Based on GLOBOCAN 2022 data, breast cancer accounted for 2.3 million new cases and 666,000 deaths globally, remaining the most frequently diagnosed cancer and the leading cause of cancer-related mortality among women (1). A notable proportion, ~3-10%, of cases are diagnosed with distant metastases at the time of initial diagnosi (2-4). The disease comprises a heterogeneous group of subtypes, each with distinct molecular features, clinical behavior, and prognostic implications. Among these, hormone receptor-positive (HR⁺) and human epidermal growth factor receptor 2 (HER2)-negative (HER2^-) BC accounts for nearly 70% of all BC cases (5).

In the metastatic setting, systemic therapy remains the cornerstone of treatment, as recommended by current international guidelines. For patients with metastatic HR⁺/HER2^- disease, the combination of endocrine therapy and cyclin-dependent kinase 4/6 (CDK4/6) inhibitors has emerged as the standard first-line approach, supported by multiple large-scale clinical trials demonstrating significant improvements in both progression-free and overall survival (6-8).

The potential role of locoregional surgery in patients with de novo stage IV BC remains a matter of ongoing investigation. Several retrospective studies and a few prospective randomized trials have assessed whether resection of the primary tumor offers a survival advantage in this setting. However, the results have been inconclusive; some studies report improved outcomes in selected patient populations, whereas others have failed to demonstrate a clear benefit (4,9-11).

BC exhibits significant biological diversity not only between individuals but also within a single tumor. Intratumoral heterogeneity (ITH), characterized by the coexistence of genetically and phenotypically distinct subclones, has emerged as a major challenge in oncology practice (12). This diversity within tumors contributes to diagnostic uncertainty, therapeutic resistance and disease recurrence (13). Subpopulations with resistant features may persist and drive tumor progression (14). A deeper understanding of ITH is therefore essential for optimizing clinical management and developing more effective, personalized therapeutic approaches in BC.

In the present case report, two cases of de novo metastatic HR⁺, HER2^- BC in which an initial favorable response was achieved with standard first-line systemic therapy, were presented. Despite this apparent clinical benefit, subsequent resection of the primary tumor revealed the emergence of HER2-positive (HER2⁺) tumor clones that had not been detected in baseline diagnostic samples. These findings prompted a change in systemic treatment strategy and illustrate how limited initial sampling may fail to capture clinically actionable subclonal populations or to fully reflect tumor biology under therapeutic pressure, as it remains uncertain whether these HER2⁺ clones were pre-existing or arose as a result of treatment-driven clonal selection.

Although intratumoral and intertumoral heterogeneity and receptor discordance are well recognized in BC, most prior studies have focused on receptor changes occurring after disease progression or metastatic relapse. By contrast, the present cases demonstrate that biologically relevant HER2⁺ clones may be present or emerge even in patients with HR⁺, HER2^- metastatic disease who show an early and favorable response to systemic therapy. These observations underscore the potential clinical consequences of ITH and highlight the need for individualized and dynamic treatment strategies, even in patients with initial treatment sensitivity.

The role of locoregional surgery in de novo stage IV BC remains controversial. While retrospective studies have suggested a potential survival benefit in selected patients with low tumor burden, these findings are limited by inherent selection bias (20). By contrast, most randomized controlled trials have failed to demonstrate a significant survival advantage associated with surgery (21-23). However, a recent meta-analysis suggested that patients with hormone receptor-positive, HER2^- tumors in selected clinical scenarios may derive benefit from locoregional treatment, supporting a hypothesis-generating role for surgery rather than a general treatment strategy (4). Additionally, the prospective BOMET MF 14-01 study demonstrated a significant survival benefit from locoregional surgery in patients with de novo stage IV BC presenting with bone-only metastases, a finding that further informs discussion in selected patient populations (10). In this context, locoregional surgery was performed in two patients with de novo metastatic hormone receptor-positive, HER2^- BC with bone-only metastases who had achieved a favorable response to first-line systemic therapy. In these cases, surgery provided local control and enabled a more detailed pathological assessment of the primary tumor.

BC is widely recognized for its pronounced heterogeneity, which plays a pivotal role in tumor progression, therapeutic response and clinical outcomes. This heterogeneity manifests at two distinct levels: Intertumoral heterogeneity, which refers to the biological differences between tumors of different patients or between primary and metastatic lesions; and ITH, which denotes the coexistence of diverse subclonal populations within a single tumor (24). Both forms of heterogeneity contribute to the complexity of disease management and highlight the need for individualized treatment strategies.

In clinical practice, the significance of this biological complexity becomes particularly evident in metastatic BC. When disease progression occurs, repeat biopsy is typically recommended to assess intertumoral heterogeneity, such as receptor status discordance between primary and metastatic sites (25). However, identifying ITH is considerably more challenging (26). While broader tumor sampling may provide additional biological information, surgical resection is not routinely performed in metastatic disease and is not intended as a tool to reveal ITH; rather, it may be feasible only in selected cases. In the present study, HER2⁺ subclones were identified only after surgical resection in both cases.

Two cases of hormone receptor-positive, HER2^- de novo metastatic BC with bone metastases, were presented. After a favorable response to first-line systemic treatment with endocrine therapy and CDK4/6 inhibitors, both patients had persistent residual primary breast tumors. To achieve local control and obtain a more comprehensive pathological assessment, surgical resection of the primary lesions was performed. Postoperative histopathological analysis revealed unexpected ITH, specifically the presence of HER2⁺ subclones that had not been detected in the initial diagnostic biopsies. These findings prompted a change in therapeutic strategy.

In the first case, although the patient had shown a favorable systemic response to CDK4/6 inhibitor therapy, it was decided to continue trastuzumab in addition to endocrine therapy to target the HER2⁺ component identified in the surgical specimen. The patient has no radiologic evidence of active disease and remains under follow-up.

In the second case, CDK4/6 inhibitor therapy was initially continued due to the favorable clinical response. However, the patient later developed progressive disease with new liver metastases. A biopsy of the liver lesion revealed a HER2-low phenotype (IHC score 1+). Despite this, based on the HER2⁺ clone found in the breast tumor, dual HER2 blockade with trastuzumab and pertuzumab was initiated. Unfortunately, the patient did not respond to this approach. Local therapy was required to control the liver lesion, and systemic chemotherapy was resumed as the main treatment strategy. Subsequently, considering the HER2-low phenotype in the liver metastasis, treatment was continued with T-DXd, a novel option for HER2-low disease. In this setting, identification of a HER2-low phenotype in the metastatic lesion provided a rationale for considering trastuzumab deruxtecan, in line with emerging evidence supporting the role of antibody-drug conjugates in HER2-low metastatic BC (17,18). Overall, this treatment sequence illustrates that, in cases with HER2 discordance, therapeutic decisions may be guided by reassessment of tumor biology.

An important consideration in interpreting these findings is whether the observed HER2⁺ subclones reflect true biological evolution under systemic therapy or sampling limitations of the initial core needle biopsy. In both cases, HER2 positivity was identified only in the surgical specimen and not in the initial biopsy, precluding a definitive distinction between these mechanisms.

This issue is particularly evident in Case 2, where HER2 amplification detected in the surgical specimen was followed by a HER2-low profile in a subsequent liver biopsy, together with limited clinical benefit from anti-HER2 therapy. This pattern may reflect spatial heterogeneity, temporal changes in tumor biology under treatment-related selective pressure, or technical variability in HER2 assessment across different tissue samples. Given the case-based nature of this report, it remains unclear which mechanism predominated; however, these findings underscore the dynamic and context-dependent nature of HER2 expression in metastatic BC.

These two cases highlight two important aspects: First, the decision to perform surgery in metastatic BC following a favorable response to initial systemic treatment; and second, the unexpected identification of ITH, specifically HER2⁺ subclones, through pathological evaluation of the surgical specimens. To the best of our knowledge, no previous studies have described the emergence of HER2⁺ subclones revealed solely by surgery after systemic therapy in HR⁺/HER2^- de novo metastatic BC. Although ITH is a well-recognized biological phenomenon, the emergence of HER2⁺ subclones after an initial favorable response to endocrine-based therapy in metastatic HR⁺/HER2^- BC, identified following surgery, presents a therapeutic challenge. The present cases emphasize how such findings, revealed only through more comprehensive tumor assessment, may complicate treatment decisions and underscore the dynamic nature of tumor biology in HR⁺/HER2^- de novo metastatic BC.

In conclusion, these two cases highlight the potential clinical impact of ITH in de novo metastatic HR-positive/HER2^- BC. While limited by their case-based nature, they serve as a practical reminder that tumor biology may be underestimated by initial diagnostic sampling. Reassessment of tumor characteristics, when feasible, may offer additional clinically relevant information and should be considered in selected scenarios where it may help inform treatment decisions.