Per-Fraction Positional and Dosimetric Performance of Prone Breast Tangential Radiotherapy Assessed with Daily Rapid Kilo-voltage Cone Beam Computed Tomography: Key Statistics, Predictors, and Recommendations

Background This study investigates daily breast geometry and delivered dose to prone-positioned patients undergoing tangential whole breast radiation therapy (WBRT) on an O-ring linear accelerator with 6X flattening filter free mode (6X-FFF), planned with electronic compensation (ECOMP) method. Most practices rely on skin marks or daily planar image matching for prone breast WBRT. This system provides low dose daily CBCT, which was used to study daily robustness of delivered dose parameters for prone-positioned WBRT. Methods Eight patients treated with 16-fraction prone-breast WBRT were retrospectively studied. Planning CTs were deformed to daily CBCT to generate daily synthetic CTs, on which delivered dose distributions were calculated. A total of 8 × 16 = 128 synthetic CTs were generated. Consensus ASTRO definition was used to contour Breast PTV Eval for each daily deformed CT. Breast PTV Eval coverage (V90%) and hotspot (V105% and Dmax) were monitored daily to compare prescription dose with daily delivered dose. Various predictors including patient weight, breast width diameter (BWD), and Dice similarity coefficient (DSC) were fit into an analysis of covariance model predicting V90% and V105% deviation from prescribed (ΔV90%, ΔV105%). Statistical significance is indicated with asterisks (* for p < 0.05; ** for p < 0.001). Results Daily delivered Breast PTV Eval V90% was moderately smaller than prescribed (median ΔV90%=-0.1%*), while V105% was much larger (median ΔV105%=+10.1%** or + 92.4cc**). Patient’s weight loss correlated with significantly increased ΔV105% (+ 4.6% / -1% weight., R2 = 0.4**) and moderately decreased ΔV90% (-0.071%/-1% wt., R2 = 0.2**). Comprehensive ANCOVA models indicated three factors affect ΔV90% and ΔV105% the most: (1) BWD decrease (-0.09%* and + 10%**/-1cm respectively), (2) PTV Eval volume decrease (-0.4%** and + 9%**/-100 cc), and for ΔV105% only, (3) the extent of breast deformation (+ 10%**/-0.01 DSC). Breast PTV Eval volume also decreased with time (-2.21*cc/fx), possibly indicating seroma resolution and increase in V105% over time. Conclusions Daily CBCT revealed key delivered dose parameters vary significantly for patients undergoing tangential prone breast WBRT planned with ECOMP using 6X-FFF. Patient weight, BWD, and breast shape deformation could be used to predict dosimetric variations from prescribed. Preliminary findings suggest an adaptive plan based on daily CBCT could reduce excessive dose to the breast.