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Diagnostic Quality CT with Soft Tissue Alignment for IGRT Can Safely Reduce Planning Margins For Prostate Cancer: Implication For SBRT

W Li

W Li*, A Vassil, L Muhieddine Mossolly, Q Shang, P Xia, The Cleveland Clinic Foundation, Cleveland, OH

WE-G-141-3 Wednesday 4:30PM - 6:00PM Room: 141

To evaluate whether the use of daily diagnostic quality CT for image-guided radiation therapy (IGRT) can safely reduce the planning margin using soft tissue alignment for prostate cancer treatment, which can be applied to SBRT.

Nine patients with prostate cancer, who underwent IMRT and daily IGRT using the CT-on-rails system, were selected for this study. For all patients, a planning margin of 8mm/5mm posterior was used in the clinically approved plans. For this study, three additional plans were generated with planning margins of 6mm/4mm posterior, 4mm/2mm posterior, and 2mm uniform, resulting a total of four plans for each patient. On the first seven and last seven daily CTs, the prostate, bladder, and rectum were delineated by the same radiation oncologist. Subsequently, for each plan, the corresponding dose distribution on these daily CTs was calculated. The endpoint dose of the prostate was defined as the percentage of the volume receiving prescription dose of 78Gy (V78). V65 and V70 were used for the bladder and rectum, respectively.

For 8.7% of the first seven and 13.5% of the last seven fractions (n=126), the daily prostate volumes were 10% higher than those at simulation. Correspondingly, to achieve V78> 95%, the minimum margins of 4mm/2mm posterior and 6mm/4mm posterior were required for the first seven and last seven fractions, respectively. With margin reduced from 8mm/5mm posterior to 6mm/4mm posterior and 4mm/2mm posterior, the average bladder V65 decreased from 17.0±10.6% to 15.2±10.9% (p<0.01) and 9.9±7.4% (p<0.01), respectively. The average rectum V70 decreased from 15.2±8.9% to 13.6±7.9% (p<0.01) and 10.5±6.5% (p<0.01), respectively.

A planning margin of 4mm/2mm posterior is sufficient for prostate SBRT with daily IGRT using soft tissue alignment with diagnostic quality CT. Drastic change in daily prostate volumes may increase the planning margin to 6mm/4mm posterior, requiring daily offline dose monitoring.

Funding Support, Disclosures, and Conflict of Interest: Siemens Medical Solution

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