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.

背景：人表皮生长因子受体2（human epidermal growth factor receptor 2, HER2）阳性乳腺癌通常预后较差，但对HER2靶向治疗应答良好。典型的HER2阳性肿瘤表现为实性肿块，而囊性特征极为罕见。
病例报告：我们报告1例37岁女性患者，左侧乳腺影像学检查发现巨大囊性肿块（9.9×1.8 cm）。病理学检查证实为Ⅲ级非特殊型浸润性癌（no special type, NST），伴HER2阳性及激素受体（hormone receptor, HR）阳性，肿瘤浸润淋巴细胞（tumor-infiltrating lymphocytes, TILs）比例较低（5%），程序性死亡受体配体1（programmed death-ligand 1, PD-L1）高表达（综合阳性评分CPS=25）。患者接受改良根治性乳房切除术联合腋窝淋巴结清扫术，术后病理显示15枚腋窝淋巴结中7枚存在转移，且伴淋巴管血管侵犯。患者随后接受TCbHP方案（多西他赛、卡铂、帕妥珠单抗、曲妥珠单抗）辅助治疗，计划共6个周期，后续予以曲妥珠单抗联合帕妥珠单抗（HP）维持治疗1年。截至目前，患者治疗耐受性良好，未出现远处复发或转移。
结论：本病例凸显了乳腺癌影像学表现与病理学表现之间的不一致性，强调了HER2阳性肿瘤中低TILs水平与高PD-L1表达的临床意义，并突显了个体化手术与辅助治疗策略的重要性。