ECP 2023 Abstracts

S380 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 by two pathologists, independently. The cut-off points established were <5% vs > 5%. We used univariate analysis with Kaplan Meier curves and Long Rank test to determine association between cribiform pattern and time to recurrence. Then, a Cox regression was performed. Results: 30.6% of patients presented recurrence. In the univariate analysis, the percentage of cribriform pattern greater than 5% was sig- nificantly associated with time to recurrence (p= 0.02). The large cri- briform pattern was significantly associated with recurrence (p= 0.02) but not the small cribriform pattern (p= 0.155) with cut-off values of <3% vs > 3. The Cox model showed that more than 5% cribriform pattern on biopsy carries a significantly increased risk of recurrence (HR = 1.8, 95% CI: 1.1-3.18, p = 0.02), regardless of other pathologi- cal findings of posterior radical prostatectomy (ISUP grade group 2 vs ISUP 3, R- vs R+, % tumour < 10% vs > 10%). Conclusion: The presence in the biopsy of a percentage of cribriform pattern greater than 5 provides predictive information for biochemical recurrence and is independent of other surgical pathological findings in radical prostatectomy performed later. These association may lead to a change in the follow-up and treatment of patients with intermediate- risk prostatic acinar adenocarcinoma (ISUP grade 2 and 3). E-PS-24-053 Primary renal mesenchymal neoplasms - a retrospective analysis over a 8 year period from a tertiary care centre in South India S. Naresh Shah*, S. Kar, M. Menon *Apollo hospitals, India Background & objectives: Primary renal mesenchymal tumours although rare, can be of varied morphology and subtypes. They present as renal masses and are often misdiagnosed as carcinomas, clinically and radiologically. Methods: We retrospectively analysed the renal tumours received in the histopathology laboratory at Apollo hospitals, Chennai, India, from 2012 to 2020 and herein describe their frequency, incidence and demographical features. Histopathological examination was done in all cases, with confirmation by immunohistochemistry and molecular studies when required. Results: Out of 2050 renal tumours, 67 (3.2%) cases of mesenchymal renal tumours were identified. Of these 51(76.1%) cases were benign, 14(23.8%) cases were malignant, and 2(0.1%) cases were unclassifi- able. There was female (64%) preponderance over males (36%). Angio- myolipoma (43, 65.8%) constituted the major bulk of these tumours, followed by leiomyosarcomas (5, 7.5%), Ewing sarcoma (4,5.9%), Lei- omyomas (3, 4.4%) and benign vascular tumours (3,4.4%). Two cases of synovial sarcoma(2.9%) were present. One case each of Schwan- noma(1.5%), solitary fibrous tumour(1.5%), angiomyoadenomatous tumour (1.5%) and sarcoma with heterologous elements (1.5%) was documented. Conclusion: Mesenchymal renal tumours cover a wide spectrum of benign and malignant tumours. Here we emphasize the role of his- topathological examination, in conjunct with immunohistochemistry and molecular studies, which remains the gold standard for diagnos- ing these tumours. Some of these tumours have an aggressive course, present at end stage and may require multidisciplinary patient manage- ment modalities. E-PS-24-054 Aberrant CK7/CK20/HMWK expression in prostatic adenocarci- noma with unusual urothelial-like morphology: a case series N.J. Nguyen*, C. Sherman, T.H. van der Kwast, M.R.D. Downes *University of Toronto, Canada Background & objectives: Prostatic adenocarcinoma typically expresses markers of prostatic lineage while lacking CK7, CK20 and high molecular weight keratin (HMWK). Occasionally prostate cancers have unusual morphology which requires immunohistochemistry (IHC) to determine lineage. We report here a series of CK7/CK20/HMWK- positive prostatic adenocarcinoma. Methods: We describe 11 cases of prostatic adenocarcinoma with unusual IHC expression and urothelial-like morphology, seen between 2018 and 2023 at two Toronto academic centres. We collected patient age, prior androgen deprivation therapy (ADT), tumour site, histomor- phology, Grade group (GG), and results of IHC, including prostatic, urothelial and neuroendocrine markers. Results: Patients’ age ranged from 41 to 85 years (median 80). The tumour sites were: prostate (n=6), bladder (n=2), liver metastases (n=2), and lung metastasis (n=1). Six cases had prior ADT. The GG were: GG3 (n=1), GG5 (n=7); metastases were not graded. Nine cases had diffuse urothelial-like morphology, and two cases had focal urothelial morphology. CK7 (n=10) was strong/diffuse in seven cases, and weak/focal in one case. CK20 and HMWK showed patchy/moderate positivity in 3/6 and 4/7 cases, respectively. Eight cases expressed at least one prostate-specific marker. 6/9 cases had focal to diffuse positivity for neuroendocrine markers. Molecular test- ing (n=6) showed one TMPRSS2:ERG fusion (lung) and one ATM deletion (bladder). Conclusion: This case series illustrates a cohort of high grade and/or metastatic prostatic adenocarcinomas with distinctive urothelial-like morphology and aberrant immunoprofiles (expression of CK7/CK20/ HMWK) that initially suggest urothelial origin, a potential diagnos- tic pitfall with significant clinical consequences. This highlights the importance of (1) clinical history and (2) utilization of broad IHC panels, as 6/11 cases had prior ADT and 8/11 had expression of at least one marker of prostatic lineage, to aid in determining the cor- rect diagnosis. E-PS-24-055 Nephrogenic metaplasia: a pitfall on frozen section of urethral strictures A. Oberc*, C. Sherman, M.R.D. Downes *University of Toronto, Canada Background & objectives: Urethral strictures (US) are a common cause of urinary obstruction which can be treated with surgical resec- tion. Frozen sections are rare and pose a diagnostic challenge to pathologists due to the presence of benign lesions such as nephrogenic metaplasia (NM). Methods: We retrospectively examined all cases of US resections submitted to pathology at our institution from 2012 to 2022 (n=258). Final pathology reports were searched to identify cases with dysplasia, carcinoma, or NM. When available, frozen section (FS) reports were also examined and compared to the final report, and additional clinical history and microscopic images were collected for cases with NM. Results: NM was identified in the final report of 3.8% (10/258) of US resections. Dysplasia was identified within US in a single patient who underwent two separate resections, and squamous cell carcinoma (SCC) was found in one case. Intraoperative FS was requested in 3.4% of all cases (9/258). In two of these cases, an initial diagno- sis of SCC was favoured by the intraoperative pathologist, however when reviewed with a genitourinary pathologist the diagnosis was changed to "reactive process" with a final diagnosis of NM. NM can be challenging on FS due to variable architectural patterns, eosino- philic cytoplasm and surrounding inflammation and additional reac- tive changes. Conclusion: While US are relatively common, their assessment by intraoperative FS is rare and pathologists may lack familiarity with the variable morphology of benign entities such as NM that can be seen on FS resulting in their misinterpretation. Here we present two cases of NM that were initially misclassified as SCC to highlight this potential diagnostic pitfall at FS and the value of a second opinion prior to definitive FS diagnosis of malignancy.