Prostate Cbct Dose Optimization : From An Iterative Mas Reduction To A Sytematic Exposure Reduction

Introduction A daily repositioning Cone Beam Computed Tomography image (CBCT) for prostate radiotherapy is realized using exposure templates (mAs, kV) which affect image quality and imaging dose. Settings should be optimized to minimize patient exposure while maintaining sufficient image quality to register the initial planning CT with CBCT using soft tissue matching. Methods 20 prostate patients (without hip prosthesis) with daily CBCT (40 fractions) acquired on a TrueBeam™ (Varian Medical Systems) machine were selected. After the first fraction using the standard pelvis template (125 kV 1080 mAs CTDIw 14 mGy), the therapists manually applied, day after day, a low mAs reduction and assessed if the CBCT image quality was good enough for patient repositioning. The iterative process stopped when image quality was assessed too bad and the last proper mAs were selected. The link between the mAs reduction and corpulence (patient volume inside CBCT FOV) was studied. For one example patient 23 therapists registered CBCT images with CT for 3 fractions: the first fraction (S 0% ), a fraction with 50% mAs reduction (S −50% ) and the fraction with maximum mAs reduction (S −71% ). Fisher’s test was applied to every direction, to compare the variance between S 0% /S −50% and S 0% /S −71% . Results The median mAs reduction for all patients was 64% (13–85%). Patient corpulence was not correlated to the mAs reduction achieved (Spearman’s correlation r s  = 0.465). Variance analysis, for every direction, shows no significant difference ( p 0% /S −50% and S 0% /S −71% . Conclusion mAs reductions recorded across the 20 patients are highly variable, due to the subjective assessment of CBCT image quality, but a median reduction of 64% indicates a great potential for reducing imaging dose. For one patient it has been demonstrated that image quality deterioration has no impact on interobserver variability. A 50% mAs reduction for the Pelvis CBCT template is therefore considered.