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Investigating Dose Accumulation During VMAT Treatment for Prostate Cancer Using CBCT-CT Deformable Image Registration

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J Hammers

J Hammers1*, S Pirozzi2 , J Matney1 , O Kaidar-Person1 , R Chen1 , A Wang1 , S Das1 , P Mavroidis1 , (1) University of North Carolina, Chapel Hill, NC, (2) MIM Software Inc., Cleveland, OH


WE-RAM1-GePD-J(B)-6 (Wednesday, August 2, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Lounge - B

Purpose: This study aims to utilize daily Cone Beam Computed Tomography (CBCT) images and deformable registration to examine the effects of anatomical deformations on the accumulated dose in bladder and rectum over the course of VMAT treatments for prostate cancer.

Methods: This study includes 10 patients, who were treated with VMAT to 66.6Gy in 37 fractions. A physician segmented bladder and rectum on the daily CBCTs in approximately weekly intervals. Those contours were used to enhance the contrast in the corresponding regions in both the planning CT (pCT) and CBCTs. A MIM Software workflow was developed to estimate the daily delivered dose by translating the dose distribution from the treatment plan based on the daily shifts. The accumulated dose distribution was calculated by using the pCT as a reference by deforming each CBCT to the pCT and applying that deformation matrix to the daily delivered dose matrix. Accumulated dose volume histograms (DVHs) were computed for bladder and rectum at approximately weekly intervals. The accumulated DVHs were compared against the plan dose DVH scaled to the appropriate number of fractions.

Results: Based on the findings of a single patient, considerable variation in the shape and size of bladder affected the daily delivered dose against the treatment plan, while those effects were lesser for rectum. The mean projected dose accumulated delivered dose was higher than the plan by 6.5±1.2Gy. Respectively, for the rectum, mean accumulated delivered dose was lower by 2.2±0.8Gy. In all the fractions, the projected accumulated dose exceeded to clinical constraints for bladder but not for rectum.

Conclusion: Although the general sense is that fractional dosimetric deviations in treatment delivery cancel out during the course of the treatment, it is shown here that considerable shape and size deformations may lead to considerable deviations between the total planned and delivered doses.

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