ECP 2023 Abstracts

S283 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 Methods: A 58-year-old man presented with a history of mandibular cyst for 2 years. The first biopsy showed inflammatory odontogenic cyst. He was treated by marsupialization. After 6 months, he had recur- rent symptoms, and the size of the lesion had increased. Computed Tomography scan showed expansile lytic mandibular lesion with focal cortical defect. The lesion was excised and sent for histopathology. Results: Histology showed a cystic lesion lined by mucin cell-rich mixed epithelial aggregate containing basal cells and squamoid cells. They showed sharp delineation to the subepithelial fibrous stoma which con- tained variable degree of chronic inflammatory infiltrates. The other frag- ments showed a multicystic lesion composed of haphazard admixture of monomorphic epidermoid cells, intermediate cells and focally prominent mucinous (goblet) cells in a fibrous stroma. Focal area with a predomi- nance of clear cells was seen. No high-grade nuclear features were seen. The diagnosis of a predominantly cystic low-grade CMC developing from a pre-existing GOC was made. Fluorescence In Situ Hybridization (FISH) testing showed MAML2 translocation which confirmed the diagnosis. Conclusion: Cystic low-grade CMC shares common features with the GOC. Careful histological examination and – in some cases – molecu- lar tests are required for the definite diagnosis. Rare forms of CMC can develop from GOC. However, the nature of the pre-existing lesion – whether it is a GOC or a genuine bland looking mucoepidermoid carcinoma– carries no prognostic significance. E-PS-12-004 Ameloblastoma featuring plexiform granular cell odontogenic tumour: an under-recognized entity R. AlRasheed*, R. AlShagroud *King Saud University, Saudi Arabia Background & objectives: Plexiform granular cell odontogenic tumour (PGCOT) has been initially described as plexiform strands composed of granular cells. Few cases of ameloblastoma featuring PGCOT have been previously reported. Herein, we report additional case of ameloblastoma with granular cells in plexiform architecture. Methods: 31-year-old male presented to the clinic with a 4-month his- tory of a swelling of the right posterior mandible. The patient indicated that the mass was painless and was associated with a yellowish discharge. Radiographic examination showed a multilocular radiolucent lesion with scalloped borders. Incisional biopsy of the mass was performed. Results: Grossly, the received specimen consisted mainly of multiple soft tissue fragments that were firm in consistency. Histopathological examina- tion revealed long thin anastomosing cords and strands of closely packed, large, epithelial cells with granular eosinophilic cytoplasm in a collagenized connective tissue stroma. These strands and cords occupied almost the entire specimen and consisted of a parallel arrangement of tall cylindrical cells with the nucleus exhibiting hyperchromasia and palisading, reverse polarization, occasional basilar cytoplasmic vacuolation, and intranuclear inclusion. In addition, the tumour showed formation of neoplastic follicles demonstrating similar nuclear features as the strands within the peripheral layers while the centre of the follicles exhibited "stellate reticulum-like" structure. Conclusion: The presentation of ameloblastoma as PGCOT has been only rarely reported in the literature. Typically, the granular cells in ameloblastoma are present focally and limited to the areawithin the neoplastic islands. Interest- ingly, the presented case showed granular changes within the cells forming the plexiformstrands and cordsmimickingother neoplasms andposing a diagnostic challenge. It is important for pathologists to recognize this entity inorder to avoid misdiagnosis and consequently provide patients with the proper management. E-PS-12-005 Intratumour molecular heterogeneity in mucosal melanoma of the head and neck region M.V. Altavilla*, D. de Biase, C. Ricci, B. Corti, E. Pasquini, L. Presutti, A.M. Baietti, L. Amorosa, T. Balbi, C. Baldovini, F. Ambrosi, M. Grillini, A. D’Errico, M. Fiorentino, M.P. Foschini *Pathology Unit, IRCCS Azienda Ospedaliero-Universitaria di Bolo- gna, Italy; School of Anatomic Pathology, Department of Biomedical and Neuromotor Sciences (DIBINEM), University of Bologna, Italy Background & objectives: Mucosal melanoma of the head and neck (MM-H&N) is a rare but aggressive histotype, with limited data on its genetic landscape. We tested a series of MM-H&N to evaluate the intratumor molecular heterogeneity (IMH) among samples from the same patients. Methods: 39 histological samples of MM-H&N obtained from 24 patients (2003-2023) were collected and reviewed to confirm its pri- mary nature and exclude metastases. The cases were analysed with a multi-gene NGS panel (covering 28 genes involved in the pathogenesis of melanoma), and patients with sequential samples (excision of the primary tumour and excision of residual tumour) were compared to investigate IMH. Results: In 35 histological samples (22 patients) the obtained material was suitable for NGS analysis. The most commonly detected muta- tions involved RAS (KRAS and NRAS) (5/35, 14.3%), TP53 (3/35, 8.6%), and KIT (3/35, 8.6%) genes; by contrast, BRAF (p.Asn581Ile), IDH1, and GNA11 mutations were detected in only 1 sample (2.9%), and 17 samples (48.6%) resulted wild-type (wt). In 9 patients with multiple/sequential samples and at least two suitable for NGS analysis, 5 (55.6%) showed IMH, as follows: patient #1 (KRAS and TP53), patient #6 (NRAS and EIF1AX), patient #11 (GNA11, NRAS, and wt), patient #21 (BRAF and wt), patient #24 (KIT and wt). Conclusion: Our study confirms that the most commonly involved mutated genes in MM-H&N are RAS (KRAS and NRAS), with a higher frequency of wt cases compared to the cutaneous counterpart. Furthermore, we showed that more than half of patients with multiple/ sequential samples showed IMH. This data increases our knowledge of the genetic landscape of MM-H&N and underlines how the NGS analy- sis of multiple/sequential samples could be required to better stratify the prognosis of these patients and plan the best therapeutic approach. E-PS-12-006 Adenoid cystic carcinoma ex-pleomorphic adenoma: a case report J.L. Amaral*, J. Pimentel, A. Alves, C. Courelas, L. Carvalho *Pathology Department, Coimbra Hospital and University Centre (CHUC), Portugal Background & objectives: Carcinoma ex pleomorphic adenoma (CXPA) is rare and occurs in 1.5 to 13.8% of pleomorphic adenomas, most commonly in parotid/major salivary glands. The most frequent malignant components are salivary duct carcinoma and adenocarcinoma NOS, followed by epithelial myoepithelial/myoepithelial carcinoma. Methods: We present a case of adenoid cystic carcinoma arising in previous pleomorphic adenoma. Results: A 75-year-old woman presented with paresthesia in left pre- auricular area. A hypodense mass with 26 x 17 mm was detected in a cervical CT scan, in left parapharingeal area, without a cleav- age plane with adjacent muscular and bone structures. A first biopsy showed pleomorphic adenoma component, predominantly with cells of myoepithelial phenotype (CK5/6, calponin and p40 positives). Con- sidering the aggressive behaviour of the tumour, a second biopsy was made that confirmed pleomorphic adenoma and showed three areas with a cribriform pattern and heterogeneous expression for CD117, suggesting adenoid cystic carcinoma arising in a previous pleomorphic adenoma. The patient has been proposed for treatment with radio- therapy which is yet to start. Conclusion: Carcinoma ex pleomorphic adenoma can pose a diagnos- tic challenge, therefore, it must be kept in mind, given the impact it has on patient prognosis. Furthermore, this case raises the importance of articulation of clinical and imagiological information with histopatho- logical patterns in order to make a precise diagnosis.