ESTRO 2022 - Abstract Book

S1516

Abstract book

ESTRO 2022

0.0 mm [-4.2 – 3.7], and -0.6 mm [-3.5 – 1.9]. Degradations in CTV and PTV coverage relative to planned dose were generally limited. Mean relative dose differences were -0.1% [-1.8 – 1.0] for CTVD99% and -0.2% [-1.6 – 0.7] for PTVD95% in case A, and -1.2% [-8.8 – 0.8] and -1.2% [-5.9 – 0.7] in case B. Urethra planning organ at risk volume (uPRV) was slightly degraded after taking motion into account, with larger than 1% differences in uPRVD10% observed only for 1 patient in case B. Rectum and bladder dosimetric parameters showed major variations between reconstructed and original plans, with a favourable underexposition of rectum and an undesirable overdose to bladder. Nevertheless, no protocol dose constraints violations were observed for bladder due to posterior displacement of the prostate. Conclusion Current CTV to PTV margins, robustness of original treatment plans, and fast FFF beams delivery do not result in significant degradation of dose metrics for target and OARs due to intrafraction prostate motion in both cases. Anyway, beam gating and motion correction ensured superior results and are recommended to use in dose-escalated prostate SBRT. The dosimetric impact of daily anatomy will be also explored in future studies.

PO-1716 Intrafraction prostate motion management in dose-escalated linac-based SBRT

D. Panizza 1,2 , R. Lucchini 3,2 , V. Faccenda 1,4 , M.C. Daniotti 5 , P. Caricato 1,4 , S. Trivellato 1 , S. Arcangeli 3,2 , E. De Ponti 1,2

1 ASST Monza, Medical Physics Department, Monza, Italy; 2 University of Milan Bicocca, School of Medicine and Surgery, Milan, Italy; 3 ASST Monza, Radiation Oncology Department, Monza, Italy; 4 University of Milan, Department of Physics, Milan, Italy; 5 University of Milan Bicocca, Department of Physics, Milan, Italy Purpose or Objective This study reports the pioneering clinical experience using an electromagnetic (EM) tracking device for intrafraction prostate motion management during dose-escalated linac-based SBRT. Materials and Methods 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. Results 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.