Assessing Axis-specific Set-up Errors in Rectal Cancer Radiotherapy: A Prospective Cone-beam Computed...

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Planning

Background:

• Image-guided radiotherapy (IGRT) is crucial for precise tumor targeting.
• Accurate patient positioning is essential to minimize radiation dose to healthy tissues.
• Understanding set-up errors is key to optimizing treatment margins.

Purpose of the Study:

• To quantify set-up errors in rectal cancer patients undergoing radiotherapy.
• To derive optimal clinical target volume (CTV) to planning target volume (PTV) margins.
• To evaluate the influence of body mass index (BMI) on set-up accuracy.

Main Methods:

• Analysis of 1102 daily cone-beam computed tomography (CBCT) scans from 41 rectal cancer patients.
• Measurement of mediolateral (X), craniocaudal (Y), and antero-posterior (Z) translational shifts.
• Calculation of systematic (Σ) and random (σ) errors using the van Herk formula for PTV margin derivation.

Main Results:

• Low mean systematic set-up errors observed across all axes (X: 0.04 mm, Y: 0.57 mm, Z: 0.13 mm).
• Derived PTV margins were 7.4 mm (X), 9.1 mm (Y), and 9.7 mm (Z).
• A significant positive correlation between BMI and Z-axis set-up error (r=0.55, P=0.002) was found, with 23.1% of fractions needing >5 mm corrections.

Conclusions:

• Axis-specific PTV margins are necessary for rectal cancer radiotherapy due to anatomical variability.
• Daily CBCT significantly improves set-up precision and treatment accuracy.
• Individualized planning considering factors like BMI is recommended for optimal outcomes.