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

BACKGROUND: Radiotherapy for vulvar carcinoma often employs the "frog-leg" position to minimize skin toxicity, but this position is biomechanically unstable and prone to setup errors using conventional skin markings. PURPOSE: To evaluate the clinical efficacy of Surface-Guided Radiotherapy (SGRT) for setup accuracy in vulvar cancer patients treated in the frog-leg position and to quantify the biomechanical correlation between Body Mass Index (BMI) and setup errors. METHODS: A retrospective cohort study of 20 vulvar cancer patients treated in the frog-leg position was conducted. All patients were treated using a uRT-linac 506c accelerator with integrated helical Fan-Beam CT (FBCT). Daily image guidance was performed using a two-step registration protocol (bony anatomy followed by soft-tissue fine-tuning). Setup errors from 500 FBCT datasets were compared between the conventional laser group (n = 10) and SGRT group (n = 10). The correlation between BMI and setup errors was analyzed using Spearman's rank correlation. RESULTS: The SGRT group demonstrated significantly superior setup accuracy across multiple key dimensions (p < 0.001). Notably, the Pitch error was reduced from 1.21° ± 0.48° in the Conventional Group to 0.07° ± 0.32° in the SGRT Group (Cohen's d = 2.79). Longitudinal (LNG) error decreased from 0.49 ± 0.26 cm to 0.04 ± 0.24 cm (d = 1.82), and vertical (VRT) errors were similarly reduced. BMI CORRELATION: In the Conventional Group, BMI showed a strong significant correlation with Pitch and Yaw errors (ρ = -0.81, P = 0.005), indicating that obesity may exacerbate pelvic rotational instability. Conversely, in the SGRT Group, no statistically significant correlation was observed between BMI and setup errors in any dimension (p > 0.05), demonstrating the system's robustness against body habitus variations. CONCLUSION: The frog-leg position is characterized by significant biomechanical instability due to distinct anatomical factors. Traditional skin marking methods are susceptible to the "skin traction effect" and interference from BMI. SGRT, through real-time topographical matching, effectively corrects pelvic tilt and longitudinal sliding. Crucially, it eliminates the setup uncertainty associated with BMI, providing a solid evidence-based foundation for precision radiotherapy and individualized PTV margin reduction.