SU-E-T-434: Fixed Margin Or Online Adaptation for Intermediate-Risk Prostate Stereotactic Body Radiation Therapy? A Dosimetric Study




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).


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).


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).


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.

This work is partially supported a master research grant from Varian Medical Systems.