Clinical validation of the Varian Truebeam intra-fraction motion review (IMR) system for prostate treatment guidance.

This study quantified the performance of Intra-fraction Motion Review (IMR) during prostate Stereotactic Body Radiotherapy (SBRT) treatments. IMR was evaluated using prostate motion data from patients treated in an SBRT clinical trial (PROMETHEUS, NCT00587990).IMR measured prostate displacements were compared to those of two 3D motion management methods: Kilovoltage Intra-fraction Motion management (KIM) and MV/kV triangulation. A planning study assessing the impact of a defined prostate motion (2-5 mm) on the PTV  coverage with and without IMR was performed. A clinically relevant IMR search region for prostate cancer SBRT treatments was determined using a customised anthropomorphic pelvis phantom with implanted gold seeds and a motion platform. IMR showed submillimeter agreement with corresponding 2D projections from both KIM and MV/kV triangulation. However, IMR detected actual displacements consistently in considerably fewer frames than KIM (3D), with the actual numbers depending on the settings. The Default Search Region (DSR) method employing a circular search region proved superior to user-contoured structures in detecting clinically relevant prostate motion. Reducing the DSR search region radius can reduce the impact of the 2D nature of IMR and improve the detectability of actual motion (by 10% per 0.5 mm reduction) but must be balanced against increased beam interruptions from minor, clinically irrelevant motion. The use of IMR for SBRT prostate treatments has the potential to improve target dose coverage (minimum dose to 98% of the PTV, D98%) by > 20% compared to treatment without IMR. Calculated D98% of IMR monitored treatments with motion was within 1.5% of plans without motion.