## 1 Introduction  
Linear accelerator (LINAC)-based cancer treatment was first implemented at Hammersmith Hospital in London, England, in the 1950s. The treatment started with a 6 MV LINAC in 1956 [1]. A medical LINAC accelerates electrons using a waveguide, in which the RF power generates a standing wave. However, modern LINAC machines have a shorter vertically mounted waveguide and a longer horizontal waveguide for a higher energy output, which has a bending magnet that directs the beam vertically toward the patient [2]. A LINAC can generate a stable and accurate radiation beam using various electromechanical parts. A LINAC with a single-energy electron beam between 4 and 25 MeV can provide an X-ray output including electron energy when electrons strike a high-density (e.g., tungsten) target [2]. Brain, spine, lung, breast, prostate, liver, and skin tumors, among others, can be treated by electron or photon beams generated using a LINAC with appropriate treatment plans [3]. A LINAC is mainly composed of a gantry head, body, and treatment couch. Some differences exist in some parts depending on the manufacturer, for example, the bore and robot type. In detail, it consists of jaws, multileaf collimators (MLCs), a flattening filter, a primary collimator, an X-ray target, an accelerating waveguide, an electron gun, a feed wavelength, a magnetron, a vacuum pumping line, a vacuum pump, and another motion system [4]. The onboard imager and electronic portal imaging device are attached to LINAC's body for image-guided radiation therapy. Additionally, the body-tracking device of the patient is mounted on the ceiling and floor of the treatment room to perform additional accurate imaging, while the patient treatment in fractionation is performed using a surface-image-guided radiotherapy system [5; 6; 7]. Meanwhile, abrupt discontinuation of a medical LINAC when treating patients using high-energy radiation has serious consequences in that the planned dose cannot be delivered to the patient. Thus, performing quality assurance procedures for consecutive patient care without sudden LINAC downtime composed of complex electromechanical components is a top priority for medical institutions. Nevertheless, LINAC machine downtime is caused by harsh operating environments, such as large accumulated amounts of monitor unit (MU) use, a prolonged operation period, and higher cumulative dose rate operations. Therefore, the task group (TG)-100 of the American Association of Physicists in Medicine (AAPM) provided guidelines to each medical institution on establishing a quality management program that determines the risk of LINAC failures according to its quality control procedures [8]. Each medical institution can perform TG-142, which provides detailed items for periodic quality assurance of medical accelerators [9]. Thus, the best way to avoid downtime is for those responsible for quality control, such as radiation therapy technologists and medical physicists, to follow the LINAC's guidelines for quality control processes, risk analysis, and management practices [8; 9; 10]. Currently, all medical institutions comply with the strict radiation management regulations of their respective countries for the safety of patients and healthcare professionals, and stringent quality control is stipulated. Nevertheless, LINAC failures remain difficult to predict, and it is impossible to determine which component will cause a failure at a certain time. Several studies have published the results of operating status analysis to maintain the optimal condition of LINACs. [PERSON] et al. developed a stacked long short-term memory prediction model to predict the machine operating status and records of daily quality control lists (beam output, symmetry, etc.) for two radiotherapy machines [11]. The goal was to boost the confidence of medical staff or physicists in predicting the future malfunctioning behavior of machines before they reach their tolerance levels and taking planned preventive action.