ECP 2023 Abstracts

S24 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 mitoses: sensitivity/specificity 42.9%/97.2% all DM-PTC, 18.8%/97.2% synchronous DM-PTC, 54.6%/97.2% metachronous DM-PTC. Cutoff ≥5 has optimal parameters based on sensitivity/specificity AUC values for angioinvasion: sensitivity/specificity 36.2%/91.7% all DM-PTC, 25%/91.7% synchronous DM-PTC, 41.9%/91.7% metachronous DM- PTC. Positive/negative predictive values (PPV, NPV) were: necrosis 21.2% (PPV), 98.2% (NPV); ≥5 mitoses 32.3% (PPV), 98.2% (NPV); ≥5 angioinvasive foci 11.8% (PPV), 97.9% (NPV). After multivari- able analysis only necrosis and ≥5 mitoses remained associated with DM-PTC, necrosis also to synchronous and metachronous DM-PTC. Conclusion: Necrosis of any extent and mitoses (cutoff ≥5) are the best predictors of DM-PTC, including those cases in which metas- tases develop after initial diagnosis and staging (metachronous DM- PTC). Angioinvasion (cutoff ≥5 foci) predicts DM-PTC, but it is not independent after multivariable analysis. Our data statistically validate empirically established criteria to identify poor prognosis PTC. OFP-06-005 MCT4 expression correlates with tumour progression and high- lights metabolic heterogeneity in Pancreatic Neuroendocrine Tumours (PanNETs) K. Bräutigam*, J. Straub, P. Kirchner, V. Andreasi, C. Saganas, R. Maire, M. Falconi, I. Marinoni, M. Sadowski, A. Perren *Institute of Pathology, University of Bern, Switzerland Background & objectives: Recent transcriptome and epigenome analyses suggest that epigenetic and metabolic changes are associated with progression, increased proliferation and aggressiveness of Pancre- atic Neuroendocrine Tumours (PanNETs). However, specific metabolic markers, their role and therapeutic potential are currently unknown. Methods: Monocarboxylate transporters (MCT) 1 and 4, carboanhydrase 9 (CA9), Glucose transporter 1 (GLUT1) were analysed in two independ- ent PanNET cohorts (n(Bern)=110; n(Milano)=71) by immunohistochem- istry. Metabolism of 2D and 3D models of PanNET cell lines as well as patient-derived tumoroids was functionally assessed by PrestoBlue assay and quantitative fluorescence microscopy. MCT1/4 were investigated as potential therapeutic targets with small molecule inhibitors. Results: Immunohistochemistry of metabolic enzymes demon- strates three protein expression patterns in both PanNET cohorts: homogeneous, heterogeneous and absent expression. Homogeneous and heterogeneous expression of MCT4 and/or CA9 significantly correlate with higher tumour stage and size, as well as relapse. Lower microvessel density is associated with heterogenous expres- sion and shorter time to relapse and presence of metastasis. Simi- lar to PanNET tumours, our 3D PanNET models show regional hypoxia and spatial metabolic heterogeneity. Inhibition of MCT1/4 leads to reduced metabolic activity in PanNET cell lines, i.e. block- age of glycolysis and diminished uptake of oxidative fuels. Finally, through these mechanisms, MCT1/4 inhibition results in lower proliferation of PanNET spheroids and patient-derived tumoroids. Conclusion: PanNETs exhibit considerable metabolic heterogeneity. Protein data suggest that metabolic enzymes are robust prognostic and predictive markers of PanNET aggression. MCT4 stratifies subtypes according to their expression pattern. Interestingly, in our in vitro 2D and 3D models, metabolic inhibitory agents show similar to better effects in reducing tumour growth compared to conventional inhibi- tors (e.g. mTOR). MCT4 is a prognostic biomarker and a potential surrogate for promising anti-metabolic therapies. OFP-06-006 Considerable interlaboratory variation in PD-L1 assessment for head and neck squamous cell carcinoma in the Netherlands – a nationwide evaluation study M.A. Hempenius*, B.M. Koomen, I.A. Deckers, S.M. Willems, B. van der Vegt *University Medical Center Groningen, The Netherlands Background & objectives: Patients with recurrent or metastasized head and neck squamous cell carcinoma (HNSCC) are selected for pembrolizumab treatment by determining the PD-L1 combined positive score (CPS). This nationwide study, using real-world data, investigates interlaboratory variation in PD-L1 assessment for HNSCC. Methods: Pathology reports of HNSCC patients mentioning PD-L1 testing between 1/7/2019 and 31/6/2022 were extracted from the Dutch nationwide pathology databank Palga. Pseudonymized tumour and PD-L1 testing characteristics were analysed per year to evaluate the testing landscape. Using CPS≥1 and CPS≥20 cutoffs, variation in PD-L1 positivity between laboratories was assessed using funnel plots with 95% confidence limits around the overall mean. Results: A total of 640 PD-L1 tests were reported in 561 HNSCC patients across 20 laboratories. 83.4% of the tests were positive using the ≥1 cutoff. There were no differences in national PD-L1 positivity rates between the three years (p = 0.581). In these years, use of the recommended scoring method CPS increased from 80.0% to 98.0% and 22C3 antibody use increased from 51.7% to 75.2%. 529 PD-L1 tests on histological specimens from 12 laboratories were analysed to evaluate interlaboratory variation. Three (25%) out of 12 laboratories signifi- cantly deviated from the national mean of PD-L1 positive cases while using the CPS≥1 cutoff; 2 (16.7%) laboratories significantly deviated for the CPS≥20 cutoff. Conclusion: In the first 36 months after introduction of PD-L1 testing in HNSCC, assessment has become more uniform in the Netherlands, regarding the used PD-L1 scoring method and antibody. However, interlaboratory variation in PD-L1 positivity between Dutch labora- tories was substantial. Guidelines and additional training in PD-L1 scoring for pathologists might help to further reduce this variation. This work was supported by MSD (grant number not applicable). MSD had no role in study design, data collection and analysis, writing the manuscript or the decision to submit this abstract for publication. OFP-06-007 Comparative analysis of PLAG1 rearranged cutaneous mixed tumours and pleomorphic adenomas of salivary glands: identifica- tion of distinct recurrent TRPS1::PLAG1 gene fusion in cutaneous mixed tumours Z. Alsugair*, N. Macagno, J. Tantot, O. Harou, M. Battistella, P. Sohier, T. Kervarrec, A. de la Fouchardière, A. Champagnec, D. Pissaloux, J. Lopez, M. Laé, F. Descotes, V. Costes-Martineau, N. Benzerdjeb *Department of Pathology, Institut of Pathologie Multisite, Groupe- ment Hospitalier Sud, Hospices Civils de Lyon, Pierre-Bénite, France Background & objectives: PA (pleomorphic adenomas) and CMT (cutaneous mixed tumours) are virtually indistinguishable histologi- cally, making the distinction difficult in certain head & neck regions. Determining the tumour origin in a metastatic context of carcinoma ex-PA or malignant CMT is equally difficult. Methods: Two cohorts consisting of 34 cases of CMT and 39 cases of PA of the salivary glands were obtained from French institutions and reviewed by specialists in each subspecialty. RNA sequencing analysis was conducted to identify molecular features of cases har- bouring PLAG1. Clinical, pathological, and molecular data were collected. Results: CMT were mainly located on the head (mean: age 61yo/ size 11mm). Positive PLAG1 immunostaining was found in 33/34 cases (97%), associated with PLAG1 gene fusion with mainly TRPS1::PLAG1 in 25/33 (75.7%); and BUB1B::TRPS1 in one case. PA were mainly located in the parotid gland (mean: age 60yo/size 77mm) with different histological variants including conventional (66%,N=26), oncocytic (25%,N=10) and canalicular-like variants