2.1 Reactive oxygen species and oxidative stress

Oxidants, also known as ROS, include superoxide radical anions, hydrogen peroxide (H₂O₂), hydroxyl radical, and singlet oxygen and so on. ROS have both endogenous and exogenous sources, such as NADPH oxidase, mitochondria, radiation, and ultraviolet light. Intracellular ROS are mainly generated in mitochondria, and its main source is the inhalation chain. ROS accumulates in the body and eventually causes cardiovascular damage.⁵ Cardiomyocyte dysfunction caused by radiation-induced oxidative stress is an important mechanism underlying cardiotoxicity. Oxidative stress refers to a state of imbalance between oxidation and antioxidant action in the body, leading to oxidation. This imbalance results in inflammatory infiltration of neutrophils, increased protease secretion, and the production of numerous oxidative intermediates.⁶ Radiation can lead to changes in free radical metabolism by activating several signaling pathways, including ROS and nitric oxide (NO).⁷ Ionizing radiation can directly lead to the breakdown of the mitochondrial respiratory chain, resulting in respiratory chain dysfunction, thereby reducing Adenosine Triphosphate production, increasing ROS production, reducing antioxidant capacity, and inducing apoptosis.⁸ In addition, macrophages, lymphocytes, and mast cells are important sources of ROS and NO. Radiation exposure leads to inflammatory cell infiltration, which further increases ROS production.⁷

2.2 Release of inflammatory factors

Inflammatory cytokines are involved in the inflammatory response. Cytokines that plays a main role are TNF-α, IL-1β, IL-6, TGF-β, IL-8, and IL-l0. Endothelial injury leads to vascular dilation and increased vascular permeability. Damaged endothelial cells secrete adhesion molecules and growth factors, which in turn promote the secretion of TNF and interleukins (IL-1 and IL-6). This further activates the acute inflammatory response and creates a pro-inflammatory environment for cardiac cell inflammation and dysfunction.⁹ Neutrophils play an important role during the acute phase. It is distributed in all layers of the heart exposed to radiation. They promotes the secretion of profibrotic factors by inflammatory cells.¹⁰ In addition, oxidative stress increases TGF-β1 leading to radiation-induced vascular injury and endothelial dysfunction.

2.3 Myocardial fibrosis

It is a prominent late-stage pathological manifestation of RIHD. Its pathogenesis is similar to that of general fibrotic diseases in various tissues and radiation-induced fibrotic diseases. The core of this process is fibroblast transformation and continuous activation caused by early inflammation and endothelial injury, which involves the participation of many cells, factors, and signaling pathways. Myocardial fibrosis usually occurs when heart muscles receive higher radiation doses. Myocardial fibrosis usually does not show obvious symptoms in the early stages, and related clinical symptoms usually appear at least 10 years after radiotherapy.¹¹

Pathological hyperplasia of mast cells is often observed during myocardial remodeling caused by RIHD and other heart diseases, suggesting that mast cells may be involved in myocardial fibrosis. However, their role in radiation-induced myocardial fibrosis remains unclear. Boerma et al.¹² reported that mast cells play a protective role against RIHD. Mast cells reduce the degree of myocardial fibrosis. However, this effect contradicts the findings for mast cell degranulation inhibitors, which have been shown to reduce fibrosis. The role and mechanism of mast cells in RIHD myocardial fibrosis require further study.

TGF-β is a widespread cytokine belonging to the activin/bone morphing protein superfamily. Three forms of TGF-β exist in mammals (TGF-β1, TGF-β2, and TGF-β3), of which TGF-β1 is prevalent in most tissues. The roles and mechanisms of action of TGF-β1 have been studied in detail. During the development of many fibrotic diseases, the initiation of fibrosis depends on TGF-β1 and other fibrotic factors that activate progenitor cells, including fibroblasts, epithelial cells, endothelial cells, smooth muscle cells, monocytes, and bone marrow-derived mesenchymal cells, into myofibroblasts. This leads to increased synthesis and secretion of matrix molecules such as collagen and fibronectin, and promotes tissue fibrosis.^{13, 14} A study using radiation therapy rat model showed that increased levels of circulating TGF-βl could lead to significant fibrosis in the sub-Endo myocytes, indicating its role in myocardial remodeling and radiation-induced myocardial fibrosis. In addition, TGF-βl and EGFR signaling pathways play important roles in the fibrosis of RIHD. EGF and Nrg-1 are the two most common ligands of the EGFR signaling pathway.¹⁵ Nrg-1 is downregulated in the early stage of injury, involved in fibrosis formation and myocardial remodeling, and is upregulated in the late stage in an attempt to restore myocardial structure.¹⁶ Therefore, Nrg-1 may play a dual role in myocardial fibrosis. Additional molecular mechanisms underlying myocardial fibrosis need to be further explored by basic research.

3.1 Optimizing the radiotherapy target area

With an increase in radiotherapy dose, the damaged segment will also experience corresponding strain damage, showing a dose-response relationship even to 12 months. With an increase in irradiation dose, more troponin is released.¹⁷ Another study reached similar conclusions. High-sensitivity cardiac troponin T levels were measured before, during, and after left breast radiation therapy. Patients with elevated hscTnT levels had higher whole-heart and left ventricular radiation doses as well as higher left anterior descending artery (LAD) V15 and V20 values.¹⁸

Previously, we only focused on the overall cardiac dose, but researchers found that when the heart and planning target volume (PTV) are in close proximity, limiting the dose to another structure, such as the LAD, can provide better protection for the heart. Therefore, the receptivity of the heart substructure is more important than that of the heart as a whole.^{19, 20} In a recent small prospective study, Gkantaifi et al.²¹ assessed the radiation doses to the whole heart, LAD, and left ventricle (LV). The early results showed low radiation exposure to the whole heart and LV, but high exposure to the LAD. Therefore, the LAD exposure dose may be a better predictor of RIHD. When mapping the target area and creating a radiotherapy plan, the dose to the heart substructure should be strictly limited while paying attention to the overall cardiac dose. Our team²² investigated the effects of the LV and LAD as organs at risk on the dose volume distribution in the target area and organs at risk in the original radiotherapy plan. The results showed that the target area coverage was similar for both the treatment and optimization groups, and both met the dosimetry requirements of the actual clinical radiotherapy target area. Compared to the treatment group, the optimization group achieved a significant reduction in the radiotherapy dose to the heart, LAD, and LV. This is particularly important in radiotherapy centers without deep inspiration breath-hold (DIBH) technology, and the optimization group did not increase the radiation dose to other organs at risk (such as the lungs, right breast, and spinal cord). Therefore, to reduce long-term radiotherapy-related cardiotoxicity, we need to focus not only on the average dose to the heart as a whole, but also on the dose to the heart substructure.

3.2 Radiotherapy technique

DIBH is a breathing control technique that allows the chest to expand and the heart to move away from the wall of the chest, reducing the irradiated area/volume of the heart, thereby reducing the cardiac radiation dose. The average dose to the LAD is reduced to 9.3 Gy by respiratory gating.²³ Knöchelmann et al.²⁴ retrospectively analyzed the tangential 3D-planned radiation of 357 patients with left-sided breast cancer and found that the mean heart dose and LAD dose were significantly reduced. In addition, the volume and dose of the target area and different organs at risk (e.g., heart and lungs) were also changed in the prone position. Mulliez et al.²⁵ evaluated the cardiac effects and feasibility of DIBH in the prone position during left-sided whole-breast irradiation (WBI). The results demonstrate the ability and feasibility of prone DIBH to achieve optimal heart and lung sparing in left-sided WBI. A comparison of the free-breathing prone position with DIBH and supine position showed that the lung dose was lower in the prone position, while the heart dose was lower in the supine position.^{26, 27} The supine position resulted in a significant reduction in the mean heart dose for most treatment techniques. The supine position leads to a more substantial separation effect on the heart.²⁸

Proton therapy is a new technique to treat tumor using the characteristic of “Bragg peak” of proton beam, which has the advantage of exposing the tumor area to high doses while protecting surrounding organs. This technique is gradually being used in breast cancer radiotherapy. Hong et al.²⁹ compared the adverse effects of conventional and low-dose proton therapies for breast cancer. Proton radiotherapy can significantly reduce the dose to organs at risk in patients with breast cancer, while ensuring the target dose. Kammerer et al.³⁰ explored the impact of proton therapy on locally advanced breast cancer and found that it often decreased the mean heart dose by a factor of two or three. This reduction could potentially decrease the occurrence of cardiotoxicity. However, proton radiotherapy is expensive, has poor accessibility in primary hospitals, and is not acceptable to the general public.

4.1 Cardiac biomarkers

The 2022 European Society of Cardiology cardio-oncology guidelines introduced the notion of cancer therapy-related cardiovascular toxicity (CTRL-CVT) and emphasized that cardiac markers play an important role in detecting early cardiotoxicity.³¹ There is increasing interest in the early application of biomarkers to predict the onset of cardiotoxicity before irreversible cardiac damage occurs. In clinical practice, the most important biomarkers of cardiac injury are cardiac troponins and natriuretic peptides.

Several studies have investigated the changes in cardiac-specific biomarkers (BNP and Troponin) in patients undergoing radiotherapy. Skyttä et al.³² analyzed the change in high-sensitivity cardiac troponin T (hscTnT) and NT-proBNP three years after radiation therapy for breast cancer. In all patients, pro-BNP levels increased compared to baseline values. In a univariate model, the increase in pro-BNP correlated with the mean heart dose, volume of the heart receiving 20 Gy, and mean dose to the LV in patients with left-sided tumors. In multivariate analysis of the entire population, BNP levels increased with age. However, pro-BNP levels did not increase in patients with right-sided tumors. In patients with left-sided tumors, hscTnT levels were elevated from baseline to 3 years after RT. In patients with right-sided tumors, a corresponding increase was observed only in a few patients. Over the 3-year period, increases in hscTnT levels were not associated with any of the other variables selected in the multivariate analysis; only age showed a tendency for association. Plasma BNP levels were studied in long-term breast cancer survivors treated with radiotherapy alone. Five years after RT, the median plasma BNP level did not exceed the upper limit of normal; however, δ-BNP levels correlated with cardiac and ventricular exposure doses.³³ Tlegenova et al.³⁴ evaluated the effectiveness of combining clinical factors and traditional biomarkers for predicting asymptomatic cancer therapy-related cardiac dysfunction (CTRCD). They designed a predictive model to assess the risk of potential cardiotoxicity during or at the end of treatment, allowing for the timely administration of cardioprotective drugs and adjustments of chemotherapy regimens. This study demonstrated the dominant role of NPs in diagnosing subclinical CTRCD. Erven et al.³⁵ investigated the role of troponin I in predicting subclinical cardiotoxicity induced by radiation therapy (RT) in patients with breast cancer. Compared with pre-radiotherapy levels, the mean TnI levels were significantly higher in patients left-sided tumors after radiotherapy, while those with right-sided tumors were unaffected. However, D'Errico et al.³⁶ reported no change in TnT levels in patients with left-sided breast cancer 5 and 22 months after RT. At present, changes in troponin and BNP levels after radiotherapy in breast cancer have not reached a consensus among various studies, and more prospective studies are needed for further exploration.

In addition to BNP and TnT levels, Aula et al.³⁷ evaluated the changes in TGF-β1 and Platelet-Derived Growth Factor (PDGF) levels in breast cancer patients treated with radiotherapy and their relationship with echocardiographic changes. They found a decrease in TGF-β1 and PDGF levels during adjuvant radiotherapy for breast cancer, which correlated with parametric changes in echocardiography. Other biomarkers also showed changes after antitumor therapy. Finkelman³⁸ identified early alterations in arginine-NO metabolite levels in patients with breast cancer undergoing doxorubicin therapy, and early biomarker changes were associated with a higher CTRCD rate. Demissei et al.³⁹ retrospectively observed early changes in cardiovascular biomarkers following chest radiotherapy and analyzed the relationship between radiation dose and volume, and changes in markers related to cardiac injury. In lung cancer/lymphoma, placental growth factor and growth differentiation factor 15 (GDF-15) levels increased from pre- to post-RT; however, no significant changes were observed in breast cancer. Moreover, changes in cardiovascular-related biomarkers were not significantly correlated with changes in echocardiographic findings. Long-term follow-ups are required to determine the relationship between these biomarkers and cardiac damage.

4.2 Electrocardiogram (ECG)

Electrocardiogram is non-invasive and easy-to-perform test that reflects the electrophysiological characteristics of heart depolarization and repolarization. It can be used to evaluate electrical changes in the early stages of cardiac injury. Teimouri et al.⁴⁰ investigated changes in ECG and echocardiography following adjuvant RT in patients with left-sided breast cancer. The results showed that at three months after radiotherapy, 47% of the patients showed T-wave inversion in their ECG. The T-wave decline was associated with mean heart radiation dose. As the left ventricular volume receiving 5 Gy (LV-V5) increased, the corresponding ST segment duration decreased and the left ventricular systolic diameter (mm) increased. Another retrospective study⁴¹ aimed to investigate early RT-induced ECG changes and compare them with functional and structural changes observed on echocardiography changes. Thus, ECG can be used for post-RT cardiac screening. Because of its simplicity and convenience, ECG is recommended for monitoring cardiac function in patients with breast cancer after RT.

4.3 Echocardiography

In clinical practice, determining left ventricular ejection fraction (LVEF) by routine echocardiography is widely used to identify associated cardiac injuries. However, a decrease in LVEF primarily reflects significant LV dysfunction. In the absence of changes in LVEF, myocardial function can undergo great changes.⁴² Speckle-tracking echocardiography (STE) can detect local reductions in myocardial function before measurable changes occur in conventional echocardiographic parameters.^43-45 STE is a new technique for studying myocardial deformations, in which global longitudinal strain (GLS) is a good parameter for evaluating early cardiac injury caused by radiotherapy. Changes in the LV systolic function were observed immediately after RT in the GLS group. GLS, LVEF, and stroke volume (SV) are significantly impaired at three years of age, and the impairment is more pronounced on the left side.³² Similarly, our group conducted a meta-analysis⁴⁶ to evaluate the value of GLS in evaluating cardiotoxicity induced by radiotherapy in breast cancer. GLS is a good parameter for identifying early radiation-induced heart disease in patients with left-sided breast cancer. Thus, for patients receiving radiotherapy, especially those with left-sided breast cancer, we should focus on the cardiac substructural radiation dose and regularly monitor the changes in GLS before, during, and, after radiotherapy to detect radiation heart damage in a timely manner and avoid the occurrence of long-term radiation heart disease.

Diastolic dysfunction has also been explored in several studies. Sritharan et al.⁴⁷ used conventional and novel echocardiographic parameters to study changes in left ventricular diastolic function in patients with breast cancer after acute-phase radiotherapy. A significant reduction in diastolic function was observed in the subgroup of patients with ≥10% reduction in systolic function, reinforcing the concept of diastolic function as a potential indicator of systolic dysfunction. Further multicenter studies are required to verify the prognostic value of these observations. Early identification and appropriate measures are required to minimize the damage. However, challenges remain in assessing the underlying constitutive remodeling of the myocardium (e.g., fibrosis and edema).

4.4 Cardiac magnetic resonance (CMR)

CMR is multiparametric, highly reproducible imaging technique that can accurately detect myocardial edema related to acute injuries.^{48, 49} CMR is the gold standard for assessing LV function.⁵⁰ A prospective study⁵¹ investigated the changes in T1 and T2 relaxation times in CMR after chemoradiotherapy for breast cancer. Studies have shown that myocardial T1 and T2 relaxation times increase after chemotherapy but do not increase after left breast radiotherapy. At 13 months follow-up, CTRCD appeared in 20% of the chemotherapy patients, suggesting that myocardial damage is associated with left ventricular dysfunction. CMR findings were normal in patients undergoing radiotherapy because radiotherapy leads to long-term cardiac toxicity, which requires a longer follow-up for detection. Moisander et al.⁵² conducted a prospective follow-up study to evaluate the early signs of diffuse fibrosis after RT and its progression over a six-year period. By analyzing multimodal imaging data, it was found that diffuse myocardial fibrosis was related to the radiation dose. CMR plays an important role in detecting myocardial fibrosis.

4.5 Myocardial radionuclide imaging

Myocardial perfusion scintigraphy (MPS) is a nuclear imaging method using Thallium-201-chloride (201Tl) or Technetium-99m methoxyisobutylisonitrile (Tc-99m MIBI) to measure myocardial perfusion and evaluate left ventricular function.⁵³ In a study involving left-sided breast cancer patients with a radiation field containing at least 1 cm of the heart, Seddon et al.⁵⁴ found that nearly one-third of patients with perfusion defects had regional wall motion abnormalities in the presence of no more than a 5% decrease in LVEF. However, because of its high price, its use is limited to clinical applications. For patients who can accept this test, it is of great value to assess cardiac injury after RT.