2 groups, one classical/exocrine-like, and the other squamous-like; and (6) SRC carcinoma-derived organoids can be successfully generated, and their cultures preserve the histologic and molecular features of parental SRC carcinoma. Although pancreatic SRC carcinoma shares similarities with conventional PDAC regarding the most important genetic drivers, it also exhibits important differences. A personalized approach for patients with this tumor type should consider the clinical relevance of histologic determination of the SRC component and the presence of potentially actionable molecular targets.

RET-Altered Cancers-A Tumor-Agnostic Review of Biology, Diagnosis and Targeted Therapy Activity

RET alterations, such as fusions or mutations, drive the growth of multiple tumor types. These alterations are found in canonical (lung and thyroid) and non-canonical (e.g., gastrointestinal, breast, gynecological, genitourinary, histiocytic) cancers. RET alterations are best identified via comprehensive next-generation sequencing, preferably with DNA and RNA interrogation for fusions. Targeted therapies for RET-dependent cancers have evolved from older multikinase inhibitors to selective inhibitors of RET such as selpercatinib and pralsetinib. Prospective basket trials and retrospective reports have demonstrated the activity of these drugs in a wide variety of RET-altered cancers, notably those with RET fusions. This paved the way for the first tumor-agnostic selective RET inhibitor US FDA approval in 2022. Acquired resistance to RET kinase inhibitors can take the form of acquired resistance mutations (e.g., RET G810X) or bypass alterations.

Microsatellite instability in pancreatic and ampullary carcinomas: histology, molecular pathology, and clinical implications

Microsatellite instability (MSI)/defective DNA mismatch repair (dMMR) represents an important molecular alteration with diagnostic, prognostic, and predictive value. The increasing interest toward this genetic alteration is given to the high response rate of MSI/dMMR tumors to immunotherapy. There are different cancers in the periampullary region that can harbor MSI/dMMR, and significant morphological-molecular correlates should be acknowledged in this district: (1) pancreatic ductal adenocarcinoma (PDAC): in this tumor category, the prevalence of MSI/dMMR is about 1-2%, and medullary and colloid variants are the most typically involved; (2) ampullary adenocarcinoma: here the prevalence of MSI/dMMR is up to 18%, and in this neoplastic group, MSI/dMMR is more commonly found in the intestinal subtype; (3) pancreatic acinar cell carcinoma: here the prevalence of MSI/dMMR is up to 14%; and (4) pancreatic and ampullary neuroendocrine carcinoma: in this tumor category, the prevalence of MSI/dMMR is up to 5-8%, and this molecular alteration should be assessed also in cases of mixed neuroendocrine-non-neuroendocrine neoplasms. Given the clinical importance of MSI/dMMR and its not-negligible prevalence among the different carcinomas arising in this district, its assessment should become part of the routine diagnostic workflow at least for the most typical histotypes. The test of choice is represented by immunohistochemistry for PDAC and ampullary carcinomas, and by direct molecular analyses including MSI-based polymerase chain reaction and next-generation sequencing for acinar cell and neuroendocrine carcinomas.

AGAP3: A novel BRAF fusion partner in pediatric pancreatic-type acinar cell carcinoma

Most available molecular data on pancreatic acinar cell carcinoma (PACC) are provided by studies of adult cases. BRAF, RAF1, or RET rearrangements have been described in approximately 30% of cases. To the best of our knowledge, only seven cases with molecular data have been reported in pediatric PACC. We report here the comprehensive study of a pancreatic-type ACC from a 6-year-old patient. We detected an AGAP3::BRAF fusion. This result showing a BRAF rearrangement demonstrates a molecular link between adult and pediatric PACC. Moreover, it identifies AGAP3, a gene located at 7q36.1 that encodes a major component of the N-methyl-d-aspartate (NMDA) receptor signaling complex, as a partner gene of BRAF. The variability of BRAF partners is consistent with a driver role of BRAF alterations in PACC. The identification of such alterations is noteworthy for considering the use of MEK inhibitors in metastatic cases. We did not detect associated genomic instability. The better outcome of pediatric cases might be related to their stable genomic background.

Pancreatoblastomas and mixed and pure acinar cell carcinomas share epigenetic signatures distinct from other neoplasms of the pancreas

Pancreatic neoplasms are heterogenous and have traditionally been classified by assessing their lines of cellular differentiation using histopathologic methods, particularly morphologic and immunohistochemical evaluation. These methods frequently identify overlapping differentiation along ductal, acinar, and neuroendocrine lines, raising diagnostic challenges as well as questions regarding the relationship of these neoplasms. Neoplasms with acinar differentiation, in particular, frequently show more than one line of differentiation based on immunolabeling. Genome methylation signatures, in contrast, are better conserved within cellular lineages, and are increasingly used to support the classification of neoplasms. We characterized the epigenetic relationships between pancreatoblastomas, acinar cell carcinomas (including mixed variants), pancreatic neuroendocrine tumors, solid pseudopapillary neoplasms, and pancreatic ductal adenocarcinomas using a genome-wide array platform. Using unsupervised learning approaches, pancreatic neuroendocrine tumors, solid pseudopapillary neoplasms, ductal adenocarcinomas, and normal pancreatic tissue samples all localized to distinct clusters based on their methylation profiles, whereas all neoplasms with acinar differentiation occupied a broad overlapping region located between the predominantly acinar normal pancreatic tissue and ductal adenocarcinoma clusters. Our data provide evidence to suggest that acinar cell carcinomas and pancreatoblastomas are similar at the epigenetic level. These findings are consistent with genomic and clinical observations that mixed acinar neoplasms are closely related to pure acinar cell carcinomas rather than to neuroendocrine tumors or ductal adenocarcinomas.

Molecular Characterization of KRAS Wild-type Tumors in Patients with Pancreatic Adenocarcinoma

PURPOSE

KRAS mutation (MT) is a major oncogenic driver in pancreatic ductal adenocarcinoma (PDAC). A small subset of PDACs harbor KRAS wild-type (WT). We aim to characterize the molecular profiles of KRAS WT PDAC to uncover new pathogenic drivers and offer targeted treatments.

EXPERIMENTAL DESIGN

Tumor tissue obtained from surgical or biopsy material was subjected to next-generation DNA/RNA sequencing, microsatellite instability (MSI) and mismatch repair status determination.

RESULTS

Of the 2,483 patients (male 53.7%, median age 66 years) studied, 266 tumors (10.7%) were KRAS WT. The most frequently mutated gene in KRAS WT PDAC was TP53 (44.5%), followed by BRAF (13.0%). Multiple mutations within the DNA-damage repair (BRCA2, ATM, BAP1, RAD50, FANCE, PALB2), chromatin remodeling (ARID1A, PBRM1, ARID2, KMT2D, KMT2C, SMARCA4, SETD2), and cell-cycle control pathways (CDKN2A, CCND1, CCNE1) were detected frequently. There was no statistically significant difference in PD-L1 expression between KRAS WT (15.8%) and MT (17%) tumors. However, KRAS WT PDAC were more likely to be MSI-high (4.7% vs. 0.7%; P < 0.05), tumor mutational burden-high (4.5% vs. 1%; P < 0.05), and exhibit increased infiltration of CD8+ T cells, natural killer cells, and myeloid dendritic cells. KRAS WT PDACs exhibited gene fusions of BRAF (6.6%), FGFR2 (5.2%), ALK (2.6%), RET (1.3%), and NRG1 (1.3%), as well as amplification of FGF3 (3%), ERBB2 (2.2%), FGFR3 (1.8%), NTRK (1.8%), and MET (1.3%). Real-world evidence reveals a survival advantage of KRAS WT patients in overall cohorts as well as in patients treated with gemcitabine/nab-paclitaxel or 5-FU/oxaliplatin.

CONCLUSIONS

KRAS WT PDAC represents 10.7% of PDAC and is enriched with targetable alterations, including immuno-oncologic markers. Identification of KRAS WT patients in clinical practice may expand therapeutic options in a clinically meaningful manner.

Imaging and Clinicopathological Features of Acinar Cell Carcinoma

Background

Acinar cell carcinoma (ACC) is a rare pancreatic epithelial malignancy that poses a significant threat. However, there are few related clinical studies. The present study aimed to analyze the imaging and pathological features of ACC to provide a reference for better diagnosis and treatment planning.

Methods

Thirty-nine with ACC, referred to Qianfoshan Hospital, Qilu Hospital and Provincial Hospital in Shandong Province from December 2012 to December 2020, were enrolled. Their imaging and clinicopathological features were analyzed. They were followed up for 1 year, and Cox regression was used to analyze the factors affecting patient prognosis.

Results

ACC was more common in the middle-aged and elderly and peaked at approximately 60 years. The clinical manifestations of the patients were mostly flatulence and upper abdomen pain. The tumor was located in the head of the pancreas in 19 cases, with an average size of 5.8 cm. We found nerve invasion and liver metastasis in one case each. 8 patients showed irregular amorphous tumor calcification on plain computed tomography and 5 showed high and low signals on T1- and T2-weighted images, respectively. Immunohistochemistry revealed 100.0% positive rates for CK, β-catenin, and Ki-67. Thirty-three patients underwent surgical resection, and the 2-year overall mortality rate was 25.6%. Cox analysis revealed that smoking was an independent risk factor affecting patient prognosis.

Conclusion

An in-depth understanding of the imaging and clinicopathological features of ACC is conducive to better diagnosis and treatment planning for ACC and subsequent improvement in patient prognosis.

Gene Fusions in Gastrointestinal Tract cancers

Fusion genes have been identified a wide array of human neoplasms including hematologic and solid tumors, including gastrointestinal tract neoplasia. A fusion gene is the product of parts of two genes which are joined together following a deletion, translocation or chromosomal inversion. Together with single nucleotide variants, insertions, deletions, and amplification, fusion genes represent one of the key genomic mechanisms for tumor development. Detecting fusions in the clinic is accomplished by a variety of techniques including break-apart fluorescence in situ hybridization (FISH), reverse transcription-polymerase chain reaction (RT-PCR), and next-generation sequencing (NGS). Some recurrent gene fusions have been successfully targeted by small molecule or monoclonal antibody therapies (i.e. targeted therapies), while others are used for as biomarkers for diagnostic and prognostic purposes. The purpose of this review article is to discuss the clinical utility of detection of gene fusions in carcinomas and neoplasms arising primarily in the digestive system. This article is protected by copyright. All rights reserved.

A Performance Comparison of Commonly Used Assays to Detect RET Fusions

PURPOSE

Selpercatinib and pralsetinib induce deep and durable responses in patients with advanced RET fusion-positive lung and thyroid cancer. RET fusion testing strategies with rapid and reliable results are critical given recent FDA approval. Here, we assess various clinical assays in a large pan-cancer cohort.

EXPERIMENTAL DESIGN

Tumors underwent DNA-based next-generation sequencing (NGS) with reflex to RNA-based NGS if no mitogenic driver or if a RET structural variant of unknown significance (SVUS) were present. Canonical DNA-level RET fusions and RNA-confirmed RET fusions were considered true fusions. Break-apart FISH and IHC performance were assessed in subgroups.

RESULTS

A total of 171 of 41,869 patients with DNA NGS harbored RET structural variants, including 139 canonical fusions and 32 SVUS. Twelve of 32 (37.5%) SVUS were transcribed into RNA-level fusions, resulting in 151 oncogenic RET fusions. The most common RET fusion-positive tumor types were lung (65.6%) and thyroid (23.2%). The most common partners were KIF5B (45%), CCDC6 (29.1%), and NCOA4 (13.3%). DNA NGS showed 100% (46/46) sensitivity and 99.6% (4,459/4,479) specificity. FISH showed 91.7% (44/48) sensitivity, with lower sensitivity for NCOA4-RET (66.7%, 8/12). A total of 87.5% (7/8) of RET SVUS negative for RNA-level fusions demonstrated rearrangement by FISH. The sensitivity of IHC varied by fusion partner: KIF5B sensitivity was highest (100%, 31/31), followed by CCDC6 (88.9%, 16/18) and NCOA4 (50%, 6/12). Specificity of RET IHC was 82% (73/89).

CONCLUSIONS

Although DNA sequencing has high sensitivity and specificity, RNA sequencing of RET SVUS is necessary. Both FISH and IHC demonstrated lower sensitivity for NCOA4-RET fusions.

Why did they change that? Practical implications of the evolving classification of neuroendocrine tumours of the gastrointestinal tract

Neuroendocrine neoplasms (NENs) of the gastrointestinal tract (GIT) comprise neuroendocrine tumours (NETs) and neuroendocrine carcinomas (NECs). During the last decade the classification and grading of GIT NENs has undergone significant changes, culminating in the World Health Organisation (WHO) 2019 classification. These changes, some of which are attributable to an only partially successful attempt to achieve uniform nomenclature among different organs, include: slight changes to the cut-off used for the Ki-67 proliferative index to distinguish grade 1 from grade 2 NETs; an emphasis on the distinction between grade 3 NETs (low-grade NETs with a high proliferative rate) and NECs which, by definition, are all high grade; classification of tumours with mixed non-neuroendocrine and neuroendocrine differentiation as MiNENs; and replacement of the term 'goblet cell carcinoid' with 'goblet cell adenocarcinoma'. While some of these changes seem minor, even semantic, each was made for very specific reasons which reflect an improved understanding of neuroendocrine neoplasia. The changes have definite implications for pathologists in clinical practice, not all of which may be readily apparent. This review is an attempt to explain the background behind each of the recent changes to the classification of neuroendocrine neoplasms of the gastrointestinal tract and summarise their impact on surgical pathologists - including a guide on how to approach certain recurrent difficulties encountered with the WHO 2019 system in routine clinical practice.

A Proposed Grading Scheme for Medullary Thyroid Carcinoma Based on Proliferative Activity (Ki-67 and Mitotic Count) and Coagulative Necrosis

We investigated the prognostic value of a range of histologic parameters in medullary thyroid carcinoma (MTC) to design a grading system to predict overall survival. We assessed 76 patients with MTCs undergoing primary tumor resection for age, sex, tumor size, vascular space invasion, lymph node metastasis, multiple endocrine neoplasia type 2 (MEN2) status, mitotic count, Ki-67 proliferative index, spindled morphology, sheet-like growth pattern, coagulative necrosis, incipient necrosis, nuclear grade, multinucleation, prominent nucleoli, fibrosis, and amyloid deposition. In addition to the clinical features of age and the diagnosis of MEN2, the only histologic features that significantly predicted reduced overall survival were Ki-67 proliferative index, mitotic count, and the presence of coagulative necrosis. Using a combination of these 3 variables, we propose a 3-tiered grading system based solely on proliferative activity (Ki-67 proliferative index and mitotic count) and necrosis. There were 62 (82%) low-grade MTCs (low proliferative activity, no necrosis), 9 (12%) intermediate grade (low proliferative activity and necrosis present, or intermediate proliferative activity and no necrosis), and 5 (7%) high grade (intermediate proliferative activity and necrosis present, or high proliferative activity with or without necrosis). The mean overall survival was 193, 146, and 45 months, respectively (P=0.0001) for the 3 grades. The grading system remained prognostic when controlled for other factors associated with survival including age and known MEN2 syndrome. We conclude that this proposed grading system, which uses only a combination of proliferative activity (Ki-67 index, mitotic count) and coagulative necrosis, is a strong predictor of overall survival in MTC.

RAF1 rearrangements are common in pancreatic acinar cell carcinomas

There is now evidence that gene fusions activating the MAPK pathway are relatively common in pancreatic acinar cell carcinoma with potentially actionable BRAF or RET fusions being found in ~30%. We sought to investigate the incidence of RAF1 fusions in pancreatic malignancies with acinar cell differentiation. FISH testing for RAF1 was undertaken on 30 tumors comprising 25 'pure' acinar cell carcinomas, 2 mixed pancreatic acinar-neuroendocrine carcinomas, 1 mixed acinar cell-low grade neuroendocrine tumor and 2 pancreatoblastomas. RAF1 rearrangements were identified in 5 cases and confirmed by DNA and RNA sequencing to represent oncogenic fusions (GATM-RAF1, GOLGA4-RAF1, PDZRN3-RAF1, HERPUD1-RAF1 and TRIM33-RAF1) and to be mutually exclusive with BRAF and RET fusions, as well as KRAS mutations. Large genome-wide copy number changes were common and included 1q gain and/or 1p loss in all five RAF1 FISH-positive acinar cell carcinomas. RAF1 expression by immunohistochemistry was found in 3 of 5 (60%) of fusion-positive cases and no FISH-negative cases. Phospho-ERK1/2 expression was found in 4 of 5 RAF1-fusion-positive cases. Expression of both RAF1 and phospho-ERK1/2 was heterogeneous and often only detected at the tumor-stroma interface, thus limiting their clinical utility. We conclude that RAF1 gene rearrangements are relatively common in pancreatic acinar cell carcinomas (14.3% to 18.5% of cases) and can be effectively identified by FISH with follow up molecular testing. The combined results of several studies now indicate that BRAF, RET or RAF1 fusions occur in between one third and one-half of these tumors but are extremely rare in other pancreatic malignancies. As these fusions are potentially actionable with currently available therapies, a strong argument can be made to perform FISH or molecular testing on all pancreatic acinar cell carcinomas.

One size does not fit all for pancreatic cancers: A review on rare histologies and therapeutic approaches

Exocrine pancreatic neoplasms represent up to 95% of pancreatic cancers (PCs) and are widely recognized among the most lethal solid cancers, with a very poor 5-year survival rate of 5%-10%. The remaining < 5% of PCs are neuroendocrine tumors that are usually characterized by a better prognosis, with a median overall survival of 3.6 years. The most common type of PC is pancreatic ductal adenocarcinoma (PDAC), which accounts for roughly 85% of all exocrine PCs. However up to 10% of exocrine PCs have rare histotypes, which are still poorly understood. These subtypes can be distinguished from PDAC in terms of pathology, imaging, clinical presentation and prognosis. Additionally, due to their rarity, any knowledge regarding these specific histotypes is mostly based on case reports and a small series of retrospective analyses. Therefore, treatment strategies are generally deduced from those used for PDAC, even if these patients are often excluded or not clearly represented in clinical trials for PDAC. For these reasons, it is essential to collect as much information as possible on the management of PC, as assimilating it with PDAC may lead to the potential mistreatment of these patients. Here, we report the most significant literature regarding the epidemiology, typical presentation, possible treatment strategies, and prognosis of the most relevant histotypes among rare PCs.