2826 - Assessing the Dosimetric Impact of Bolus Positioning Variability in Vulvar Cancer Radiotherapy

Purpose/Objective(s): Definitive radiotherapy for unresectable vulvar cancer frequently requires the use of a bolus material to ensure adequate target coverage. A virtual bolus is created during radiation planning, which is later replaced by a tissue equivalent bolus (real bolus) at the time of treatment. This study aims to evaluate the impact of the real bolus placement relative to the planned (virtual) bolus on treatment planning for dosimetric objectives and constraints in vulvar cancer patients receiving definitive radiotherapy.

Materials/Methods: Definitive vulvar cancer radiotherapy plans (2018–2024) were retrospectively reviewed. Five weekly cone beam CT (CBCT) scans were fused with the planning CT to assess bolus placement. The bolus, as visualized on CBCT, was contoured, transferred to the planning CT, and assigned an appropriate density prior to recomputing the plan. Generated plans were evaluated based on target coverage for gross target volume (GTV), clinical target volume (CTV) and planning target volume (PTV) as well as dose to organs at risk (OARs). Body mass index (BMI), age, treatment position (frog-leg vs straight-leg), and GTV volume were analyzed for their influence on bolus placement. For target coverage, the volume receiving 95% of prescription (V95) was evaluated. Paired differences between the original plan and treatment fractions were analyzed using the Wilcoxon signed-rank test, while mixed-effects models assessed target coverage variations across fractions. Spearman’s rank test was used to evaluate correlations.

Results: In 25 patients, 125 plans were recomputed using the real bolus and compared to the original plan. Target coverage was lower compared to the virtual bolus, with a mean coverage decrease by approximately 2% (V95 range: 84.0-100.0%) for GTV and 4% for both CTV (V95 range: 85.0-99.7%) and PTV (V95 range: 88.1-99.1%; p<0.001), while doses to bladder (V45 <35%) and rectum (V45 <60%) remained stable. In the first 3 weeks of treatment, a larger baseline GTV in cm³ was associated with a further decrease in GTV coverage (p = 0.005). GTV and CTV coverage were less affected when patients were positioned in the straight-leg position rather than the frog-leg position (r=0.24, 0.54, respectively).

Conclusion: Bolus positioning variations negatively impacted target coverage but not OAR constraints. Planning should account for these variations, and daily CBCT verification is recommended. Further research is needed to determine if under coverage impacts clinical outcomes.