Adaptive radiotherapy: The Elekta Unity MR-linac concept

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• The MR-linac enables visualization of anatomical changes during radiotherapy for plan adaptation.
• A trade-off exists between adaptation time and dosimetric gain in adaptive radiotherapy.
• State-of-the-art 1.5 T MRI-guided online adaptive radiotherapy methods require evaluation.

Purpose of the Study:

• To present and apply various daily plan adaptation methods for MR-linac radiotherapy.
• To gain insight into the behavior of 1.5 T MRI-guided online adaptive radiotherapy.
• To evaluate the feasibility and robustness of different adaptive planning strategies across various tumor sites.

Main Methods:

• Simulated online plan adaptation for five diverse cases: prostate, rectum, esophagus, and lymph node oligometastases (single and multiple targets).
• Evaluated plans based on clinical dose constraints.
• Measured the time required for plan adaptation and optimization.

Main Results:

• Plan adaptation time varied significantly, ranging from 17 to 485 seconds.
• More advanced adaptation methods generally yielded more plans meeting clinical dose criteria.
• Full online replanning successfully created plans meeting all clinical dose constraints for all tested cases.

Conclusions:

• Daily full online replanning is the most robust adaptive planning method for the Unity MR-linac.
• This method is feasible for specific tumor sites within clinically acceptable timeframes.
• Faster adaptive methods necessitate thorough dosimetric exploration for specific use cases before implementation.