The SMC-PTS can be divided into following subsystems: the proton beam generator, irradiation system, patient positioning system, patient position verification system, respiratory gating system, and operating and safety control system.

The proton beam generator includes the accelerator, energy selection system (ESS), and beam transport system (BTS). And the irradiation system is composed of a rotating gantry and a nozzle. Each of the treatment rooms has its own irradiation system (a multi-purpose nozzle or a dedicated scanning nozzle). Details of each subsystem will be described in following sections and the layout of the treatment level of the Proton Therapy Center at SMC is shown in Fig. 1.

The proton accelerator at SMC-PTS is a normal conducting azimuthally varying field (AVF) cyclotron. The magnetic field strength in the cyclotron is designed to be radially increasing to correct the increased relativistic mass of accelerated protons. At the same time, the vertical defocusing of the protons due to the radially increasing magnetic field is adjusted by the azimuthally varying magnetic field configuration (hill and valley structures). The magnetic field of the cyclotron ranged from 0.9 T (valley) to 2.9 T (hill). The diameter of the cyclotron is 4.3 m and the weight of the cyclotron is approximately 220 tons.

The accelerator is composed of electromagnet system, radio frequency (RF) system, ion generator, and beam extractor. The electromagnet system generates a magnetic field inside the cyclotron and the RF system accelerates protons at a high frequency (106 MHz). There are two RF cavities located in the cyclotron. The ion generator ionizes hydrogen gas molecules in plasma state and generates protons to be accelerated in the cyclotron. In the beam extractor, the deflector applies a high voltage to deflect protons toward the outer extraction orbit using an electric field. After the deflector, the proton beams are focused by a pair of permanent quadrupole magnets. The cyclotron accelerates protons up to the energy of 230 MeV.

The ESS is composed of an energy degrader, an energy analyzer electromagnet, and a beam diagnosis system. The energy of proton beam can be adjusted by the thickness of the degrader in the range of 70 MeV to 230 MeV.

Although the extracted proton beams from the cyclotron are highly focused by the pair of permanent quadrupole magnets in the cyclotron, the energy width and emittance of proton beams will be increased by the degrader material. In the downstream of the degrader, the ESS composed of a set of quadrupole magnets and a bending dipole magnet deflected at an angle of 30° is located to re-focus the beam and select the proton beams only in the defined energy window by using a copper beam slit.

After the ESS and BTS is located to deliver and distribute proton beams to each treatment room. The BTS consists of a series of quadrupole electromagnets for beam focusing, bending dipole magnets for changing beam direction, and steering magnets for fine-tuning of the beam paths to the treatment rooms. It also includes profile monitors and beam current monitors.

Both treatment rooms have isocentric gantries, which are rotating steel structures that include the irradiation nozzles. The maximum radius of the gantry is 5.3 m and the mass of rotating part is 120 tons. The gantry rotates ±180° with 5° over-travel around the patient with the maximum rotation speed of the gantries is 1 rpm. The gantry system has the braking system equipped with 6 air-operation brakes. The 6 brakes assure an emergency stop within 5° at the maximum rotation speed.

Treatment nozzles are located at the end of the treatment room beam-lines and consist of various components to amend beam characteristics for clinical uses. There are two types of nozzle systems installed at SMC-PTS: a multi-purpose nozzle and a dedicated scanning nozzle. The multi-purpose nozzle has both wobbling and scanning function with corresponding nozzle components alignment. The dedicated scanning nozzle has a helium gas filled pipe, which minimizes unwanted interactions of proton beam in the beam path.

The patient positioning system consists of a robotic couch (Forte Automation Systems Inc., Machesney Park, IL, USA) and laser markers. One of the laser markers is located on the nozzle so that the center of the proton beam flux can be marked. There are additional laser makers located in the enclosure (horizontal) and the ceiling (vertical) to mark the isocenter. The robotic couch is composed of a table unit and a tabletop. The table unit has 6° of freedom and equipped with load cells for collision detection and deflection compensation. The robotic couch can support the maximum weight of 200 kg.

The VeriSuite (MedCom, Darmstadt, Germany) patient position verification system is composed of two orthogonal X-ray imaging systems. Each of the X-ray imaging system includes an X-ray generator, an X-ray imaging panel, and an image-processing device. Neither of the X-ray imaging axes are aligned with the proton beam axis as the X-ray imaging beam axis is off by 45° from the beam axis. The VeriSuite system can be operated in 3 different image acquisition modes: digital radiography (DR), fluoroscopy with respiratory signals, and cone beam computed tomography (CBCT). The acquired images will be compared with planning CT images and a correction vector will be calculated for a precise patient positioning.

At SMC-PTS, AZ-733V (Anzai Medical, Tokyo, Japan) system is installed for gating of a respiratory motion. The system is mainly composed of a respiratory sensor (RS), a sensor port (SP), and a wave deck (WD). The RS detects abdominal motions (pressure changes) of a patient and the SP amplifies and transmits the analog signal from RS. The WD receives the signal from the SP and converts the signal to digital so that it can be sent to a personal computer.

The respiratory gating system should be installed not only in the irradiation system but also in the simulation CT system in order to take images synchronously with a patient's respiration. An identical AZ-733V respiratory gating system will be implemented in Discovery CT590 RT (GE Healthcare, Milwaukee, WI, USA) located in the simulation room.

As the proton therapy system is composed of many different devices, the control of the whole system can be more efficient if the system is segmented into several groups. The segmentation can be performed according to function, procedures, and location. At SMC, the proton therapy system is mainly divided into two groups: the accelerator group and the treatment group. The accelerator group includes the proton beam accelerator, the ESS, and BTS. The treatment group includes the irradiation system, the patient positioning system, and the patient position verification system. Two groups meet at the exit of the bending magnet in the gantry. The operation of two groups is physically separated at different locations, the accelerator control room and the treatment control room.

The main control system is composed of encoders connected via a network to control both groups. Under normal operation, oncology information system is in operation connected with the control system.

A treatment safety unit is connected to each control unit of the proton therapy system via DeviceNet network, and the treatment safety unit is also connected to the door interlock system, the emergency stop buttons, and the radiation dose monitor system. The treatment safety unit controls overall safety of the system operation.

Based on the early beam data measurements, selected proton beam specifications can be described as following: the maximum range in G1 wobbling nozzle is varying with difference field size configurations (LARGE, MIDDLE, and SMALL) as the optimized total thickness of scatterers depends on the field size. The minimum range in the LARGE field configuration is measured to be 25.7 mm and the maximum range in the SMALL field configuration is measured to be 308.8 mm. The lateral penumbra (80% to 20%) at the middle of SOBP is also a function of the field size, beam energy, and the ridge filter selection. In the beam data measurement set, the minimum value of the lateral penumbra is achieved to 3.26 mm (70 MeV, SMALL, 1 cm ridge filter). The distal fall-off (distal 80% to 20%) is also a function of the beam energy, field size, and the ridge filter selection. The measured distal fall-off values are in the range of 1.2 mm to 6.8 mm. The dose uniformity in the lateral direction has been verified to be within 2% (80% of the field size region). The dose uniformity in the depth direction has less physical meaning as a single physical ridge filter covers a certain energy range: a slight slope in the depth direction cannot be avoided unless it is used with the optimal condition. The average dose rate in G1 wobbling nozzle is verified to be greater than 2 Gy/min for 1-liter volume (above 110 MeV). More detailed dosimetric characteristics of SMC-PTS will be reported in a separate article in the future.

The angular accuracy of gantry rotation is less than 0.2° and the positional accuracy of isocenter of G1 wobbling nozzle is less than 1 mm. The positional accuracy of the robotic couch has been meticulously tested and it is below 1 mm. The laser alignment system has been adjusted to the DR system within 0.2 mm accuracy. The simple quadratic sum of these uncertainties yields a total uncertainty of 1.4 mm and this is a conservative estimation as the correlation among the items has been ignored. In addition to the orthogonal X-ray imaging system, CBCT was integrated in to the gantry, therefore the setup using the internal landmark more relevant to target position will be possible.

For oncology information system, SMC made a choice on MOSAIQ (Elekta AB, Stockholm, Sweden) based on following requirements; support of SHI's beam delivery information, RayStation planning information, SMC Hospital Information System (HIS) and oncology communication, remote access (open network), integration of IGRT for proton, and integrated plan-comparison functionality.

The installation of MOSAIQ system and the first integration test with Sumitomo treatment control system (TCS) have been finished in March 2015. The final integration test among RayStation, MOSAIQ, and Sumitomo TCS will be coming soon.

According to the original proton project, there will be 6 physician staff dedicated to the proton therapy at the beginning. The number of medical physicists has been gradually increased up to 10 since the ground breaking. The engineering team has also grown up to 5. In each treatment room, CT simulation room, and the machine shop, 2 radiation therapists will be assigned. One biologist has joined the project and started to work on researches related to the proton therapy.

The volume of staffing will be related to the volume of proton therapy patient as well as the stage of the project. Up to starting of patient treatment, the physicists staff have been gradually increased for beam commission. In the clinical commissioning stage at least two medical doctors dedicates to proton therapy are required.

For therapist, assigning more than 2 radiation therapists for each treatment room is necessary due to the complexity of patient positioning and proton beam delivery.

At the beginning, focus of the clinical indications of the proton therapy will follow an evidence-based approach: the tumor sites demanding a better dose conformality while the tumor is contiguous to critical structures, pediatric cancers, and so on. The comparison studies with IMRT for all candidate disease sites are currently going on.