Understanding the Benefit of Magnetic Resonance-guided Adaptive Radiotherapy in Rectal Cancer Patients: a...

Area of Science:

• Radiation Oncology
• Medical Physics
• Gastrointestinal Oncology

Background:

• Neoadjuvant chemoradiotherapy followed by surgery is standard for rectal cancer.
• Current treatment planning uses 10 mm margins to account for motion.
• Magnetic resonance-integrated linear accelerators (MR-linacs) enable adaptive radiotherapy (MRgART).

Purpose of the Study:

• To investigate the motion of the clinical target volume (CTVA) in rectal cancer.
• To determine if reduced planning target volume (PTV) margins are feasible with MRgART.
• To develop a simultaneous integrated boost protocol for MR-linac treatment.

Main Methods:

• Nine patients with rectal cancer received MR-linac treatment.
• Intrafraction motion of CTVA was measured using pre- and post-treatment MRI scans.
• PTV margins (1-10 mm) were evaluated for coverage of CTVA using MRgART and compared to CT-based planning.

Main Results:

• Median intrafraction motion was 0 mm, indicating minimal movement.
• PTV margins of 3 mm and 5 mm were acceptable in 96% and 98% of fractions, respectively.
• CT-based planning with 5 mm and 10 mm margins covered only 51% and 76% of fractions, respectively.

Conclusions:

• PTV margins can be reduced to 3-5 mm with MRgART on MR-linacs for rectal cancer.
• This reduction is feasible within a simultaneous integrated boost protocol.
• MRgART allows for more precise targeting and potentially reduced toxicity.