Main Session
Oct 01
QP 28 - Radiation and Cancer Physics 14: Image-guided Planning, Novel Delivery Techniques and QA

1166 - Pioneering Proton FLASH-Minibeam Therapy: First Experimental Demonstration of Feasibility on a Clinical Proton System

12:25pm - 12:30pm PT
Room 160

Presenter(s)

Yuting Lin, PhD - Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS

Y. Lin1, A. Wang1, W. Wu1, J. Setianegara1, N. Gerard2, J. Nys2, R. Labarbe2, G. N. Gan1, R. C. Chen1, and H. Gao1; 1Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS, 2Ion Beam Applications, Louvain-la-Neuve, Belgium

Purpose/Objective(s): Proton minibeam radiation therapy (pMBRT) employs spatial dose modulation at submillimeter resolution, for reducing normal tissue toxicity and enhancing tumor control. However, the high monitor-unit (MU) requirements of multi-slit collimators (MSC) may result in extended delivery time, posing a significant challenge. To overcome this limitation, this study, for the first time, explores the feasibility of integrating pMBRT with FLASH, on a widely available clinical proton therapy system. In addition, FLASH-pMBRT may synergize potential biological benefits from pMBRT and FLASH to further reduce normal tissue toxicity.

Materials/Methods: The study utilized a compact proton therapy machine equipped with two MSCs (center-to-center distances: 2.8 mm; slit widths: 1.0 mm; thicknesses: 6.5 cm and 10 cm). UHDR delivery was achieved with 228 MeV protons at 125 nA, compared to clinical beams operating at 226 MeV with 1–5 nA. Depth-dose and lateral-dose profiles were measured using radiographic films in solid water phantoms, and delivery times were compared between ultra-high-dose-rate (UHDR) FLASH beam and clinical beam.

Results: The study successfully demonstrated the feasibility pMBRT under UHDR, significantly reducing treatment times to 2.5 seconds compared to 3 minutes using clinical beams. The 10 cm collimator achieved higher peak-to-valley dose ratio (PVDR) at 2 cm depth (4.36) than the 6.5 cm collimator (2.57).

The irradiation field consisted of a square field size of 3x3 cm2, with 0.5 cm spot spacing. The average dose rate of open field was 50 Gy/s for UHDR beam, and approximately 0.7 Gy/s for clinical beam. Due to the presence of MSC, a large portion of the proton beam was blocked, resulting in a reduced dose rate, e.g., the resulting peak dose rate (PDR) reduced to 12.21 Gy/s at 2 cm depth in the presence of 6.5 cm thick MSC.

The pMBRT delivered with UHDR maintained comparable physical properties to conventional clinical beams while achieving delivery times 66 times faster. The 10 cm collimator demonstrated superior PVDR especially at shallow depths, highlighting its potential for enhancing therapeutic effectiveness and reducing normal tissue toxicity.

Conclusion: A first-of-its-kind experimental demonstration is achieved on the feasibility of proton FLASH-minibeam RT using a clinical proton therapy system.

Abstract 1166 - Table 1

 Collimator Thickness

Depth 2 cm

 Depth 4 cm

 Depth 6 cm

 Delivery Time

Conventional 226 MeV

 6.5 cm

 PVDR

 2.66 ± 0.07

 2.8 ± 0.15

 2.16 ± 0.13

 166 seconds
PDR

 0.2 Gy/s

 0.1 Gy/s

 0.1 Gy/s
10 cm

 PVDR

 4.64 ± 0.21

 3.81 ± 0.15

 2.40 ± 0.06
PDR

 0.1 Gy/s

 0.1 Gy/s

 0.1 Gy/s

FLASH 228MeV

 6.5 cm

 PVDR

 2.57 ± 0.15

 2.64 ± 0.13

 2.20 ± 0.05

 2.5 seconds
PDR

 12.2 Gy/s

 9.9 Gy/s

 7.9 Gy/s
10 cm

 PVDR

 4.37 ± 0.15

 3.40 ± 0.12

 2.55 ± 0.02
PDR

 6.8 Gy/s

 5.8 Gy/s

 4.7 Gy/s