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The 5-year survival for pancreatic ductal adenocarcinoma (PDAC) exceeds 74% if diagnosed at stage I; however, this represents just 12% of resected cases.1Blackford A.L. et al.J Natl Cancer Inst. 2020; 112: 1162-1169Crossref PubMed Scopus (115) Google Scholar Detection of cancer at this stage—or before this, at the point of high-grade dysplasia (HGD)—represents a significant opportunity to improve survival. Gallium-68–linked fibroblast activation protein inhibitor (68Ga-FAPI) is a novel radiotracer used with positron emission tomography/computed tomography (68Ga-FAPI-PET/CT). It targets fibroblast activation protein (FAP), which is a cell surface receptor expressed on cancer-associated fibroblasts (malignant stroma) as well as epithelial cancer cells in PDAC.2Wen Z. et al.Ann Transl Med. 2019; 7: 532Crossref Google Scholar,3Cohen S.J. et al.Pancreas. 2008; 37: 154-158Crossref PubMed Scopus (192) Google Scholar Invasive PDAC has higher stromal FAP expression than intraductal papillary mucinous neoplasm (IPMN) with low-grade dysplasia (LGD), and this correlates with increased uptake on 68Ga-FAPI-PET/CT.4Spektor A.M. et al.J Nucl Med. 2024; 65: 52-58Crossref Scopus (6) Google Scholar However, it remains unknown if FAP expression can make a granular distinction between LGD, HGD, stage I PDAC, and chronic pancreatitis (CP) and, therefore, aid early diagnosis. Although epithelial cells contribute more than half the tumor mass in preinvasive IPMN and early PDAC,5Kakizaki Y. et al.Pancreas. 2016; 45: 1145-1152Crossref PubMed Scopus (17) Google Scholar to our knowledge, epithelial FAP expression has never been studied in tumorigenesis. This study aimed to determine if stromal and epithelial FAP expression, as measured by immunohistochemistry or 68Ga-FAPI-PET/CT, could distinguish early malignancy (HGD, stage I PDAC) from indolent pathology (LGD, CP). Patient tissue from pancreatic resection or biopsy was biobanked with the Australian Pancreatic Cancer Genome Initiative. Immunohistochemistry was performed against FAP on tissue microarrays containing IPMN with LGD (n = 13), IPMN with HGD (n = 11), PDAC at stage I (n = 25), PDAC at stages II–IV (n = 50), and CP (n = 48). Semiquantitative analysis was used to score stromal and epithelial expression separately between 0 and 6 for each case, with a score of >3 indicating positive expression for that cell type. Overall FAP expression was then calculated as the sum of stromal and epithelial scores. HGD demonstrated the highest overall expression of FAP and all 3 groups of malignant neoplasia (HGD, stage I PDAC, and stage II–IV PDAC) had significantly higher overall expression than both LGD and CP (according to analysis of variance and the Tukey post hoc test) (Figure 1A). To understand which cells drove overall FAP expression at each level, we separately compared mean stromal and epithelial expression between indolent and malignant groups. Stromal expression in all stages of PDAC was higher than LGD and CP, but no statistically significant difference was shown between HGD and indolent pathology (Figure 1B). Epithelial expression was seen in all pancreatic neoplasia and was significantly higher in malignant neoplasia compared with CP (Figure 1C). We then compared the frequency of positive expression between malignant and indolent pathology. FAP expression on any cell type was significantly more likely in all 3 groups of malignant neoplasia than LGD (Supplementary Figure 1). PDAC of any stage was more likely to have positive expression compared with CP; however, the odds were not statistically different between HGD and CP (Supplementary Figure 1). Positive FAP expression also occurred in different cellular patterns according to the pathologic group. Positive expression in preinvasive IPMN, which increased in frequency from LGD to HGD, was seen on epithelial cells in every instance where it occurred. Concomitant stromal expression occurred in some of these cases, although more commonly among HGD. All stages of PDAC showed "stromal only" or "dual expression" at similar frequencies, whereas positively expressed CP harbored a stromal-only pattern almost exclusively. This difference in the pattern of positive expression between groups was evaluated using multinomial logistic regression (Supplementary Figure 1). We then prospectively performed 68Ga-FAPI-PET/CT in 6 patients with pancreatic cystic neoplasm. Focal 68Ga-FAPI uptake was detected in 4 of these patients, 3 of whom had HGD or PDAC later confirmed on tissue sampling or interval imaging (Supplementary Table 1). Most notably, 1 participant (ID: 5) showed positive expression on 68Ga-FAPI-PET/CT, underwent pancreatoduodenectomy revealing IPMN with HGD, and had epithelial and stromal score, of 6 and 1, respectively, on immunohistochemistry against FAP. Epithelial expression was therefore detected on 68Ga-FAPI-PET/CT in this patient. Our results indicate, for the first time to our knowledge, that epithelial FAP expression is a unique feature of neoplasia that precedes malignant transformation. Importantly, the summation of FAP expression on stromal and epithelial cells becomes up-regulated at the point of HGD and is maintained through all subsequent stages of PDAC (Figure 1D). We have also shown that epithelial and stromal expression both contribute to uptake values observed on 68Ga-FAPI-PET/CT, which is consistent with our observation that focal avidity in a small cohort was associated with subsequent malignant diagnosis. FAP expression, it appears, is a marker of the abnormal epithelial-stromal cell signaling that occurs in the late stages of PDAC tumorigenesis. This is supported by previous studies suggesting that FAP expression is driven by transforming growth factor β secretion2Wen Z. et al.Ann Transl Med. 2019; 7: 532Crossref Google Scholar,6Kahounová Z. et al.Cytometry A. 2018; 93: 941-951Crossref PubMed Scopus (52) Google Scholar and that dysregulation of the SMAD4/transforming growth factor β pathway occurs in the transition of LGD to HGD.6Kahounová Z. et al.Cytometry A. 2018; 93: 941-951Crossref PubMed Scopus (52) Google Scholar,7Scarlett C.J. et al.Precursor lesions in pancreatic cancer: morphological and molecular pathology.Pathology. 2011; 43: 183-200Abstract Full Text PDF PubMed Scopus (65) Google Scholar The premise that neoplastic epithelial cells initiate this signaling is also supported in the fact that stromal FAP expression is more likely to occur adjacent to cancer cells in PDAC.2Wen Z. et al.Ann Transl Med. 2019; 7: 532Crossref Google Scholar,3Cohen S.J. et al.Pancreas. 2008; 37: 154-158Crossref PubMed Scopus (192) Google Scholar Localization of this signaling in vivo using 68Ga-FAPI-PET/CT could play a significant role in surveillance and surgical decision making for patients with pancreatic cystic neoplasm, increased familial risk, or an uncertain diagnosis of PDAC. There were limitations to this study. First, we were unable to examine the most common precursor lesion to PDAC, pancreatic intraepithelial neoplasia. Although pancreatic intraepithelial neoplasia specimens were included with the Australian Pancreatic Cancer Genome Initiative bioresource, the microscopic nature of this pathology made analysis using tumor microarrays unfeasible. Second, the prospective imaging component of this study was too small to draw statistical conclusions. Third, no threshold for positive expression on immunohistochemistry has been established with reference to findings on 68Ga-FAPI-PET/CT, nor is it known if epithelial expression can be distinguished from stromal expression on PET scan. Future studies that correlate findings of 68Ga-FAPI-PET/CT with pathologic outcome in a large prospective cohort are warranted. In conclusion, we have found that the transition to HGD and subsequent PDAC is marked by increased epithelial followed by stromal FAP expression. 68Ga-FAPI-PET/CT detects expression on either cell and therefore may facilitate diagnosis at a curable stage. The authors sincerely thank the following individuals associated with the Australian Pancreatic Cancer Genome Initiative, who all made notable contributions to the design, analysis, and writing of this work: John D. Hooper,1 Paul A. Thomas,2 Caroline Cooper,3 Thomas O'Rourke,4 Nick Butler,4 Shinn Yeung,4 Thomas Kryza,1 Brook Gulhane,2 Melissa J. Latter,2 Nicola Waddell,5 Jaswinder S. Samra,6 Fiona Simpson,7 and Marina Pajic8,9; from the 1Mater Research Institute, The University of Queensland, Translational Research Institute, Woolloongabba, Queensland, Australia; 2Department of Nuclear Medicine and Specialised PET Services, Royal Brisbane and Women's Hospital, Herston, Queensland, Australia; 3Pathology Queensland, Princess Alexandra Hospital, Brisbane, Queensland, Australia; 4Department of Hepatobiliary and Pancreatic Surgery, Princess Alexandra Hospital, Woolloongabba, Queensland, Australia; 5Cancer Program, QIMR Berghofer Medical Research Institute, Brisbane, Queensland, Australia; 6Department of Surgery, Royal North Shore Hospital, St Leonards, Sydney, New South Wales, Australia; 7Frazer Institute, The University of Queensland, Woolloongabba, Queensland, Australia; 8The Kinghorn Cancer Centre, Garvan Institute of Medical Research, Darlinghurst, New South Wales, Australia; 9Faculty of Medicine, St Vincent's Clinical School, UNSW Sydney, Sydney, New South Wales, Australia. The authors would also like thank Ruth Lyons and Nicola Blackburn for their support relating to tissue microarrays from the Australian Pancreatic Cancer Genome Initiative. The authors also thank Marita Prior and Karen Lindsay for technical and logistical support in conducting positron emission tomography imaging at Herston Imaging Research Facility. In addition to this, the authors thank Matthew Grant from BHP Billiton Limited for his technical assistance with data processing and computing. This work is dedicated to Dr Tim McGahan, who was taken too early by this terrible disease. William McGahan, MBBS (Conceptualization: Lead; Data curation: Lead; Formal analysis: Lead; Funding acquisition: Equal; Investigation: Lead; Methodology: Lead; Project administration: Supporting; Resources: Supporting; Visualization: Supporting; Writing – original draft: Lead). Madeline Gough, Bachelor of Applied Science (Conceptualization: Supporting; Formal analysis: Equal; Investigation: Equal; Methodology: Equal; Writing – review Methodology: Equal). Sharon Hoyte, Master of Bioinformatics (Formal analysis: Supporting; Methodology: Supporting; Writing – review Formal analysis: Supporting; Methodology: Supporting; Writing – review Formal analysis: Equal; Methodology: Supporting; Supervision: Equal; Writing – review & editing: Equal). Download .pdf (.07 MB) Help with pdf files Supplementary Figure 1 Download .docx (7.62 MB) Help with docx files Supplementary Table 1
McGahan et al. (Mon,) studied this question.