Session Item

This study reports the pioneering clinical experience using an electromagnetic (EM) tracking device for intrafraction prostate motion management during dose-escalated linac-based SBRT.

Thirteen patients with organ-confined prostate cancer underwent dose-escalated SBRT using VMAT techniques with FFF beams on Linac platform.  An integrated Foley catheter with a transmitter in a dedicated lumen was used for intrafractional tracking. Patient setup and target localization were achieved by CBCT matching. Treatment was interrupted when the transmitter signals exceeded a 2 mm threshold in any of the three spatial directions and, unless the offset was transient, treatment target position was re-defined by repeating CBCT. A C++ code was implemented to perform retrospective data analysis of the displacements along the three directional axes. Moreover, adjusting setup and delivery phases duration, the developed program enabled simulation of the displacements that would have occurred without any organ motion management.

Overall, intrafractional tracking was successfully carried out in all the treatment sessions. In 31 out of 56 monitored fractions (55%), no intervention was required to correct the target position as a result of an excessive displacement. In 25 (45%) treatment sessions, a correction was mandated, but only in 10 (18%) the beam delivery was interrupted. Overall mean treatment time was 10.2 minutes [range 5.5 - 22.7] with on average 3.5 minutes [range 2.5 - 7.3] for the gated beam delivery. The mean value of the target average deviation during the whole session was -0.18 mm, -0.01 mm, and -0.26 in lateral, longitudinal, and vertical direction, respectively. The prostate was found inside the 2 mm threshold from its initial position in 96% of the treatment time, i.e. in 94% of the time during the setup phase and in 98% during the delivery phase (beam on + interruptions). Without any intrafraction organ motion management, the overall mean treatment time and the mean delivery time would have been 6.7 minutes [range 7.7 - 6.6] and 3.2 minutes [range 2.5 – 4.2], respectively. The prostate would have been found outside the tolerance in 9% of the session total time, i.e. in 4% of the time during the setup phase and in 16% during the beam-on phase. The differences in the percentage of time spent by the prostate outside the threshold in the three spatial directions between the analysis without and with the organ motion management are shown in Figure 1.

Our findings show that EM tracking is a reliable technique for real-time non-ionizing prostate monitoring during dose-escalated SBRT, allowing to keep the target within 2 mm, by interrupting the beam delivery when the prostate was in an unsafe position. Without any intrafraction motion management, both the setup and the delivery phases would have been shorter, but significant displacements would have occurred leading to potential target missing and overdose to organs at risk.