Online Adaptive MRI-Guided Radiotherapy for Primary Tumor and Lymph Node Boosting in Rectal Cancer.

Area of Science:

• Radiation Oncology
• Medical Imaging
• Cancer Treatment

Background:

• Online adaptive radiotherapy (ART) is crucial for precise cancer treatment.
• Accurate definition of planning target volume (PTV) margins is essential for dose escalation.
• Mesorectal lymph node motion impacts treatment accuracy in rectal cancer.

Purpose of the Study:

• To characterize motion and define treatment margins for pathological mesorectal lymph nodes (GTV_ln) using two online adaptive MRI-guided strategies.
• To determine margins for the primary gross tumor volume (GTV_prim).
• To compare adapt-to-shape (ATS) and adapt-to-position (ATP) workflows for rectal cancer radiotherapy.

Main Methods:

• Twenty-eight patients treated on a 1.5T MR-Linac were analyzed.
• Gross tumor volumes (GTV_ln and GTV_prim) were delineated on T2-weighted MRI scans before and after irradiation.
• Intrafraction and interfraction motion were assessed, and PTV margins were calculated using the Van Herk recipe for ATS and ATP workflows.

Main Results:

• The adapt-to-shape (ATS) workflow resulted in smaller margins for GTV_ln (3-6 mm) compared to adapt-to-position (ATP) (10-19 mm).
• Margins for GTV_prim with ATS were 1.7-5.6 mm.
• Daily delineation with ATS for both target volumes yielded the smallest margins.

Conclusions:

• Daily delineation using the ATS workflow is recommended for safe dose escalation to the primary tumor and lymph nodes in rectal cancer.
• ATS effectively corrects interfraction motion, leaving only intrafraction motion as the primary concern for margin definition.
• The findings support the use of ATS in MRI-guided radiotherapy for optimizing treatment margins and enabling dose escalation.