Dynamic intrafractional position monitoring with implanted fiducial markers for enhanced accuracy in radiotherapy of prostate cancer

Introduction: Recent advances in the radiation therapy of prostate cancer have brought a shift toward moderate- and ultra-hypofractionated treatment schedules. Reducing safety margins can broaden the therapeutic window in stereotactic treatments and alleviate concerns for toxicity in high dose-per-fraction treatment schedules. Management of intrafractional motion is a necessity for stereotactic body radiation therapy (SBRT). It can be achieved by performing intrafractional image guidance and position corrections. We evaluate the suitability of such a novel prostate motion management system and its potential benefit for treatment accuracy. Methods: Intrafractional IGRT was performed for 22 patients during 149 treatment sessions using repeated orthogonal kV-XR imaging of implanted fiducial markers with the ExacTrac Dynamic (EXTD) system. Position measurements were taken four times during each arc of the applied volumetric modulated arc therapy (VMAT). Position correction was performed if translational deviation exceeded 2 mm in any direction. Results: Of 677 single EXTD measurements, 20.6% exceeded the predefined threshold of 2 mm 3D deviation. Without intrafractional corrections, 39.4% of all individual measurements would exceed the threshold. The 3D accuracy could thus significantly be improved, reducing mean 3D shifts from 1.97 (& PLUSMN; 1.44) mm to 1.39 (& PLUSMN; 1.01) mm by performing intrafractional IGRT. In total, 34% of all treatment sessions required correction of intrafractional position shifts. Conclusion: Monitoring of prostate motion using repeated intrafractional orthogonal kV-X-ray-based position measurements of implanted fiducial markers proved to be a reliable method to improve precision of stereotactic irradiations of the prostate. It can prevent unacceptable translation deviations in one third of all sessions.