Treatment plan adaptation for MRI-guided radiotherapy using solely MRI data: a CT-based simulation study.

Area of Science:

• Medical Physics
• Radiation Oncology
• Medical Imaging

Background:

• Integrated MRI-accelerator systems offer real-time imaging during radiation therapy.
• Adapting treatment plans requires accurate patient anatomy information.
• MRI data lacks electron density information crucial for accurate dose calculation.

Purpose of the Study:

• To assess the feasibility of adapting prostate cancer treatment plans using only MRI data.
• To evaluate the impact of missing density information in MRI on treatment adaptation.
• To determine the importance of physical path length versus tissue density for MRI-based adaptation.

Main Methods:

• Treatment plans for five prostate cancer patients were evaluated.
• Simulated prostate shifts (3-10 mm) were analyzed in multiple directions.
• Dose distributions were recalculated with and without CT-derived density information, mimicking MRI data.
• Monitor unit (MU) rescaling was performed using physical path lengths from MRI data.

Main Results:

• Physical path lengths derived from MRI are more critical than tissue density for treatment plan adaptation in prostate cancer.
• MU rescaling based on the central beam axis physical path length accurately represents the entire beam.
• Geometrically accurate MRI data is sufficient for effective treatment plan adaptation.

Conclusions:

• MRI data, when geometrically accurate, can be utilized for adaptive radiation therapy in prostate cancer.
• Physical path length information in MRI is a key factor for successful treatment adaptation.
• This approach minimizes the need for CT data in adaptive MRI-guided radiotherapy.