SU-E-J-11: Characterization of Interfractioanal Anatomical Variations in Post-Operative Radiation Therapy for...

Area of Science:

• Radiation Oncology
• Medical Imaging
• Anatomy

Background:

• Post-operative radiation therapy (RT) for prostate cancer aims to eradicate residual disease.
• Accurate tumor targeting is crucial, but interfractional anatomic variations can compromise treatment efficacy.
• Understanding these variations is essential for developing improved treatment strategies.

Purpose of the Study:

• To quantitatively assess interfractional anatomic variations in the prostate bed after surgery.
• To inform the development of strategies that effectively manage these observed variations during RT.

Main Methods:

• 102 daily pre-treatment CT scans from 10 patients undergoing post-operative intensity-modulated radiation therapy (IMRT) for the prostate bed were analyzed.
• Deformable registration was used to contour the planning target volume (PTV), rectum, and bladder on daily CT scans.
• Interfractional variations in volume, shape, and position (using displacement of the center of mass and maximum overlap rate) were quantified.

Main Results:

• Significant interfractional volume variations were observed for the rectum (average 116±41%) and bladder (average 67±26%).
• Substantial interfractional positional shifts were recorded for the rectum (e.g., -3.41±1.14cm vertically) and bladder (e.g., 3.63±1.10cm longitudinally).
• These variations exceeded the capabilities of standard image-guided radiation therapy (IGRT) repositioning.

Conclusions:

• Post-operative RT for the prostate bed is characterized by significant interfractional changes in organ volume, shape, and position.
• Current standard IGRT repositioning methods are inadequate to fully compensate for these observed anatomic variations.
• Further research and strategy development are needed to optimize treatment delivery and improve outcomes.