ECP 2023 Abstracts

S261 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 *National Institute of Oncology, Morocco Background & objectives: Uterine tumour resembling ovarian sex- cord tumour (UTROSCT) is a rare tumour characterized by morpho- logical patterns resembling those in ovarian sex-cord tumours, without an endometrial stroma component. Diagnosis of UTROSCT can be challenging, in order rule out other common uterine lesions. Methods: We report the case of a 65-year-old post-menopausal female that presented with irregular uterine bleeding and pelvic pain. Ultrasonography and MRI revealed a heterogenous mass measuring 4,8x4cm, located in the lumen and infiltrating partially the uterine wall. An endometroid carcinoma was suggested in front of the clinical and radiological presentation and the patient underwent non conservative hysterectomy with bilateral annexectomy. Results: Macroscopic examination revealed a whitish, polypoid intracavitary mass. Microscopic analysis showed a proliferation of tumour cells organized in sheets, cords and trabeculae. Neoplas- tic cells were small, round with a scant eosinophilic cytoplasm and nuclei with minimal atypia. This neoplasm seemed to infiltrate less than 50% of the myometrial thickness. Immunohistochemically, the tumour cells showed focal positive stain- ing for Alpha-inhibin, CD10 and Melan A, a diffuse positive staining for calretinin, desmin and CK AE1/AE3, while cyclin D1, Synapto- physin and WT1 were all negative. Given all those morphological and immunohistochemical criteria, the diagnosis of UTROSCT was confirmed. The patient did not receive any adjuvant therapy, with no evidence of recurrence during follow-up. Conclusion: Although rare, it is essential to considered UTROSCT in the differential diagnosis of uterine neoplasms, such as endometrial stromal tumours with sex cord-like elements, epithelioid leiomyosar- coma, adenosarcoma, sertoliform endometrioid carcinoma, corded and hyalinized endometrioid carcinoma, and mesonephric-like adenocarci- noma. Those tumours mostly harbour different molecular abnormalities with different prognoses. E-PS-10-035 Myometrial myxoidosis; a rare case report E. Kilic Bagir*, Ö. Yaprak, T. Toyran, D. Gümürdülü *Department of Pathology, Faculty of Medicine, Cukurova University, Adana, Turkey Background & objectives: Myometrial myxoidosis is an entity that has been described in recent years with case reports. It is a terminology that describes of non-neoplastic extracellular mucin deposition in the myome- trial wall. Myxoid changes can also be seen other mesenchymal tumours. Methods: A 28-year-old patient with complaints of abdominal swell- ing, abnormal uterine bleeding and menstrual irregularity; On USG, the uterus was severely enlarged and heterogeneous. Based on the radio- logical and physical examination findings, the patient is primarily diag- nosed with leiomyoma and adenomyosis. After the control smear and endometrial biopsy were normal, the patient underwent hysterectomy and bilateral salpingectomy. Results: In macroscopic examination of the resection specimen; uterus was 23x20x12 cm and myometrium was diffusely thickened, average thickness was 7 cm. Widespread myxoid changes were observed between the muscle fibres without forming a mass in the myometrium. hypocellu­ lar myxoid areas dissecting the smooth muscle bundles in a uterine sec- tion. Alcian blue pH 2.5 and mucicarmine stains were diffusely positive in the interstitial myxoid of the myometrium whereas a Periodic acid– Schiff (without diastase) stain was negative. immunohistochemically CD34,CD10 were focal positive and negative for desmin, S100, ALK, H-caldesmon. As a result of immunohistochemical and microscopic find- ings, the case was diagnosed as myometrial myxomatosis. Conclusion: Myxoid changes in the uterus have been described in several neoplastic conditions such as leiomyomas, leiomyosarcomas, adenosarcomas, malignant peripheral nerve sheath tumours and endo- metrial stromal sarcomas. It has been reported that patients with myo- metrial myxomatosis have systemic diseases such as SLE and NF-1, which may contribute to the pathogenesis. Since it has an important place in the differential diagnosis of myxoid neoplasms of the uterus, the presence of this entity should not be forgotten by pathologists. E-PS-10-036 Copy number does not differ in HER2 amplified endometrial can- cer to breast cancer M. Kinloch*, H. Li *University of Saskatchewan, Canada Background & objectives: HER2 amplification exists in Endome- trial Cancer (EC) and Breast Cancer (BC). However, the mechanism of amplification [LH1]differs, and no quantitative comparisons have been reported. We aim to investigate if differences exist in HER2 Copy Number between EC and BC. Methods: HER2 CNV was [LH1]quantified via in-situ hybridization (ISH). Each ISH was scored by 2 independent reviewers. HER2 was considered amplified if the average number of HER2 copies per cell exceeded 4.0 and the ratio of HER2 to Chr17 exceeded 2.0. HER2 copy number and ratio was compared between EC and breast cancer. Results: We reviewed 55 EC and 242 BC cases. The average HER2 copy number of all cases was 3.09 for EnCa and 2.87 for breast can- cer (p = 0.21). Of these, 20% (11; 7 HER2 IHC 3+, 4 HER2 2+) EC and 8.3% (20; all HER2 2+) BC were HER2 amplified. In the HER2 amplified population, the average HER2 copy number was 6.21 vs. 6.83 for EC and BC (p = 0.22). The HER2/Chr17 ratio for EC and BC was 3.27 and 3.09 respectively (p = 0.33). The Human Protein Atlas HER2 mRNA expression data showed HER2 expression of 25.2 nTPM vs 56.0 nTPM for EC and BC respectively. Conclusion: We show, at the DNA level, there is no significant differ- ence in HER2 copy number in amplified EC and BC. This study does not consider differences in HER2 at gene expression levels that may exist downstream of DNA, which may be important in EC where factors such as TP53 have been shown to affect HER2 expression. Looking ahead to refining companion diagnostics, it may be more accurate to measure EC HER2 mRNA to better correlate with response to anti-HER2 therapy. E-PS-10-037 Incidence and clinicopathological features of mismatch repair defi- cient (MMR-d) endometrioid type endometrial adenocarcinomas: a tertiary single-centre data from Turkey K. Kurt*, A. Orgen Calli, A. Akder Sari *Izmir Katip Celebi University, Ataturk Training and Research Hospi- tal, Department of Pathology, Turkey Background & objectives: Mismatch repair immunohistochemistry (MMR-IHC) is suggested for all newly diagnosed endometrioid-type endometrial adenocarcinomas (EECs). This study aims to evaluate the incidence of MMR-deficient (MMR-d) endometrial carcinomas and clinicopathological features. Methods: A total of 30 cases diagnosed as pure EECs of the uterus between February 2020 – March 2023 were included in the study. All cases were stained immunohistochemically for MLH1, MSH2, PMS2, and MSH6. All the data were recorded from the database including MMR-IHC results. Results: The mean age of patients was 61 years (min35-max74). MMR-d tumours constituted 37% (n=11/30) of all ECs. Loss of MLH1 and PMS2 proteins was the most common, compromising %73 (n=8) of MMR-d cases. There were 2 MSH6-deficient cases (18%) and one case had a loss of three proteins (9%), MLH2, PMS2, and MSH6. No MSH2-deficient case was present. No statistically significant relation- ships were found between any of the clinicopathological parameters and the MMR status of tumours. However, the rate of grade 1 tumours