Supplementary Material for: Cystic-like HER2-positive Breast Cancer with Low TILs and High PD-L1 Expression in a Young Woman: A Case Report

Background: HER2-positive breast cancer is generally correlated with poor prognosis, whereas it demonstrates a favorable response to HER2-targeted therapies. Typically, HER2-positive tumors present as solid masses, while cystic features are exceedingly rare. Case Presentation: We present a case of a 37-year-old female with imaging findings of a large cystic mass (9.9 × 1.8 cm) in the left breast. Pathological examination confirmed grade III invasive carcinoma of no special type (NST) with HER2 positivity and HR positivity, low tumor-infiltrating lymphocytes (TILs, 5%), and high PD-L1 expression (CPS = 25%). The patient underwent a modified radical mastectomy with axillary lymph node dissection, revealing metastasis in 7 of 15 lymph nodes and the presence of lymphovascular invasion. Adjuvant therapy with the TCbHP regimen (docetaxel, carboplatin, pertuzumab, and trastuzumab) was initiated, with a total of six cycles planned, followed by maintenance therapy with trastuzumab and pertuzumab (HP) for one year. To date, the patient has tolerated the treatment well without evidence of distant recurrence or metastasis. Conclusion: This case underscores the discordance between radiological and pathological findings in breast cancer, highlighting the clinical significance of low TILs and high PD-L1 expression in HER2-positive tumors, and emphasizes the importance of individualized surgical and adjuvant treatment strategies.