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Fixed Margin Or Online Adaptation for Intermediate-Risk Prostate Stereotactic Body Radiation Therapy? A Dosimetric Study

Y Sheng

Y Sheng*, T Li , F Yin , Q Wu , Duke University Medical Center, Durham, NC


SU-E-T-434 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:To investigate the choice of fixed margin or online adaptation when treating intermediate-risk prostate cancer including seminal vesicles (SV) using stereotactic body radiation therapy (SBRT).

Methods:9 prostate SBRT patients were retrospectively studied. All patients were implanted with fiducial markers in the prostate for daily localization and verification. Each patient had 5 pairs of pre-treatment and post-treatment cone-beam CT (CBCT) per protocol. SVs were contoured on planning CT and all CBCTs by one attending physician. Simultaneous integral boost (SIB) IMRT plans were developed to deliver 25Gy/5fx to the SV while delivering 37Gy/5fx to the prostate. A 3mm isotropic margin was added to the prostate while a 5 mm isotropic margin was used for the SV. The planning CT was registered to daily pre-treatment and post-treatment CBCT based on fiducial markers in the prostate to mimic online prostate localization; and the SV on daily CBCT was transferred to the CT structure set after the prostates were aligned. Daily pre-treatment and post-treatment SV dose coverage and the organ-at-risk (OAR) sparing were evaluated for the SIB regimen. At least 95% of the SV need to receive the prescription dose (5Gy per fraction).

Results:For the total of 90 daily SVs analyzed (ten CBCTs for each of nine patients), only 45 daily SVs (50%) were able to meet the coverage that 95% of the SV received 25Gy. The OAR sparing performance was acceptable for most of the dosimetric constraints in low-risk prostate SBRT protocol with only two exceptions in bladder V100 (cc).

Conclusion:A fixed 5mm margin for SV is not sufficient to provide consistent daily dose coverage due to SV’s substantial inter- and intra-fractional motion relative to the prostate. This finding calls for innovative strategies in margin design as well as online treatment adaptation.

Funding Support, Disclosures, and Conflict of Interest: This work is partially supported a master research grant from Varian Medical Systems.

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