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Proton therapy may lower the chance of causing secondary tumors or impairing white blood cells and the immune system compared to conventional radiation therapy. Despite this, the use and expansion of proton therapy is controversial, primarily due to the substantial costs associated with the technique. To reduce the cost of proton therapy, we propose a solution of a gantry less and ultra-compact proton facility. Here, we describe the detailed design and dosimetric performance of two ultra-compact and highly efficient solutions for cyclotron-based, gantry-less proton therapy facilities.

We designed a beamline by using the principle of momentum cooling using a wedge (instead of a slit) in the energy selection system (ESS) to reduce the momentum spread (generated by the energy degrader) of the beam without introducing significant beam losses. This allows us to achieve ultra-high dose rates at the isocenter and reduce the length of the ESS (Maradia et al 2022, PTCOG). The beamline performance is calculated with the BDSIM Monte Carlo simulation code. As already shown (Schippers et al 2007), fast energy changes with a degrader and dipoles are possible within 50 ms. To reduce the dead time between spots, treatment plans with the beam characteristics of these compact beamlines have been generated using spot-reduction (van de Water et. al. 2020) and compare the plans to those delivered clinically.  Gantry-less beamlines can be used for treatment of patients in seated position.

The single-room design fits in a 136 m^2 area, roughly the size of a conventional LINAC bunker, while the 3-room design can fit onto tennis court (Fig. 1). BDSIM simulations predict maximum beam currents of 100 nA and 550 nA for 70 and 230 MeV beams respectively at isocenter (when we use 800 nA (250 MeV) from cyclotron), more than a factor 100 higher compared to conventional proton facilities. Additionally, we achieved a similar beam sizes as other commercially available systems and can reach a scanning area of 40*40 cm^2. The spot reduced plans calculated using these beam parameters are comparable to the clinically used treatment plans (Fig 2) but, through the high dose rates achieved in the design, would allow to deliver individual fields to even the large tumour shown in fig. 2 (~1.2 liters) within 5 sec. This can be compared to the 30-45 minutes (with motion mitigation techniques like gated rescanning) that would be required on conventional gantries, and would allow single field delivery well within a single breath-hold. 

The use of momentum cooling enabled us to design two ultra-compact and highly efficient proton therapy solutions: a) a single room solution, the of a conventional LINAC bunker, and b) a 3-treatment room solution that fits on a tennis court. In addition, due to the high beam transmission and gantry-less design, treatment delivery time can be drastically reduced compared to conventional proton designs.