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Online CBCT Guided Prostate Cancer Radiotherapy: Is 3 Mm Target Margin Adequate?

A Qin

A Qin1*, Y Sun2, J Liang3, N Tyagi4, D Yan5, (1),(3), (5) William Beaumont Hospital, Royal Oak, MI, (2) Sun Yat-Sen University Cancer Center, Guangzhou, Guangdong,(4) Memorial Sloan-Kettering Cancer Center, New York, NY

SU-C-103-3 Sunday 1:00PM - 1:55PM Room: 103

Purpose: Evaluation of treatment dose for daily CBCT guided prostate cancer radiotherapy.

Methods:25 prostate cancer patients treated using daily IGRT with the total dose 64Gy in 20 fractions were retrospectively studied. IMRT plan was created with 3 mm CTV to PTV margin, where CTV was defined as prostate alone for low risk patients and prostate plus seminal vesicles for intermediate or high risk patients. Daily CBCT was performed before delivery for target (prostate with implanted radio-markers) localization and translational correction. Additional CBCT was taken immediately after the treatment, and used in the treatment dose construction. For each patient, deformable registration between the pre-treatment helical CT and the post-treatment CBCTs was performed. The treatment dose in each organ was accumulated using the displacement vector field and the daily dose recalculated on daily CBCT. Dose-volume parameters, as well as EUD, from both the planned and delivered doses for the CTV, rectum and bladder were used in the evaluation.

Results:Evaluation of 11 patients, 4 low risk and 7 intermediate/high risk, has been completed so far. The EUD for CTV was in [61.0%, 100.3%] of the planned target dose with mean±sd = 93.7±11.4%, D99 in [77.9%, 100.1%] with mean±sd = 91.9±5.9%, D95 in [93.8%, 100.5%] with mean±sd = 97.1±2.3%. Large target dose reduction (> 10% on the CTV EUD) occurred for two patients(> 15%), and both appeared at the posterior and superior edges of the CTV. For rectum, the treatment EDU is in [80.0%, 96.8%], with mean±sd = 88.8±6.1%. The bladder EUD is in [98.1%, 131.1%] with mean±sd = 112.1±10.2%.

Conclusion:The evaluation shows that 3 mm target margin is inadequate to compensate for intra-treatment target motion in the prostate cancer treatment with daily CBCT guided target localization and correction. More than 15% of patients could have marginal missing without a validated intra-treatment motion management.

Funding Support, Disclosures, and Conflict of Interest: This research is partially supported by Elekta R&D fund.

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