ECP 2023 Abstracts

S160 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 Results: These findings triggered a second panel of immunostains to exclude metastasis due to the neuroendocrine morphology. The tumour was positive for CD56, and Synaptophysin and negative for GATA-3 and SOX-10. Pancytokeratin showed dot-like perinuclear staining rais- ing the question of metastatic neuroendocrine carcinoma. The multidis- ciplinary team meeting discussion revealed that the patient had under- gone bilateral total thyroidectomy 14 years ago, and was diagnosed with MTC.Staging computed tomography showed another mass on the contralateral breast with the same features. Additional immunostains were positive for Calcitonin and TTF-1 and negative for PAX-8. Congo red staining highlighted amyloid deposition within the tumour. The final diagnosis was a bilateral metastasis of MTC to the breast. Conclusion: Most cases of MTC (about 75%) are sporadic while the remaining 25% have a hereditary basis. MTCmost frequently metastasizes to the regional lymph nodes, liver, lungs, and bone. Metastases to the breast from extra-mammary organs are infrequent, and MTC rarely metastasizes to the breast. It is crucial to be aware of the rare metastatic lesions that could be found in the breast in order to avoid unnecessary surgical pro- cedures and enable the appropriate management of the primary tumour. E-PS-02-029 In hormone receptor-positive breast cancers (oestrogen receptor and/or progesterone receptor positive and HER2 negative) can the Ki-67 proliferation index in the tru-cut biopsy guide us for the neoadjuvant treatment response? I. Guzelis*, B. Bolat Kucukzeybek, Y. Kucukzeybek *Izmir Katip Celebi University, Ataturk Training and Research Hospi- tal, Department of Pathology, Turkey Background & objectives: Seventy percent of breast cancers are hormone receptor-positive group. In hormone receptor-positive breast cancer, neoadjuvant treatment has a low pathologic response rate. The question is what will be the initial treatment and if the Ki-67 prolifera- tion index can guide us. Methods: In this retrospective study, 62 hormone receptor positive patients with breast cancer who received neoadjuvant therapy and underwent resection were included. Ki-67 proliferation index were evaluated in initial tru-cut biopsy specimens. The cut-off point for the Ki-67 proliferation index was designated as 27.5% by receiver operat- ing characteristic curve analysis. Clinical T was assessed according to the initial radiological tumour size. Results: The mean value of the Ki-67 proliferation index was 37.8% ± 27.4%. Of the cases, 50 were luminal-B subtypes (81%). Before the neoadjuvant treatment, the largest radiological tumour size was 3.6 ± 1.9 centimetres. The mean tumour size in the resection specimen after neoadjuvant treatment was 1.6 ± 1.5 centimetres. Lymph node metas- tasis in the resection specimen was found in 28 cases (45.2%). Patho- logical complete response (yP0N0 or yPisN0) was seen in 8 patients (13%). Of the cases, 37 (60%) had a regression in the tumour size that would change its pT. Ki-67 proliferation index over 27.5% was seen to be associated with regression in the tumour size (p=0.02). Conclusion: As in line with the literature pathological complete response rates after neoadjuvant treatment are low in hormone recep- tor-positive breast cancer cases. As we have shown 27.5% cut-off value of Ki-67 can be a useful predictive parameter for the regression in the tumour size. More data are needed to predict the response to neoadju- vant therapy in hormone receptor-positive breast cancer. E-PS-02-030 Idiopathic granulomatous mastitis (IGM) - a diagnostic challenge G. Hennessy*, K. Ryan, E. Houlihan, J. Mannion, B. Dunne, B. O’Connell, E. Connolly *Ireland Background & objectives: Idiopathic is a rare, benign, inflammatory breast disorder of unknown aetiology. Differential diagnoses include breast malignancy and granulomatous diseases. The pathological criteria for diagnosis of IGM includes granulomatous inflammation with the presence of multinucleated giant cells, fat necrosis, abscesses eosinophils. Methods: All clinically detected cases of idiopathic granulomatous mastitis over a 5-year period (2017-2022) were identified using HIPE data. A retrospective review was undertaken of the histological features: 1. Periductal or perilobular inflammation 2. Sheet like or well formed granulomas 3. Presence of neutrophils/eosinophils 4. Presence of necrosis 5. Special stains Results: A new breast lump was the most common clinical presenta- tion, 72.5% of patients. 61% of samples were biopsies of the left breast, with the remainder from the right. 19.5% were post-partum. 14.6% were current smokers. 2 patients had a history of diabetes mellitus. All 40 samples sent for histological analysis had granulomatous inflam- mation present. 32 were described as in sheets, 7 well-formed, and 1 combination of both. Neutrophils were seen in 34 samples, foreign body giant cells seen in 29, histiocytes seen in all 40 samples, and necrosis was visualised in 2 samples only. A positive gram stain was reported in 2 samples, which correlated with microbiology results. Conclusion: IGM is difficult to distinguish clinically from other inflammatory breast diseases or cancer. Clinicopathological correlation with radiology and microbiology are important, but the gold standard of diagnosis remains with histopathology. E-PS-02-031 Breast ductal carcinoma in situ diagnosed by stereotactic vacuum- assisted biopsy has favourable pathologic features and a lower upstage rate C. Huang*, C. Lee, C. Lin, R. Chang, P. Liao, P. Lin, Y. Lee *Department of Pathology, National Taiwan University Hospital, Taiwan Background & objectives: Stereotactic vacuum-assisted breast biopsy (SVAB) has a superior diagnostic accuracy than core needle biopsy (CNB) for clustered microcalcifications. We aim to compare the clin- icopathological features and the upstage rate of ductal carcinoma in situ (DCIS) diagnosed by SVAB and CNB. Methods: Data from 533 patients diagnosed with DCIS by SVAB or CNB in a single centre between 2016 and 2020 were analysed. Mam- mogram and ultrasound reports were retrieved. The biopsies were inde- pendently reviewed by two pathologists and evaluated by an in-house artificial intelligence (AI) system (Deep DCIS). The rates of upstaging to invasive carcinoma in subsequent wide excision specimens were compared. Results: In patients receiving SVAB, only 17.8% showed suspicious lesions (BI-RADS 4 or 5) on breast ultrasound, contrary to those receiving CNB (96.8%). SVAB obtained a larger tissue amount with less proportion of area involved by DCIS (10.2% vs. 32.3%). Patholo- gists graded more cases as high-grade in CNB (48.1%) than in SVAB (37.9%). All the pathological scores evaluated by the AI system (Deep DCIS) were more severe in CNB than in SVAB, including nuclear grade (51.6 vs. 40.9), necrosis (7.9 vs. 1.1), and stromal reactivity (1.5 vs. 0.5). Oestrogen receptor positivity was higher in SVAB (85.7%) than in CNB (63.5%). SVAB shows a significantly lower upstage rate (11.4%) than CNB (66.3%). Conclusion: SVAB is associated with less underestimation of DCIS than CNB, probably because more SVAB cases are impal- pable microcalcification lesions, which tend to be less aggressive (low- to intermediate-grade and oestrogen receptor-positive). The ability of SVAB to obtain a larger quantity of tissue may also contribute to the lower subsequent upstage rate. Clinical trials are