ECP 2023 Abstracts

S71 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 Background & objectives: Conventional clinicopathological char- acteristics insufficiently predict a patients’ unfavourable oncologi- cal outcome (UOO) after surgical treatment for oral squamous cell carcinoma (OSCC). We aimed to assess the added predictive value of tumour environment immune cell composition (TMICC) in addi- tion to conventional clinicopathological characteristics. Methods: Clinicopathological data of 290 OSCC patients was obtained. Primary tumour samples underwent immunohistochemi- cal staining for CD4, CD8, CD20, CD68, CD163, CD57, FoxP3 and PD-L1. Least Absolute Shrinkage and Selection Operator (LASSO) regression analyses with cross-validation were conducted to train and validate predictive models. Receiver operating characteristic (ROC) analyses were used to quantify the added predictive power of TMICC within models. UOO was defined as loco-regional recur- rence, distant metastasis and second primary tumours. Results: UOO occurred in 32.1% of the 290 patients. The LASSO regression-based predictive model contained conventional clinico- pathologic variables: tumour localisation, T-stage, N-stage, resec- tion margin, differentiation grade and perineural invasion. The area under the ROC curve (AUC) of training and test cohorts were: 0.72 (± 0.01) and 0.67 (± 0.01), respectively. Addition of TMICC to the model improved the AUC to respectively 0.80 (± 0.01) and 0.74 (± 0.01). The model showed that CD163 and FoxP3 were positively, whereas CD4 and CD8 were negatively correlated with UOO. Conclusion: This is the first study showing that addition of TMICC to conventional clinicopathological characteristics improves the per- formance of predictive models for UOO of OSCC after surgery. This may give guidance in clinical decision making. PS-04-006 Investigation into the potential role of R-loops in cisplatin resistant oropharyngeal squamous cell carcinoma (OPSCC) H. Crane*, K. Hunter, S. El-Khamisy *University of Sheffield, United Kingdom Background & objectives: OPSCC is frequently treated with a chemoradiotherapy regime including cisplatin, but resistance can develop. R-loops are three-stranded DNA:RNA hybrids which modu- late gene expression as well as being a source of genomic instability, however their role in cisplatin resistance is unknown. Methods: A HPV-positive (HPV+) and HPV-negative (HPV-) head and neck cell line were subjected to long term treatment with cisplatin and single cell clones were selected. Alterations in R-loop dynam- ics were explored using S9.6 slot blots and DRIP-qPCR. The effect of depleting a known R-loop regulator on cell viability and DNA damage was assessed using MTS assays and immunofluorescence respectively. Results: In both HPV+ and HPV- cisplatin sensitive cells, R-loops increased with cisplatin treatment. Interestingly, in HPV+ resistant cells, there was an increase in R-loops at baseline when compared to sensitive cells. This increase was also seen at specific genomic loci with DRIP-qPCR. HPV+ resistant cells were also found to upregu- late a known R-loop resolving protein known as senataxin. When senataxin was depleted using siRNA, both HPV- and HPV+ resistant cells showed reduced cell viability in response to cisplatin treatment, however the effect was greater in the HPV+ resistant cells. Follow- ing depletion of senataxin and cisplatin treatment in HPV+ resistant cells there were higher levels of DNA damage and increased R-loops. Conclusion: R-loops can modulate sensitivity to cisplatin in resist- ant cells and the effect is greater in cells which upregulate senataxin. This may represent a potential therapeutic target which warrants further investigation. Funded by a Cancer Research UK (CRUK) and the Pathological Soci- ety Joint Predoctoral Research Bursary and Wellcome Trust Clinical PhD Fellowship. PS-04-007 Technical external quality assessment schemes for p16 and high- risk human papilloma virus detection in head and neck squa- mous cell carcinoma: report of year-one results A. Dodson*, D. Nayar, S. Parry *UK NEQAS ICC & ISH, United Kingdom Background & objectives: p16 immunohistochemistry is an aid to identifying HPV-positive oropharyngeal squamous cell carcinoma (SCC), which has distinct clinicopathological features and a favourable prognosis. We report results from external quality assessment (EQA) programmes for p16 and HPV testing in this clinical setting. Methods: Sections from two FFPE Head & Neck SCC specimens positive for the expression of p16 and HPV, together with a tumour in which neither marker was demonstrable, and a piece of tonsil showing reactive changes, were prepared onto microscope slides and circulated to participating laboratories. Stained slides were returned for central assessment by an expert panel of assessors. Results: Summated data are presented from four runs conducted at regular intervals in 2022-23. p16: 143 participations, of which 140 (97.9%) were assessed as accept- able (inter-run-range, IRR=94.3-100.0%). 126 (88.1%) were further categorised as good/excellent (IRR=83.3-90.0%). The most used anti- body was CINtec E6H4 (Ventana), used by 78 (81.3%) of 96 labora- tories reporting methodology. All 78 (100.0%) produced submissions of acceptable stain quality. HPV: 43 submissions, of which 42 (85.7%) were assessed as accept- able (IRR=75.0-92.3%). 31 (63.3%) were of good/excellent quality (IRR=46.2-84.6%). The probe set most used was the HPV III Family 16 Probe (Ventana), used by 34 (82.9%) of 41 laboratories that reported their method. 29 (85.7%) produced submissions of acceptable quality. Conclusion: In the H&N SCC setting, p16 IHC is done exceptionally well in most laboratories submitting material to the EQA. Similarly, detection of HPV in the same type of material is also performed well, albeit with slightly more variability. The most commonly used primary markers in each of the EQAs both produced superior results when compared to the aggregate for all the alternatives. PS-04-008 Study of CTNNB1 mutations in a case series of 25 basal cell neoplasms of the salivary gland M. Gómez Tena*, N. Fullana Llinas, I. BLAZQUEZ, M. Gomà Gallego, M. Vazquez Pariente *Hospital Universitari de Bellvitge, Spain Background & objectives: Basal cell adenoma (BCA) and basal cell adenocarcinoma (BCAC) are infrequent neoplasms that mimic other basaloid salivary gland tumours. Nuclear expression of β-catenin and CTNNB1 mutations are useful in distinguishing BCA from other neoplasms. Methods: We reviewed 23 BCA and 2 BCAC diagnosed at our institu- tion between 1994 and 2022, assessing histological pattern (tubular- trabecular, solid, membranous), presence of biphasic cellularity, pres- ence of nuclear of β-catenin expression and of CTNNB1 mutations. Results: After reviewing the cases, two BCAs were reclassified as pleo- morphic adenomas and two as myoepitheliomas. The most common pattern in BCA was tubular-trabecular (70%), followed by solid (12%) and membranous (18%). 16 cases exhibited biphasic cellularity. Nuclear β-catenin staining was demonstrated in 15 cases (focal in 10, diffuse in 5), in 6 of them only epithelial cells showed positivity while in 9 it was present also in the stroma. CTNNB1 mutations were present in 12 out of the 17 BCA: six I35T, four T41I and one S45C. Membranous BCA didn’t show biphasic cellularity, nuclear β-catenin or CTNNB1 mutations. β-catenin nuclear expression and CTNNB1 mutations were absent in the two BCAC.