## 4 Discussion and Conclusions  
We present a conceptual design for a laser plasma accelerator-driven very high energy electron/photon beam radiation therapy robotic system comprising a drive laser system, laser plasma accelerator, electron beam focusing system, photon beam target system, main-body of robot, and laser beam stabilizing system. The drive laser system produces energy-amplified high-energy laser pulses. The intense laser pulses are guided through the inside vacuum structure of the main body of a robot into the accelerator chamber attached at the head of the main body of the robot, and focused onto the laser plasma accelerator mounted in the accelerator chamber, where they finally generate electron beams. The electron beam focusing system transports electron beams to the target in a patient, and the photon beam target system also generates photon beams for VHE electron/photon beam radiation therapy. The main body of the robot delivers the electron/photon beam to the target in a patient from multiple directions. The laser beam stabilizing system monitors laser beam positions and corrects their alignment during treatment. From a clinical point of view, since protons and heavy ions have a finite range in the radiation dose distribution along the beam path, where the dose increases slowly with the depth and then rises very steeply to produce a Bragg peak, as shown in Figure 1, generally speaking, the proton and carbon ion beams therapies are superior even to the highest technology X-ray therapy. However, in reality most radiation therapy (\(\sim\)99%) is currently carried out with X-ray beams of 6-20 MV photons because of the cost-effectiveness and the high capital cost of constructing and maintaining infrastructures to produce such proton and carbon ion beams, including large-scale particle accelerators such as cyclotrons, synchrotrons, beamlines, gantries, and radiation shielding in a clinical environment as well as the human resources for operating such a large facility. To date, Japan has the world's largest number of hadron therapy facilities (nine proton and four heavy ion therapy facilities) [36]. It is said that a new hadron facility 3000 square meters in area requires roughly $100 million for initial construction and several million dollars for annual maintenance [37], compared to $10 million for the highest technology X-ray therapy system in a clinic 100 square meters in area. In addition, the cost of treatment requires about $30,000 for proton therapy [37], compared to $6000 for X-ray therapy. Considering the current prices for a commercial multi-TW laser and robotic system, one may envisage a similar cost for the installation, operation, and treatment of the proposed laser-based VHE electron/photon beam therapy system as that of the X-ray therapy system. In this context, if electron beams with 50-250 MeV energies, which are not currently available in small-scale facilities, could be generated with a compact medical accelerator having a similar or even smaller size, such VHE electron beams would provide us with clinical effects superior to the present X-ray therapy, even comparable to proton beam therapy in terms of cost-effectiveness, compactness, and simplicity, which are important requirements for ubiquitous high-technology radiation therapy. Here we propose a new radiation therapy device that makes it possible to deliver 50-250 MeV VHE electron beams from a compact laser-driven plasma-based accelerator, which can be mounted on a gantry with laser optics installed in a robotic arm and controlled for delivering narrow collimatedbeams--so-called pencil beams--to tumors in a patient from multiple directions for intensity-modulated radiation therapy (IMRT) using pencil beam scanning (PBS), in conjunction with image guided radiation therapy (IGRT).