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Delivered Dose to Organs From CBCT-Based IGRT of the Prostate

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C Liu

C Liu*, A Kumarasiri, I Chetty, J Kim, Henry Ford Health System, Detroit, MI

SU-E-J-206 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To estimate the actual delivered dose to organs from CBCT-based image-guided radiation therapy for localized prostate cancer treatments.

Methods:
Seven localized prostate cancer treatments were retrospectively selected, each with ~40 CBCT images. All patients were treated with 9 IMRT beams. The PTV margin was 6 mm in the posterior direction, and 10 mm otherwise around the prostate. For each patient, the original plan was used to calculate treatment dose on each CBCT image at treatment position. The calculated daily dose was then warped back to the original planning CT via an intensity-based B-spline deformable image registration (DIR) algorithm. The DIR algorithm employed mutual information with a multi-resolution and multi-stage scheme. From the transferred total cumulative dose, dosimetric parameters were recorded for comparison with the original plan. The quality of prostate registration was quantified using three implanted fiducial markers. Only those cases with < 2mm error were included in the dose analysis.

Results:
The overall marker displacement was 1.8±1.8 mm after registrations. 75 of 281 registrations (27%) exceeded the 2 mm error threshold. The primary causes were the excessive noise level on CBCT images especially for large patients, presence of rectum gas, and lower abdominal motion artifacts. Excluding these cases, a total of 206 CBCT images were included in dosimetric analysis. Changes in dose coverage to the prostate was minimal (<1%). Dose deviations for rectum (Dmax, V50, V65, V75) were (0.5±1.0%, 2.6±2.5%, 3.5±3.7%, 2.3±4.2%) and (-1.1±0.9%, -2.5±5.3%, -3.0±5.3%, -3.8±4.9%) for bladder, respectively.

Conclusion:
With the conventional PTV margins, deviations in dose coverage of the prostate in the face of daily anatomic changes were minor However, relatively larger deviations were observed for rectum and bladder (1SD=~4.3%). Reduced margins may lower dose to critical organs while maintaining target coverage. However, margin reduction must be viewed cautiously and with consideration of clinical outcomes.

Funding Support, Disclosures, and Conflict of Interest: Please refer to the supporting document.

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