The goal of the radiation therapy is to destroy cancer cells, minimising the damage to healthy tissues and other side effects. The "FLASH" therapy, an innovative technique in radiation therapy, has shown that short pulses of electron and X ray beams at very high dose rates are less harmful to healthy tissues but just as efficient as conventional dose rate radiation to inhibit cancer growth.
The FLASH technique is being investigated with the use of external beams with ultra-high doses and ultra-high speeds. The FLASH effect is indeed observed at mean dose rates larger than 40 Gy/s delivered in less than one second (100-500 ms) unlike conventional radiotherapy where typical dose rates are of the order of few cGy/s released in a few to tens of minutes. Furthermore, the total number of needed sessions with FLASH therapy could be potentially reduced to only one.
In order to achieve the FLASH therapy requirements, we propose to investigate the use of a linear accelerator (Linac) for the production of electron beams with pulsed currents of the order of 100 mA in bursts of few microseconds as a viable solution. We will perform a feasibility study and carry out the prototype construction of a compact, cost-effective C-band Linac with high current and maximum energy of about 9 MeV. The prototype will be tested in our laboratory at low power to characterize all the radiofrequency properties of the structures.
The 9 MeV maximum energy is suitable for operation with the current IORT protocols to treat superficial cancer, for which we will perform a dosimetry study. Moreover, in order to design a detector capable of monitoring with the due precision beams of very high intensities, a dedicated simulation will be developed using state of the art Monte-Carlo software tools such as FLUKA developed at CERN and INFN, or FRED, developed in our department.
It is important to point out that the FLASH therapy represents a novel modality of irradiation that was just born and it is at the beginning of its development, and only few research groups all around the world have started investigating the effects of high dose rate on cancer and sane cells. However, despite its recent birth, its capacity to produce differential effect between tumors and normal tissues and its effectiveness to destroy tumors, while better protecting normal tissues and preventing side effects with respect to conventional radiotherapies, makes FLASH therapy a very attractive technique in the fight against cancer.
A second important consideration is that, nowadays, high current Linacs are not very widespread, and industries and hospitals rarely use them. Our aim is to propose an 'ad hoc' Linac designed from the outset for the purpose of FLASH therapy. This means an accelerator able to deliver high currents of the order of 100 mA, with a high level of compactness such that it can fit inside a hospital. This explains the reason for our choice of the C-band technology. It is the first time that the C-band technology is proposed for such a medical application. We believe that it has the potentiality for establishing itself in terms of compactness with respect to the nowadays more widespread S-band technology, that is at the basis of IORT Linacs produced, for example, by SIT.
Another strong point of this technology is its cheapness. A commercial electron gun can be combined with our design of C-band accelerating structures and a commercial magnetron can be used as power source. Additionally, we could investigate the possibility to make this kind of accelerating structures out of hard copper by using two halves which could be clamped together and welded on the outer surface. With this procedure it is possible to avoid any high-temperature processes like brazing or diffusion bonding. The two Linac parts can be joined together by means of specifically designed screws which ensure good vacuum and RF contacts, allowing to realise an even lower cost Linac. Since our request foresees the realisation of a prototype to be tested at low power, we do not need to follow such procedure, which is not consistent with the requested budget, but which remains an interesting option for a full operational machine.
The proposal of a low-cost and very compact Linac able to deliver high currents is very attractive in general not only for medical applications, but also for other industrial applications. For example, a similar design can also be used in industry or hospitals for sterilisation of sanitary/medical equipment and food products, as well as for water treatment. Our studies will be also useful to the design a compact C-band based higher energy electron Linac, which can be used to cure deeper tumors. The software that will be developed in order to provide a fast calculation and optimisation of the treatment plans performed using electrons of different energy will have a very large impact both on the FLASH therapy perspective (allowing to establish a protocol to treat the patient and evaluate the better technical solution to be implemented in a treatment room to deliver it) but also on the recent growing interest in replacing conventional RT with machines capable of delivering high energy electrons with high intensities. If the preliminary results obtained with the FLASH approach will be confirmed in clinical trials, the impact in the development of the infrastructures needed by the high energy electron based RT will become enormous, shaping the RT worldwide future. By providing robust and fast tools to plan the patient treatment and the indication about the best technology that has to be exploited to monitor the delivered beam, we will significantly speed up and ease the clinical FLASH approach implementation.