Setup Error Differences and Margin Expansion Among Rectal Cancer Segments: Consistency Evaluation Between EPID and...

Area of Science:

• Radiation Oncology
• Medical Physics
• Cancer Imaging

Background:

• Accurate patient setup is crucial for effective radiotherapy, especially for rectal cancer.
• Image-guided radiotherapy (IGRT) techniques minimize setup errors.
• Comparing different IGRT methods like electronic portal imaging device (EPID) and iSCOUT is essential for optimizing treatment accuracy.

Purpose of the Study:

• To investigate setup errors in upper, middle, and lower rectal cancer patients.
• To evaluate the consistency between EPID and iSCOUT image-guided techniques.
• To determine optimal planning target volume (PTV) margins based on rectal cancer location.

Main Methods:

• Retrospective analysis of 277 rectal cancer patients treated with radiotherapy.
• Categorization into upper, middle, and lower rectal cancer groups.
• Calculation and comparison of setup errors and PTV margins between EPID and iSCOUT.

Main Results:

• Significant differences in setup errors were observed in the anterior-posterior (Z) direction between upper and lower rectal cancers (p=0.013).
• Discrepancies existed between EPID and iSCOUT in the left-right (X) and superior-inferior (Y) directions.
• EPID and iSCOUT measurements showed strong correlation (p<0.001) with 95% consistency limits within millimeters.

Conclusions:

• Different margin expansions in the Y direction are recommended based on rectal cancer segment.
• iSCOUT demonstrates reliable consistency with EPID and can be used as an alternative.
• Optimizing PTV margins improves treatment precision for rectal cancer patients.