Breast cancer (BC) has become the most commonly diagnosed cancer and the leading cause of cancer death in women surpassing lung cancer [1]. Anthracyclines are a mainstay adjuvant treatment for patients with advanced disease or triple-negative neoplasms, and they significantly reduce the 10-year risk of breast cancer recurrence and mortality [2]. In addition, the rate of human epidermal growth factor receptor 2 (HER2) positivity is 15% to 30% and is generally associated with a poor prognosis [3]. Trastuzumab, the first humanized monoclonal antibody targeting HER2, significantly improved clinical outcomes in HER2-positive patients [4]. However, Cardiotoxicity (CTX) is a well-known complication associated with both anthracyclines and trastuzumab, and can determine the prognosis in cancer survivors [5]. Anthracyclines exert their effects through mechanisms such as topoisomerase inhibition and oxidative stress, which contribute to progressive myocardial fibrosis and are correlated with the cumulative dose of anthracycline [6]. In contrast, trastuzumab-induced cardiotoxicity is not dose-dependent and is reversible, and it is thought to occur through inhibition of the Neuregulin-1 (NRG-1)/HER and downstream signaling pathway [7].

Epicardial adipose tissue (EAT) is a type of visceral adipose tissue that is found between the myocardium and the visceral layer of the epicardium. It plays a crucial role in regulating metabolic activity in the surrounding area through paracrine signaling. Consequently, alterations in EAT are believed to be linked to a higher risk of adverse cardiovascular events [8, 9]. Measurement of EAT on the free wall of the right ventricular using echocardiography was the first technique used for quantification of EAT thickness. However, this method has limitations as it cannot accurately measure the entire volume of EAT and relies on the experience and technique of the operator. Cardiac magnetic resonance imaging (MRI) can accurately measure the volume of EAT without exposing patients to radiation, but is associated with high cost, long examination time, and can trigger claustrophobic events in patients, all of which limits its application in clinical practice [8, 10]. Chest computed tomography (CT) can image the entire heart to provide accurate EAT volumes and densities. Patients receiving BC adjuvant chemotherapy require regular follow-up with chest CT, so an additional risk assessment can be performed using EAT without additional radiation exposure or cost [11, 12]. A recent study has reported an increase in EAT radiodensity and volume in BC patients who received adjuvant chemotherapy with anthracyclines and trastuzumab [3, 12]. However, another study discovered a decrease in EAT radiodensity following adjuvant chemotherapy with anthracyclines [13].

Given the different mechanisms leading to CTX between anthracyclines and trastuzumab and the key role of EAT in adverse cardiovascular events, the objective of this study was to evaluate the alterations in EAT radiodensity and volume in BC patients treated with anthracyclines (AC-T) and anthracyclines combined with trastuzumab (AC-TH) adjuvant chemotherapy and to assess whether there was a difference in the changes in EAT radiodensity and volume between the two groups in order to understand the potential role of EAT changes in CTX.

This study demonstrated that changes in both the volume index and radiodensity of EAT were observed in a group of patients who did not develop CTX, regardless of whether they received AC-T or the AC-TH adjuvant chemotherapy protocol. All patients exhibited an increase in EAT volume index and a decrease in EAT radiodensity. These findings suggest that changes in EAT volume index and density may alert clinicians to focus on BC patients receiving AC-T or AC-TH chemotherapy regimens to prevent the occurrence of CTX.

Under normal conditions, EAT is cardioprotective as a unique fat depot for the heart. It contains more uncoupling protein 1 (UCP-1) and mitochondria, which can produce heat from glucose and free fatty acids. However, in pathological conditions, EAT releases adipokines that are pro-inflammatory and pro-oxidative, which can be detrimental to the heart [8, 17]. It has been well established that estimating EAT using chest CT scans is highly reproducible and shows good agreement with EAT quantified by coronary CT angiography [10, 18]. Therefore, regularly reviewing chest CT images of breast cancer patients allows for detection of EAT and may be of additional prognostic value, especially in patients who have not yet developed CTX.

Trastuzumab and anthracyclines are used as the primary chemotherapeutic agents for BC patients due to their efficacy. However, CTX is a serious complication that can greatly impact the patients’ quality of life and prognosis [2, 4]. Several conventional risk factors, such as advanced age, hypertension, hyperlipidemia, diabetes, and coronary artery disease, may be associated with EAT and may even increase the likelihood of CTX [19, 20]. In this study, the mean age of the patients was 52.15 $\pm$ 7.97 years, which falls within the peak age range of 45–54 years for the incidence of BC in Chinese women [11]. There were no significant differences in hypertension and diabetes mellitus between the AC-T and AC-TH groups (p = 0.618). Additionally, all patients included in the study were free of hyperlipidemia, and those with a history of coronary artery disease and other cardiac conditions were excluded.

Our study found that the EAT volume index was significantly higher at T1 than at T0 in patients receiving either AC-T or AC-TH adjuvant chemotherapy, which is similar to previous studies by Li et al. [3] and Kwon et al. [12]. They also found that the EAT volume and the EAT volume index were significantly increased with the use of trastuzumab or anthracyclines. Previous research has shown that increased EAT volume is strongly associated with the progression of coronary artery disease, the occurrence of adverse cardiovascular events, and the prevalence and severity of atrial fibrillation [21, 22]. Moreover, patients with heart failure with preserved ejection fraction (HFpEF) tend to have significantly higher EAT thickness or volume compared to the healthy population. This affects left ventricular function and myocardial substrate utilization, leading to decreased cardiac reserve. In contrast, heart failure with reduced ejection fraction (HFrEF) is often characterized by reduced EAT thickness or volume, which may be related to more severe LV systolic dysfunction, left ventricular remodeling, or lower food intake in critically ill patients [8, 22, 23, 24, 25]. Although they did not experience CTX after chemotherapy, all patients in our study showed a significant increase in EAT volume index. Therefore, we hypothesize that trastuzumab or anthracyclines may increase the EAT volume index as a protective mechanism, resisting early myocardial damage and ventricular remodeling.

In this study, we also observed that the EAT radiodensity at T1 was significantly lower than at T0 in all patients who received adjuvant chemotherapy with AC-T or AC-TH. This finding contradicts the results of two recent studies [3, 12], but aligns with the findings of Monti et al. [13]. It is commonly believed that increased EAT radiodensity is associated with higher brown fat content [26] and inflammation and fibrosis in pathological conditions [8, 27]. Conversely, decreased EAT radiodensity is attributed to the conversion of brown fat to white fat, often accompanied by adipocyte hypertrophy and hyperplasia [8, 9, 28]. In this study, several factors may have contributed to the reduced EAT radiodensity in patients receiving adjuvant chemotherapy with AC-T or AC-TH. First, Anthracyclines have the potential to induce oxidative stress by binding to cardiolipin in the inner mitochondrial membrane, thereby increasing the production of reactive oxygen species. Additionally, trastuzumab may exacerbate anthracycline-induced oxidative stress by inhibiting the HER-2 pathway, which would further increase the accumulation of reactive oxygen species, leading to cardiomyocyte death and a progressive increase in myocardial fibrosis [7, 29, 30]. The rise in reactive oxygen species and myocardial fibrosis may be closely associated with the conversion of brown fat to white fat, a phenomenon commonly known as ‘EAT whitening’ [9, 31]. Second, all patients in our study received 5-HT3RA and a single dose of dexamethasone for antiemetic purposes before anthracycline chemotherapy. It has been observed during the COVID-19 pandemic that EAT radiodensity was significantly reduced in COVID-19 patients treated with dexamethasone, possibly due to its anti-inflammatory effect [32]. It has been shown that reduced EAT radiodensity is associated with adverse cardiac metabolic events. Liu et al. [33] found that reduced EAT radiodensity was associated with an increased risk of readmission and composite endpoints for heart failure in patients with HFpEF. In addition to this, Franssens et al. [34] also found that reduced EAT radiodensity was associated with higher coronary artery calcium scores in men at high risk for cardiovascular disease. Therefore, reduced EAT radiodensity in the present study may be associated with a higher cardiometabolic risk after chemotherapy in BC patients.

Due to the different mechanisms of injury between anthracyclines and trastuzumab leading to CTX, the present study compared patients receiving adjuvant chemotherapy with AC-T and AC-TH to investigate the differences in EAT volume index and EAT radiodensity. At T0, there were no statistically significant differences between the two groups for either EAT volume index (p = 0.090) or EAT radiodensity (p = 0.059). Similarly, at T1, there were still no observed differences in EAT volume index (p = 0.308) and EAT radiodensity (p = 0.802) between the two treatment groups. Similarly, another recent study also found that no statistical differences were observed in extracellular volume before chemotherapy and at 1 and 5 years after the end of chemotherapy between the two groups of patients who were treated with neoadjuvant chemotherapy using anthracyclines and anthracyclines in combination with trastuzumab [30]. This may be due to the fact that anthracycline-induced type I CTX is often considered irreversible and dose-dependent, while trastuzumab-induced type II CTX is often considered reversible, non-dose-dependent, and has a favorable prognosis [35]. However, it has been found that early detection and treatment can lead to reversible changes in type I CTX [36]. In some cases, patients who developed myocardial injury with trastuzumab did not recover, even after discontinuation of the drug [37], especially when anthracycline drugs were used in combination with trastuzumab, potentially leading to a synergistic effect and minimizing the distinction between the two types of CTX [38]. It is currently believed that the mechanism by which anthracyclines induce the development of CTX include: (1) the interaction of anthracyclines with iron, which leads to iron death, and (2) the dysregulation of the transcriptome mediated by the Topoisomerase II$\beta$-Dox complex expressed only in cardiomyocytes, which work synergistically to exacerbate oxidative stress [39], and (3) the fact that trastuzumab, which is known for its ability to block HER2 in cardiomyocytes, further exacerbates oxidative stress and apoptosis through blocking the HER2 receptor and further exacerbates oxidative stress and apoptosis [40, 41]. Oxidative stress, on the other hand, has a close relationship with EAT, where increased levels of reactive oxygen species and decreased expression of antioxidant enzymes lead to increased inflammation and promotion of cardiomyocyte fibrosis in EAT, which further leads to changes in EAT volume index and radiodensity [3, 8, 27, 31]. Currently, a $>$10% reduction in LVEF from baseline with an absolute value of 53% is recognized as a diagnostic criterion for CTX [15]. However, the sensitivity and reproducibility of LVEF are poor. Studies have demonstrated that CTX occurs in 6% to 18% of patients treated with anthracyclines after a median follow-up of 9 years [42]. Therefore, the decrease in LVEF may be a delayed process in disease evolution. When a significant decrease in LVEF occurs, significant cardiac damage may have already happened, often causing patients to miss the optimal treatment window [43]. In this study, all the patients did not meet the diagnostic criteria for CTX, but they did show significant changes in EAT volume index and radiodensity. This establishes a correlation between EAT and subclinical CTX, providing an imaging biomarker for monitoring patients with possible CTX.

There are some limitations in our study. First, it was a retrospective study conducted at a single center with a small sample size due to strict inclusion and exclusion criteria. Second, both chest CT and echocardiography were performed at the same time after the completion of chemotherapy, and none of the patients experienced CTX. Therefore, we could not confirm the relationship between CTX occurrence and specific changes in EAT over time. Prior research demonstrated that myocardial velocities decreased at week 5/6 post-injection of anthracyclines, returning to baseline by week 10, indicating an acute response followed by recovery [44]. Moreover, the delayed LVEF decrease suggests that EAT changes post-chemotherapy might better reflect early CTX changes, but longer follow-up is needed for a comprehensive understanding of EAT evolution in CTX patients. Therefore, we intend to conduct extended follow-up studies to enhance early CTX diagnosis and enable timely risk mitigation by clinicians. Third, the lack of a control group of breast cancer patients not receiving chemotherapy complicates distinguishing disease effects from chemotherapy-induced EAT changes, attributed to limited eligible patients [45]. Note that breast cancer patients without anthracycline and trastuzumab drugs do not show increased heart failure or cardiomyopathy risk [46]. To mitigate confounding factors, we included healthy women as controls. Finally, our study population did not include patients treated with trastuzumab alone. Consequently, we cannot rule out the possibility of a synergistic effect between anthracyclines and trastuzumab on the heart. This makes it difficult to determine whether the changes in EAT caused by type I versus type II CTX differ. The use of semi-automated software improved the reproducibility of EAT measurements, which opens up the possibility of incorporating EAT measurements into routine clinical practice.

In conclusion, our study found that BC patients undergoing chemotherapy with the AC-T and AC-TH protocols experienced a significant increase in EAT volume index and a significant decrease in EAT radiodensity. Given that breast cancer patients undergo regular follow-up chest CT scans, utilizing these scans to monitor the potential cardiac side effects of anthracyclines and trastuzumab would be an efficient approach without subjecting the patient to additional radiation exposure or financial burden. And the use of semi-automated software improved the reproducibility of EAT measurements, which opens up the possibility of incorporating EAT measurements into routine clinical practice.

The datasets used and analyzed during the current study are available from corresponding or first author on reasonable request.

YYL and QY contributed to the study concept and design; YYL, TJZ, XH, and LS contributed to data acquisition and outlining the regions of interest; YYL and TJZ analyzed and interpreted the data and drafted the manuscript; YYL and QY critically revised the manuscript for important intellectual content; and all authors were fully involved in the work and agreed to be accountable for all aspects of the work. All authors contributed to the editing of the manuscript. The final manuscript was read and approved by all authors.

This study was approved by the Ethics Committee of Yongchuan Hospital of Chongqing Medical University (No. 2023-025). Given the retrospective nature of this study, the requirement for informed consent was waived.

Not applicable.

This research received no external funding.

The authors declare no conflict of interest.