Radiotherapy can be traced back about 125 years to the discovery of X-rays (1895) by a Germany physicist named W. C. Roentgen. After X-rays were discovered, it did not take long before X-rays were used in cancer treatments. On January 29, 1896, just three days after announcement of the discovery of X-rays, the E. H. Grubb company, a vacuum tube manufacturer, applied X-rays for the first time in cancer treatment at the suggestion of doctor Ludlam [1,2]. A single treatment was performed for about one hour in breast cancer patients. In 1900, T. Stenbeck and T. Sjogren treated patients with skin cancer at the tip of their nose [3]. In 1903, Senn [4] first attempted treatment in a leukemia patient. In 1896, Despeignes began using radiotherapy in France. Patients with stomach cancer were irradiated 15 to 30 minutes with 80 fractions and it was reported that the disease improved and pain was relieved [1].

In 1902, Guido Holzknecht of Austria initiated the first proposal for Roentgen-based dosimetry. When his chromoradiometer was exposed to X-rays, a salt mixture changed from yellow to green. The levels of change with various radiation doses were compared to chromoradiometer readings calibrated in Holznecht (H) units [5]. The use of an ionization chamber was then adopted in 1928 at the first Association of International Radiology conference held in Stockholm, Sweden [6]. Initial radiation therapy relied on low energy with significantly low penetration power. There was neither an exact concept of the radiation dose nor the location of the tumor. Moreover, there was no concept of treatment planning as is used today. Since then, Co-60 beam therapy emitting megavoltage gamma rays became the common treatment in most radiotherapy departments in the 1950s to 1960s and this led to much research. This time period has become known at the first radiotherapy period. Only in the middle of the 20th century were exact dosimetry systems developed [7].

It is difficult to obtain exact information on when radiotherapy devices were first introduced to Korea, but the book of “Past Footprints” released by the Department of Radiation Oncology, Yonsei University College of Medicine, reports 1937 as the first year. The equipment at that time was an X-ray generating device with a combined use of diagnosis and treatment. It had a brand name of Special Polex and was produced by the Shimazu Japanese Company. The system was characterized by a tube voltage of 200 kVp, tube current of 20 mA, and 10 Gy emitting power (Fig. 1) [8]. The book describing the 20 year history of the Department of Radiation Oncology of Chonnam National University Hospital reported that a radiation oncology class was founded in 1953 and it took over the superficial cancer treatment machine (uncontrollable gas tube, Polyphos, Siemens, 135 kVp/5 mA, which doubled as diagnostic equipment) from the provincial hospital, predecessor of Chonnam National University Hospital, but it is unknown whether radiotherapy was actually performed. The book published in memory of the 30th anniversary of the Radiation Oncology Department of Catholic University Hospital reports that it installed a Maximar (50 kVp and 20 mA) in 1960 and began offering radiation therapy.

Since then, medical LINAC machines with megavoltage high energy were introduced in Korea through pioneering passion and endeavors even in poor surroundings. Severance Hospital was the first to purchase a clinical LINAC machine in Korea. The book of “Past Footprints” mentioned above reports that a high energy LINAC (LMR-13, Toshiba) was first introduced in Korea in 1972 at Severance Hospital with the assistance of the Japanese Overseas Technical Cooperation Group (OTCA). The LMR-13 was an electron accelerator that adopted a traveling wave mode and produced electron energies of 8 MeV, 10 MeV, and 12 MeV, X-ray energy of 13 MV, and a dose rate of 400–600 rad. At that time, it was thought that the machine had excellent performance because the X-ray beam or electron beam could be chosen according to the patient’s needs. Following Severance Hospital’s introduction of LINACs, there are no accurate records of another introduction of high energy LINACs in Korea in the 1970s. About 10 years later, St. Mary’s Hospital in Seoul introduced a 6 MV high energy LINAC in 1983 and the Department of Radiation Oncology, Korea Atomic Energy Hospital, introduced Micron accelerators with X-ray energies of 6MV, 10 MV, and 21 MV in 1985. The book published in memory of the 30th anniversary of the Radiation Oncology Department, Seoul National University Hospital, recorded that it started radiation therapy by introducing a Clinac 6/10 (Varian Company, Palo Alto, CA, USA) in 1985.

The Cobalt teletherapy machine was used for radiation therapy before the introduction of a clinical LINAC. Since the 1980s, high energy LINACs were introduced nationwide for radiation therapy. Dr. Ji Young-Hoon, who worked at the Korea Atomic Energy Hospital, reported in his documents on the “present development status and prospect for radiation therapy machine of Korea” that high energy LINACs, as many as about 200, were introduced and used in the clinical institution as of 2017, including Cyberknife and Tomotherapy. Needless to say, this number is fewer than that of Europe or the Americas, but the quality of LINAC machines and treatment levels in Korea does not suffer by comparison with those of any medically advanced countries. High level radiation therapy machines have been introduced and used in clinical practice, but professionals such as medical physicists who work with such therapy machines are very few compared to Europe or the Americas.

Computed tomography (CT) was not applied, as it is today, in radiation therapy during the days when Co-60 and the initial model of a megavoltage LINAC were used in clinical applications. Treatment planning was performed manually by an expert with related knowledge using radiographic images of cross sections of the tumor center or anatomical center of the treatment site. As for the application of radiation imagery to radiotherapy, single photon emission computed tomography and positron emission tomography emerged in the 1960s but were not applied for radiation treatment planning. CT came into wide use around 1972 and started to be utilized in radiation treatment planning. In 1960, treatment plans using radiation images started and 2-dimentional treatment planning was carried out for a short time until CT was used. Treatment planning did not achieve significant progress until patient-specific 3-dimentional (3D) treatment planning began to be implemented. Since then, personal computers began to be widely used and led to remarkable progress in radiation treatment planning using radiation images. At this time, many researchers studied treatment planning related topics and produced many reports [9].

It was very difficult to get good information on when computed treatment planning systems were introduced in Korea. The book of “Past Footprints” by the Radiation Oncology Department of Yonsei University College of Medicine records that modern treatment planning could be implemented in August 1970 only after the introduction of measurement devices capable of measuring and plotting isodose distribution curves. Prior to the introduction of these systems, treatment plans were implemented manually. Since 1980, high energy LINACs were widely introduced and then computed treatment planning systems were also introduced. In 1994, computed treatment planning systems capable of 3D treatment planning were introduced.

Current cancer therapy includes radiotherapy using an X-ray beam. Treating cancer using a medical LINAC has become a main trend in cancer therapy. Clinical LINAC models currently used commercially in clinical practice have evolved continuously through many trials and errors. Until 1950, external beam radiotherapy has been performed mainly with X-rays generated at a maximum of 300 kV. In the1950s and 1960s, the existing kV machines were replaced gradually with high energy machines as the development of high energy photon-emitting accelerators and Co-60 therapy machines became highly popular. Nevertheless, kV machines with low energy beams did not fade away until recently and have existed for the treatment of special sites.

In this chapter, we review X-ray generators that were the starting point of high energy machine development and were used for the first radiotherapy procedures. They predicted the kind of machines that would be developed in the future. The Radiotherapeutic Research Unit of the Medical Research Council at Hammersmith Hospital, London, installed the first clinically usable high energy LINAC. It was used February 1953 for physics experiments and other tests. Patient treatment was attempted on September 7 the same year but it was not used for patient treatment exclusively [15]. The Granz Ray Therapy device, which generated X-rays of less than 20 kV, was the first machine to be used for treatment before high energy X-ray generating LINAC devices. Contact Therapy generated X-rays in the range of 40–50 kV and it was used to treat patients with contact close to the treatment lesion. This therapy was suited for superficial tumors because the maximum absorbed dose was near the skin. Superficial Therapy machines were developed next. They generated X-rays in the range of 50–150 kV and were applied for radiotherapy. Orthovoltage Therapy and Deep Therapy that generated energies of 150–500 kV were developed and utilized for cancer treatment. This system had a source-surface distance (SSD) of about 50 cm and used cones to collimate the beam to the desired size. It still had a maximum dose near the skin but 90% of the dose occurred at a depth of about 2 cm below the skin, which facilitated treating the tumor deeper than with Superficial Therapy. Fig. 2a shows graphs of the percentage depth dose for Granz to Orthovoltage with different beam qualities and maximum doses at the skin surface, while Cobalt-60 (Co-60) had its maximum dose at some distance below the skin and much less skin reaction than with the kV machines. The device (Fig. 2b) is a superficial therapy machine equipped with a cone. Fig. 2c shows a superficial skin cancer that was treated with this type of machine.

However, there have been problems when the skin was exposed to large radiation doses to treat deep-seated tumors. From the 1950s to the 1960s, many efforts were made to develop a device capable of generating high energy photon beams. Using a step just before high energy, Supervoltage Therapy was developed that could generate up to 1,000 kV. The Van De Graff Generator without Co-60 teletherapy was the first to produce MV high energy. This system was an electrostatic accelerator designed to accelerate charged particles. This machine accelerated electrons to produce high energy X-ray beams of up to 2 MV in radiation therapy. This machine is not commercially produced anymore because Co-60 teletherapy machines and medical LINACs with better technical and physical advantages came to be used.

The need for high-energy radiotherapy machines was recognized from the beginning of radiotherapy, but technical shortages hampered development. In 1953, Metropolitan-Vickers contributed to the advent of LINACs. He developed and produced a LINAC device that accelerated charged particles, such as electrons, to high energies through linear a vacuum tube using high-frequency electromagnetic waves. This LINAC produced 8 MV X-rays using 3 meter acceleration or corrugated waveguides, was used to treat patients, and was the only LINAC machine at that time [16]. This was immediately followed by the additional installation of two LINACs using 4 MV.

One of them was installed at Newcastle by Mullard Equipment Ltd, a division of Philips, and the other was installed at Manchester Christie Hospital by Met-Vic. In addition, Met-vic supplied two more in 1955. They were installed at Western General Hospital, Edinburgh, and Mount Vernon Hospital, Middlesex [17]. In addition, a Stanford University research group in California, USA installed a 6 MV LINAC at Stanford University in the Department of Radiology in 1954. They treated the first patient in 1956. At that time, seven LINACs were installed through the world. The 15 MV machines were installed at St. Bartholomew’s Hospital and a few medical centers in Chicago. At that time, these machines were said to be installed or under installation, but it was not clear whether they had been applied in clinical practices [17,18].

Three Met-Vic systems were operating in Australia from 1956 to 1957 and, not long after that, they were installed in New Zealand as well. Until 1962, the Mullard Company installed LINAC systems in Australia, Japan, and Russia. Varian Company installed the first prototype of a 6 MV isocenter LINAC at University of California, Los Angeles Medical Center. A Mevatron was installed in France in 1965 [17]. Since then, high energy LINACs have been evolving. The development of high energy radiotherapy machines has a deep connection with that of electron beam acceleration modes.

In 1928, the mode of electron acceleration, proposed by Ising, a Swedish physicist, and developed by Wideroe, a Norwegian physicist, connected a series of linear tubes that allowed electrons to gain energy whenever they went through the gaps (Fig. 3). This principle had a limitation for clinical use in that the length of the accelerator tube increased with electron speed when the frequency was kept constant.

In the mid-1930s, Hansen from Stanford, CA, USA, developed an electron accelerating machine called Rhumbatron. This device repeatedly passed electrons through a resonant microwave cavity to increase energy when passing each section [19]. However, the electric power level at that time from the microwave generator was inappropriate for clinical applications. Through World War II, war radar devices in the military required a microwave generator with MW output, and two tubes with this type of performance were developed. In 1939, Boot and Randall [20] (England) designed the Magnetron, and in 1939, the Varian brothers (USA) developed the Klystron [21]. The basic difference between these two is the Magnetron is a self-generator that generates frequencies in response to DC input, while the Klystron is, in principle, an amplifier that amplifies the low power microwave input. These war radar systems were operated at a frequency of 3 GHz, which corresponds to a wavelength of 10 cm in free space, and they were well suited for the development of electron accelerators. Since 1945, the development of LINACs using microwaves has been done independently in the United States and the United Kingdom.

For the first time, the LINAC models were developed by adopting a traveling wave mode of acceleration modes. The LINAC, which adopted the first traveling wave mode for clinical use, was installed at Hammersmith with an accelerator tube length of about 3 m and a 2 MW magnetron. The beam traveling in a horizontal direction was bent by 90 degrees to a target. It had a field size of maximum 20 cm and dose rate of around 150 rad/min. The Varian Company, USA, at first adopted the traveling wave mode as well, but in 1962, the standing wave mode was adopted by Knapp et al. [22]. The main difference in this design was it produced an improved shunt impedance, enabling significant reduction of the accelerator size at the same energy. This design could produce 4–6 MV of energy from waveguides of 30–35 cm.

In the 1970s, LINACs were not changed dramatically in basic concept or structures, but the wedge system and the precision of the beam bending system were continuously improved. Therapy machines equipped with a general safety system were developed using the latest optimization techniques, such as a magnetron, control system, and other components. At the time of development, the advantages were not clear, but the initial therapy devices were very important in the evolution of sophisticated and modern LINACs used currently in the clinic.

Clinically available devices following physicists’ research steps are the following: basic control system, safety and support system, beam correction, patient positioning accessory, beam isotropic mounting, full rotation, vacuum pump, other vacuum component development, beam bending system, energy increase, independent collimator (MLC), electronic portal imaging device (EPID), computer control, dynamic wedge, IMRT function, and IGRT system, which has become easy to use in recent years. At the same time, various, accurate, and automated patient positioning and identification systems have been developed.

As to the development of medical LINACs, dose rates are important. Medical LINACs have experienced dramatic enhancement of dose rates since their initial development. LINAC-based radiation therapy for cancer treatment began with the first patient in 1953 in London, UK, at Hammersmith Hospital, with an 8 MV machine built by Metropolitan-Vickers and installed in 1952, which was the first dedicated medical LINAC. A short while later in 1954, a 6 MV LINAC was installed at Stanford, CA, USA, which was used for treatments in 1956 (Fig. 4a). The first Stanford LINAC device produced output of 65 rad/min at the maximum of the transition curve in water. Conventional medical LINACs use fixed dose rates of, for example, 200 MU/min (=2 Gy/min) and 300 MU/min (=3 Gy/min) for 6 MV and 10 MV, respectively, at the depth of a maximum dose in a water phantom on the central axis beam when irradiated with a 10×10 cm² field at a SSD of 100.0 cm. The latest LINAC can produce a dose rate of up to 20 Gy/min. Maxim et al. [23] are developing pluridirectional high-energy agile scanning electronic radiotherapy (PHASER) as a platform for clinical translation of FLASH (mentioned briefly in Chapter 3) cancer radiotherapy (Fig. 4b), which requires dose rates exceeding ~50 Gy/s. Such a rapid rise of dose rates reflects one aspect of the future trends of clinical LINACs.

The radiation cancer therapy devices installed and used for cancer treatment in about 100 medical institutions in Korea are all imported from abroad. Many research groups in Korea have been participating in the government-led development of medical LINACs from 10 years ago to achieve technological independence.

In 2012, Dr. Young-Hoon Ji’s research team at the Korea Atomic Energy Research Institute attempted to develop a Korean LINAC supported by the Ministry of Science and ICT, and Future Planning with the aim of providing them to developing countries and Korea. They did not reach the stage of application due to a lack of research funding, but succeeded in the development of a general purpose MLC and treatment planning system. Supported by the Ministry of Knowledge Economy in 2012, professor Chae Jong-seo at Sungkyunkwan University, aimed to develop a dual-head LINAC designed to reduce 30% of the treatment time through joint participation from the Korea Atomic Energy Research Institute, Yonsei University, Pusan National University, Kyungpook National University, and Korean companies.

At the same time, the team of Dr. Kyung-Min Jeong at the Korea Atomic Energy Research Institute conducted the development of a LINAC radiotherapy machine equipped with high-performance radiation for therapy and an imaging system supported by the Ministry of Science and ICT. As a segment of leading industry and promoting industry in metropolitan economic blocks in 2012, Taesung Precision Co., Ltd led the development of a high-powered X-ray and electron beam LINAC equipped with a small accelerator using an S-band for clinical applications and bio-industry in cooperation with the Atomic Energy Medical Center in the southeast of Korea. In Korea, the government-led development of LINACs has been attempted many times. Such efforts were not put to practical use because of the lack of lasting support from the government and poor participation from companies, but constant interest from related professionals has been making continuous accumulation of related technologies.