Room: Exhibit Hall | Forum 1
Purpose: SBRT is gaining prominence as an excellent treatment option for men with favorable risk prostate cancer. Because of the high doses per fraction, advanced treatment delivery techniques to correct for target motion thus reducing planning target volume (PTV) margins are important in order to minimize bladder and rectal toxicities. This work quantifies the dosimetric advantages of using CyberKnife (CK) for prostate SBRT treatment over linac-based VMAT.
Methods: We consecutively selected 10 patients for this study. All of the patients were treated with CK for 36.25Gy in 5 fractions. As our previous study has demonstrated that a 3 mm PTV margin is adequate for CK treatment planning, due to the fact that the CK inTempo Adaptive Imaging System is equipped with a time-based motion tracking technology to compensate intrafraction motion of the target, we have adopted this margin in our clinical practice. For linac-based treatment planning, our clinical guideline is to use a 5 mm margin for the PTV, as imaging guidance associated with a linac does not offer real-time motion tracking and compensation. Treatment planning comparison was performed on Multiplan and Eclipse planning systems, respectively, using the constraints suggested in the RTOG 938 protocol.
Results: Our results show that, comparing to linac-based VMAT plans, the maximum doses received by 1-cc and 3-cc of the rectum in the CK plans are reduced by 5.9% and 6.2%, respectively. The reduction in the maximum dose received by 1-cc of bladder is moderate (2%). The doses to 90% of the bladder and the rectum are generally comparable or reduced with linac-based VMAT. The difference in the maximum dose received by the urethra is not clinical significant (1%).
Conclusion: An SBRT platform that tracks and corrects for prostate motion in real time has dosimetric advantages over other platforms for prostate SBRT.