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Purpose: Notable effectiveness of trastuzumab deruxtecan (T-DXd) in patients with HER2-low advanced breast cancer (BC) has focused pathologists' attention. We studied the incidence and clinicopathologic characteristics of HER2-low BC, and the effects of immunohistochemistry (IHC) associated factors on HER2 IHC results. Materials and Methods: The Breast Pathology Study Group of the Korean Society of Pathologists conducted a nationwide study using real-world data on HER2 status generated between January 2022 and December 2022. Information on HER2 IHC protocols at each participating institution was also collected. Results: Total 11,416 patients from twenty-five institutions included in this study. Of these patients, 40.7% (range: 6.0%-76.3%) were classified as HER2-zero, 41.7% (range: 10.5%-69.1%) as HER2-low, and 17.5% (range: 6.7%-34.0%) as HER2-positive. HER2-low tumors were associated with positive ER and PR statuses (p<0.001 and p<0.001, respectively). Antigen retrieval times (≥ 36 min vs. < 36 min) and antibody incubation times (≥ 12 min vs. < 12 min) affected on the frequency of HER2 IHC 1+ BC at institutions using the PATHWAY HER2 (4B5) IHC assay and BenchMark XT or Ultra staining instruments. Furthermore, discordant results between core needle biopsy (CNB) and subsequent resection specimen HER2 statuses were observed in 24.1% (787/3259) of the patients. Conclusion: The overall incidence of HER2-low BC in South Korea concurs with those reported in previously published studies. Significant inter-institutional differences in HER2 IHC protocols were observed, and it may have impact on HER2-low status. Thus, we recommend standardizing HER2 IHC conditions to ensure precise patient selection for targeted therapy.
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RATIONALE: Bones are the most common site of prostate cancer metastasis. Other common sites of metastases include the distant lymph nodes, liver, thorax, brain, and digestive system. However, cutaneous metastases from prostate cancer are extremely rare. PATIENT CONCERNS: We present a case of a 61-year-old man with scalp nodules without any cancer history. DIAGNOSIS: The patient was diagnosed with metastatic prostate adenocarcinoma through an incisional biopsy for his scalp nodules. The patient presented with a serum prostate-specific antigen level of 10.2 ng/mL; imaging examinations revealed extraprostatic extension, lymph node involvement, and multiple bone metastases. INTERVENTION: The patient was treated with androgen deprivation therapy with leuprolide acetate (7.5 mg every month) and abiraterone acetate (1000 mg daily). OUTCOMES: The scalp metastases resolved without adverse effects, and the serum prostate-specific antigen level decreased to 0.02 ng/mL. LESSONS: Cutaneous metastasis, especially scalp metastasis from prostate cancer, is extremely rare. If there is a rash or nodule on the skin, it is necessary to evaluate it carefully and to confirm it through a biopsy.