# A Dosimetric Analysis of Reduction Cardiac Dose with Lead Shielding in Breast Cancer Radiotherapy
## 1 Introduction  
In radiotherapy for breast cancer, tangential fields that avoid irradiation to normal organs such as the heart and the lungs are traditionally adopted to reduce doses to these critical organs [1]. Multiple studies have indicated that intensity-modulated radiotherapy (IMRT) can produce a uniform dose distribution within the target volume as well as provide decreasing doses to the surrounding critical organs, thereby reducing complications, greatly enhancing prognosis, and improving the local tumor control rate [2, 3, 4, 5, 6]. Owing to the superior beam direction, the concept of tangential beams is still being used in some 3D conformal radiotherapy (3D-CRT) and IMRT treatments after modification [7, 8, 9, 10]. Literature has indicated that female patients undergoing postoperative radiotherapy for breast cancer are likely to receive higher doses to the surrounding normal tissues such as the lungs and the heart. It is also challenging for postoperative radiotherapy plans to achieve the desired tumor control curve without compromising on the doses to adjacent normal tissues. At present, there are three postoperative radiotherapy techniques for breast cancer patients, namely (1) 3D-CRT; (2) IMRT; and (3) Volumetric-modulated arc therapy (VMAT). The difference between these three techniques lies in the number and the angles of the beams, which results in different dose distributions. The rapid development of radiotherapy techniques has made it possible to increase the target dose while minimizingorgan-at-risk (OAR) doses as well as shortening the treatment duration [11]. In addition to the radiotherapy technique, the cardiac dose can be reduced by the use of the prone position or the deep-inspiratory breath-hold (VBH) method for managing the movement of the patient's internal organs due to breathing [12]. Radiotherapy for breast cancer requires not only the achievement of good target coverage, but also the reduction of normal tissue complications in the lungs and the heart. Since 1976, [PERSON] et al. started performing tumor resection combined with radiotherapy on patients with stage I and II tumors of sizes less than 4 cm. Subsequently, they carried out a statistical analysis on these patients, which found that the mortality of breast cancer patients receiving combined treatment was slightly lower than that of those receiving tumor resection alone. However, the side effects of radiotherapy increased the number of deaths from other causes, thereby compromising the overall survival rate [13]. A study by [PERSON] et al. [14] identified the correlation between the lung volume in patients receiving more than 20 Gy and pulmonary complications and suggested that when the lung dose was less than 20 Gy, there was no incidence of radiation-induced injury. However, in clinical practice, it is often inevitable that approximately 10-20% of the irradiated areas receive 30-40 Gy or even over 40 Gy. The probability of radiation pneumonia would increase with increasing dose. In radiotherapy, the primary tool used to assess the dose received by tumors and normal tissues in different treatment plans is the dose volume histogram (DVH). The lung V\({}_{\text{20 Gy}}\) parameter is adopted to evaluate the probability of radiation-induced lung injury. In a study in 1999 that analyzed the correlation between DVH and radiation pneumonia, [PERSON] et al. suggested that when the lung V\({}_{\text{20 Gy}}\) was less than 25%, the probability of radiation pneumonia approached 0. When the lung V\({}_{\text{20 Gy}}\) was between 25-37%, an alternative plan was necessary to reduce V\({}_{\text{20 Gy}}\). Therefore, it is important to ensure that the lung V\({}_{\text{20 Gy}}\) of a breast plan is within a reasonable range to reduce the probability of subsequent radiation pneumonia in breast cancer patients [15]. Similarly, the mortality due to radiation-induced heart disease is correlated with the irradiated volume and dose of the heart.