Masking Enhances Accuracy of Bladder Deformation in Multi-Fraction Adaptive Brachytherapy as a First Step Toward Composite Dose Estimation
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BACKGROUND: GEC-ESTRO guidelines for cervix HDR brachytherapy advocate measurement of the minimum dose to the 2cc volume of organs at risk (OAR) receiving the highest amount of radiation and summation across multiple treatment fractions to give a worst-case-scenario cumulative dose estimate. If the OAR from different fractions could be accurately co-registered using deformation, then a more accurate composite dose could be obtained. OBJECTIVE: As a first step toward composite dose estimation, we sought to assess and improve the quality of bladder deformation using a technique called masking, which involves resetting the pixel values within a contour. METHODS: CT scans from nine cervical cancer patients with bladders contoured by radiation oncologists were obtained, and the urethra near the bladder was contoured using the catheter as a fixed reference point. Three copies of each CT were made: the first was unaltered, the second had the bladder masked, and the third had the bladder masked at one pixel value and the rest of the body masked at a different pixel value. Using VelocityAI 2.8.1, the bladder was deformed onto the target (Fraction 1) planning CT from subsequent planning CTs in an attempt to match up the tissue from different fractions. To assess the accuracy of the deformation, several indices were used: the percent error of the deformed bladder volume from the expected volume, the conformality index, the Hausdorff distance, and the distance between the centers of the deformed urethra and the target urethra. RESULTS: Deformation quality improved with masking. The standard deviation of the percent error was reduced from 18.1% with no masking to 4.3% with masking. Mean conformality increased from 0.83 with no masking, to 0.91 with the bladder masked, to 0.93 with body and bladder masked (P<0.001). The mean Hausdorff distance decreased from 13.8mm without masking, to 9.1mm with the bladder masked, to 5.7mm with body and bladder masked (P<0.001). The mean error in the urethra deformation increased from 4.3mm without masking, to 5.2mm with the bladder masked, to 6.6mm with body and bladder masked (P=0.08). CONCLUSION: The accuracy of bladder deformation can be significantly improved by masking. With masking, the volume and location of the deformed bladder more closely approached that of the target bladder than without masking. Thus masking has the potential to improve the accuracy of dose deformation and composite dose calculation in adaptive brachytherapy.