Speaker
Description
FLASH radiotherapy is a rapidly growing topic, with most evidence at the cell/animal level. It may spare normal tissue while maintaining tumor control comparable to conventional RT, but clinical progress remains slow. A major bottleneck is the lack of suitable accelerators/radiators. Protons from cyclotrons offer ultra-high dose rate (UHDR) and penetration but not rapid energy modulation; carbon synchrotrons lack UHDR; electron LINACs provide UHDR but limited penetration; X-ray LINACs offer penetration and need no energy modulation but currently lack UHDR. For protons/heavy ions, magnet tuning/switching takes 50–100 ms, still insufficient and lowering average dose rate in some tissues. Electrons can reach kGy/s–MGy/s, but conventional LINACs are 5–20 MeV; VHEE ≥100 MeV requires large, expensive facilities, though compact X-band/S-band designs are emerging. Laser plasma wakefield accelerators are compact but low dose rate.
X-rays were selected for narrow penumbra (10–90%: 6 mm), large fields, conformality, easier dosimetry/shielding, and compact economic potential (6–10 MeV, <1 m). Challenges include low dose rate (0.1–0.2 Gy/s vs FLASH threshold ≥40 Gy/s) and multi-angle irradiation. UHDR X-ray animal experiments use short SSD and single angle; clinical use needs large SAD and multiple angles. Gantry rotation within FLASH time is nearly impossible, so static multiple irradiators are proposed: one accelerator with multiple beamlines or multiple compact accelerators. Reported approaches include LLNL’s induction accelerator with four parallel beams, UCLA/RadiaBeam fast-rotating VMAT-FLASH, and CAEP’s superconducting accelerator, but 15 Gy/s at SAD=50 cm remains insufficient.
Tsinghua developed a compact S-band BTW linac for UHDR X-ray. A standing-wave S-band linac achieved 10 MeV, 7.5 MW peak power, 431 mA measured pulse current, 1% duty cycle, and 72 Gy/s at SAD=70 cm. A clinical concept uses five linacs with carefully chosen angles, suitable for IMRT and rotatable across fractions. A 2025 prototype is 3 m diameter and 2.5 m long, fitting conventional bunkers, with 45 Gy/s at SAD=80 cm and planned upgrade to 100 Gy/s. Compactness and cost-effectiveness remain key advantages of X-ray FLASH-RT.