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Evaluation of the Inter-Fractional Treatment Delivery Accuracy of Prostate IMRT/VMAT Plans Using Organ Gamma Indices and Dosimetric Constraints
E Pryser1*, L Rankine2 , J Hammers3 , J Matney4 , R Chen5 , S Das6 , P Mavroidis7 , (1) University of North Carolina at Chapel Hill, Chapel Hill, NC, (2) University of North Carolina at Chapel Hill, Chapel Hill, NC, (3) University of North Carolina, Chapel Hill, NC, (4) UNC Health Care, Chapel Hill, NC, (5) University of North Carolina, Chapel Hill, NC, (6) University of North Carolina, Chapel Hill, NC, (7) University of North Carolina, Chapel Hill, NC
Presentations
SU-H2-GePD-J(B)-3 (Sunday, July 30, 2017) 3:30 PM - 4:00 PM Room: Joint Imaging-Therapy ePoster Lounge - B
Purpose: To estimate the inter-fractional accuracy in treatment delivery of prostate cancer patients using IMRT regarding the doses to bladder and rectum. To evaluate the correlation of the accumulated gamma indices of bladder and rectum against their normal tissue complication probability (NTCP) values.
Methods: Ten prostate cancer patients were treated with VMAT/IMRT plans and daily CBCT scans were acquired. The planning dose was rigidly transferred to the planning CBCTs, which were subsequently deformably registered to the planning CT. Using this registration, the dose from each CBCT was also deformed. The planned and delivered doses and the contours of bladder and rectum were transferred to 3DVH software for the 3-D gamma analysis. The respective accumulated dose volume histograms (DVHs) were obtained and were used to calculate the NTCP for bladder and rectum. In this study, accumulated doses to the bladder and rectum were evaluated using gamma analysis criteria of 3%/3mm and 2%/2mm. For the calculation of the NTCP values, the Relative Seriality model was used.
Results: Based on the findings of a single patient, the gamma analysis between the planned bladder dose distribution and the CBCT-reconstructed dose yielded gamma passing rates of 41.4±3.3% and 53.2±4.1% using the 2%/2mm and 3%/3mm criteria, respectively. The respective values for rectum were 62.1±8.2% and 82.2±7.8%. The bladder NTCP from the CBCT-reconstructed dose was 10.2±1.0% (higher than that from the plan 7.0%), whereas for rectum it was 1.9±0.3% (lower than that of the plan 4.3%). Simple linear regression analysis between the gamma and NTCP values showed high correlation mainly for bladder.
Conclusion: The dosimetric differences appear to have a measurable impact on the gamma values of both organs. The low gamma values of bladder and rectum were found to correlate with the deviation of their delivered NTCP values from the planned ones.
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