Setup accuracy for prone and supine whole breast irradiation

Area of Science:

• Radiation Oncology
• Medical Imaging
• Radiotherapy Physics

Background:

• Accurate patient setup is crucial for effective radiation therapy.
• Cone-beam computed tomography (CBCT) is increasingly used for image-guided radiotherapy.
• Determining appropriate margins is essential to ensure adequate dose coverage while minimizing toxicity.

Purpose of the Study:

• To evaluate the setup accuracy and determine necessary margins for prone and supine whole breast irradiation (WBI) using CBCT.
• To identify patient-specific factors influencing setup margins.

Main Methods:

• Analysis of 3559 CBCT scans from 242 patients undergoing WBI.
• Calculation of uncertainty margins using the van Herk formula.
• Uni- and multivariate analyses to assess the impact of age, BMI, and cup size on margins.

Main Results:

• Significantly larger margins were required in the lateral (LA) and longitudinal (LO) directions for prone WBI compared to supine WBI (p < 0.01).
• Population-based margins were 10.4/9.4/9.4 mm (vertical/lateral/longitudinal) for supine and 10.5/22.4/13.7 mm for prone.
• Higher BMI correlated with larger LO margins in supine and larger VE/LA margins in prone positions (p < 0.05).

Conclusions:

• Patient setup accuracy is significantly poorer in the prone position for WBI.
• Uncertainty margins of approximately 1 cm are necessary for supine WBI even with image guidance.
• Increased margins are recommended for patients with higher body mass index (BMI).