Session Item

Previously we demonstrated that prostate intrafraction motion during MR-guided radiotherapy hampers planning margin reduction below 4 mm, even with daily on-line contour adaptation and re-planning. We therefore developed a tool that allows to adapt to this motion by splitting a single fraction dose delivery into multiple subfractions in a regular time slot. We present results on the clinical performance of this method.  

65 prostate cancer patients were treated on a 1.5T MR-linac (Unity, Elekta AB) to 5 x 7.25 Gy using a ‘subfractionation’ workflow: the delivery of a single fraction (7.25 Gy) was split into two subsequent deliveries of 3.625 Gy plans (fig. 1). While performing daily online contour adaptation and re-planning on the first MR scan of the day (MR1), a second scan (MR2) was acquired. The MR1 based plan was adapted to MR2 by a final dose shift (<1 min calculation time) and delivered. During delivering of the 1st plan a new MR scan (MR3) was acquired and a 2nd plan was generated to correct for shifts between MR3 and MR2. The timing of this process is such that the 2nd plan is ready for delivery once the 1st plan has been delivered. During delivery of the 2nd plan a final MR scan (MR4) was obtained. Based on favorable results obtained in 15 previous patients treated with subfractionation, CTV-PTV margins of 2, 2, and 3 mm for LR, SI and AP directions were used in the current patient group.     
Using the four daily MR scans, the residue intrafraction motions were calculated. To detect ‘outlier’ patients with unexpectedly large motions, a traffic light protocol based on these residue motions in each fraction was applied and if necessary a switch to larger (4-5 mm) margins was made.  

58 out of 65 patients completed treatment with the small (2,2,3 mm) margins. The cumulative distribution of the residue 3D intrafraction shift per fraction is depicted in fig. 2. In addition, the corresponding distribution in case a standard single 7.25 Gy plan would have been delivered is shown, demonstrating the benefit of subfractionation. The SDs of residue systematic errors () were 0.2, 0.4 and 0.4 mm for LR, SI and AP directions while the corresponding values without subfractionation would have been 0.5, 0.7 and 0.7 mm. A margin calculation that also included other remaining uncertainties (e.g. intrafraction rotation) confirmed the validity of the applied small margins. The 7 patients with a red traffic light switched to larger margins after 1-3 fractions but retrospective analysis on all 5 fractions showed this would not have been necessary in 3 patients.

We clinically confirmed that margins of 2-3 mm can be safely applied with subfractionation in at least 89% of patients. We expect this percentage to grow further with more loose traffic light criteria based on growing experience.