Surface-Guided Radiotherapy for Vulvar Cancer in the Frog-Leg Position: Setup Accuracy and BMI Effects

Area of Science:

• Radiation Oncology
• Medical Physics
• Gynecologic Oncology

Background:

• The frog-leg position is standard for vulvar carcinoma radiotherapy to reduce skin toxicity.
• This position presents biomechanical instability and setup inaccuracies with conventional skin markings.

Purpose of the Study:

• To assess Surface-Guided Radiotherapy (SGRT) efficacy for setup accuracy in vulvar cancer patients using the frog-leg position.
• To investigate the correlation between Body Mass Index (BMI) and setup errors in this patient group.

Main Methods:

• A retrospective study of 20 vulvar cancer patients treated in the frog-leg position.
• Comparison of setup errors between conventional laser marking and SGRT groups using daily helical Fan-Beam CT (FBCT).
• Spearman's rank correlation analysis was used to assess the relationship between BMI and setup errors.

Main Results:

• SGRT group showed significantly improved setup accuracy (p < 0.001) compared to the conventional group.
• Pitch error reduced from 1.21° to 0.07°, and longitudinal error decreased from 0.49 cm to 0.04 cm with SGRT.
• SGRT eliminated the significant correlation between BMI and setup errors observed in the conventional group.

Conclusions:

• The frog-leg position's inherent instability and skin traction effects challenge conventional radiotherapy setups.
• SGRT effectively corrects pelvic tilt and sliding, mitigating setup uncertainties related to BMI.
• SGRT provides a robust foundation for precise radiotherapy and personalized planning target volume (PTV) margin reduction.