Which linear accelerator (linac) can be modified for flash radiation therapy (RT)?

Linear Accelerators That Can Be Modified for FLASH Radiation Therapy

Both clinical linear accelerators (LINACs) from manufacturers like Varian and Elekta can be modified for FLASH radiation therapy delivery, with successful modifications demonstrated for electron FLASH without requiring hardware changes.

Types of LINACs Suitable for FLASH Modification

• Clinical LINACs can be adapted for ultra-high dose rate (UHDR) electron FLASH delivery by modifying the electron beam configuration without hardware changes, primarily through tuning the program board of the electron energy delivery system 1
• Varian LINACs have been successfully modified for FLASH by replacing the target and flattening filter with an electron scattering foil, allowing for UHDR electron beam delivery 2
• Elekta Precise LINACs have been adapted for FLASH through upgrades to the beam control system and beam tuning process, achieving average dose rates exceeding 160-200 Gy/s at isocenter distance 3
• The Novalis TX LINAC (BrainLab AG) has been demonstrated to achieve intensity-modulated radiation therapy in animal models, suggesting potential for FLASH applications 4

Technical Modifications Required

• For electron FLASH, modifications include:

  • Removal or replacement of scattering foils or targets in the gantry head 5
  • Tuning of the program board to enable UHDR electron delivery 1
  • Implementation of pulse delivery control using respiratory gating interfaces 1
  • Short source-to-surface distance (SSD) configurations to achieve higher dose rates 1

• For more advanced FLASH applications:

  • C-band systems with high accelerating gradients can be designed to reach 60-160 MeV energy range needed for VHEE (Very High Energy Electron) FLASH 6
  • Beam control systems that can interrupt based on monitor units, pulse counting, or preset delivery time enhance safety and precision 3

Dosimetry and Treatment Planning

• Monte Carlo beam models can be implemented in clinical treatment planning systems (such as Varian Eclipse) for FLASH-RT planning 5
• Gafchromic EBT-XD film has been validated for dosimetry at ultra-high dose rates up to 2×10^4 Gy/s 2
• Ionization chambers require dose rate correction models for accurate measurements at FLASH dose rates 2
• Treatment planning for FLASH can be integrated into conventional workflows with minimal deviation, facilitating clinical translation 5

Clinical Feasibility and Applications

• Short SSD setups with standard clinical LINACs can deliver flat, homogenous UHDR electrons over clinically relevant field sizes (3-20 cm) 1
• Modified clinical LINACs can achieve FLASH dose rates while maintaining the ability to be easily reverted to standard clinical use 1
• FLASH-RT has shown promising results in preclinical models with reduced toxicity while maintaining tumor control compared to conventional dose-rate RT 1, 4

Considerations and Limitations

• Energy spectrum and beam source emittance must be carefully modeled for accurate dose delivery 5
• Pulse-to-pulse stability is critical for precise dose delivery, with synchronization to the pulse forming network (PFN) charging cycle improving dosimetric precision 3
• For VHEE FLASH applications, specialized high-gradient accelerators may be required to reach energies of 60-160 MeV 6
• Beam tuning processes must be established to maximize output while ensuring delivery stability and reproducibility 3

Future Directions

• Ongoing development of compact C-band systems aims to achieve the high accelerating gradient and high current necessary for VHEE FLASH applications 6
• Integration of FLASH capabilities into treatment planning systems will facilitate clinical translation 5
• Further refinement of beam control systems will enhance safety and precision for clinical FLASH trials 3