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Using Daily Images to Verify Dosimetric Advantage of Dual-Isocentric Technique for High-Risk Prostate Cancer Treated with Pelvic Lymph Node

T Cui

T Cui*, Q Shang , W Li , A Vassil , P Xia , The Cleveland Clinic Foundation, Cleveland, OH


SU-F-T-534 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To evaluate a dual-isocentric technique to compensate for the daily independent movements of prostate and pelvic lymph nodes (PLN) of high-risk prostate cancer patients.

Methods:Three patients, who received 78 Gy to the prostate, 66 Gy to the seminal vesicle (SV), and 50.4 Gy to the PLN in 39 fractions under CT-on-rail (CTOR) imaging guidance, were retrospectively selected. Each patient was planned with volumetric modulated arc therapy (VMAT) techniques using (1) single isocenter with a 6 mm margin except 4 mm posteriorly (6/4 mm), to the prostate-CTV and a uniform 5 mm margin to the PLN-CTV, and (2) two isocenters, located at the centroids of the prostate or the PLN, with a 5/3 mm margin to the prostate-CTV and a 5 mm uniform margin to the PLN-CTV. Both plans met the established clinical guidelines, and were subsequently applied to the daily CTOR images, where prostate, SV, rectum and bladder were contoured by the same physician. On each of 117 CTOR images, two plans were created by registering only the bony anatomy for the single-isocentric one, or by registering both the bony anatomy and the prostate for the dual-isocentric one. These plans were then compared based on V78Gy of prostate-CTV, V66Gy of SV-CTV, and V50.4Gy of PLN-CTV. The paired Student’s t-test was performed to test for significance.

Results:The V78Gy of prostate-CTV in the dual-isocentric plans from 117 CTOR images was 99.32±0.89%, significantly (p<0.01) better than that in the single-isocentric ones (98.29±2.11%). Both V66Gy of SV-CTV and V50.4Gy of PLN-CTV were increased from 99.15±3.28% and 99.38±0.74% in the dual-isocentric plans to 99.31±2.02% and 99.48±0.90% in the single-isocentric ones, respectively, but neither were statistically significant (p=0.47 and 0.34).

Conclusion:By targeting prostate and PLN accordingly in the daily verification images, the dosimetric advantage of dual-isocentric technique is verified even with a reduced margin.

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