ESTRO 37 Abstract book

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ESTRO 37

1 VU University Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands 2 VU University Medical Center, Department of Radiation Oncology Medical Physics, Amsterdam, The Netherlands Purpose or Objective Verification of intra-fractional tumor position during gated delivery is of utmost importance. We integrated gated breath-hold delivery under MRI guidance with only 3 mm margins using video-feedback to patients, and report on the accuracy of the real-time gated delivery system and reproducibility of tumor position. Material and Methods MR-guided gated breath-hold SBRT delivery in combination with visual feedback was implemented with the MRIdian system (Viewray Inc, Mountain View, USA). During treatment delivery, real-time tumor tracking is realized through repeated fast planar MR imaging in a single sagittal plane at 4 frames-per-second with 3.5mm x 3.5mm in-plane resolution via deformable image registration. An in-room MR-compatible monitor allows projection of this sagittal image with visualization of both the tracked GTV contour and gating boundary, which is the PTV (GTV+3mm). For each delivery, a threshold ROI% is set which determines the maximum allowed percentage of the GTV-area that can be outside the gating window before triggering a beam-hold. Accuracy of the real-time gated delivery and reproducibility of tumor position during repeated breath- holds was analyzed for 15 patients with five lung-, adrenal- and pancreas tumors each for a total of 87 fractions, resulting in a cumulative 33.4 hours of MR-cine. For image analysis, bi-cubic interpolation was applied to all acquired frames resulting in images with 0.8mmx0.8mm resolution. For each fraction we analyzed: [1] reproducibility of the GTV-centroid (GTV-COM) position within the PTV; [2] the geometric coverage of the GTV-area within the PTV; [3] treatment duty cycle efficiency; [4] effects of threshold ROI%-settings on treatment duty cycle efficiency and GTV-area coverage. Results Figure 1 shows a single 2D planar tracking image for a lung patient with observed GTV-COM positions, relative to the PTV-COM during beam-on. Grouped results showed 5 th –95 th percentile distributions of GTV-COM positions, relative to PTV-COM, in AP [ventral-dorsal] direction of [- 3.3mm,+2.8mm], [-2.5mm,+3.7mm] and [- 4.4mm,+2.9mm] for lung-, adrenal- and pancreas tumors, respectively. The corresponding distributions in CC [caudal-cranial] direction were [-2.6mm,+4.6mm], [- 4.1mm,+4.4mm] and [-4.4mm,+4.5mm], respectively. However, the mean GTV-areas that were encompassed by the PTV during beam-on across all fractions were 94.6%, 94.3% and 95.3% for lung-, adrenal- and pancreas tumors. The mean treatment duty cycle efficiency ranged from 67% to 87% for tumor sites. Selecting a higher threshold ROI% resulted in increased duty cycle efficiency, at the cost of a slight decrease in GTV area coverage.

Results The range of motion of the three OAR with respect to the pCT had an average surface-to-surface and center-of- mass displacement between 6 - 10 mm and 8 - 12 mm, respectively, while all OAR had a mean Dice Coefficient of 0.8 ± 0.1. In 13-29% of the FxCTs, depending on the OAR, the V35Gy dose constraint was violated by more than 1 ml (Table 1). Through PCA, the first three principal components mainly revealed anterior-posterior deformations for the stomach and the duodenum whereas the bowel mostly deforms in the cranio-caudal direction (Fig 1). Thirty-three modes were required to describe the 90% of the variance.

Conclusion Analysis of delineated OAR in high-quality daily CTs shows the significance of OAR motion even when target tracking is used. A PCA model was successfully built and might provide valuable information to clinicians to improve management of daily OAR motion. To this end, PCA is now being used to explore planning restrictive volume margins, adaptive plan-of-the-day strategies and probabilistic treatment planning. OC-0185 SBRT using MR-guided, video-assisted gated treatment delivery during patient breath holds. J.R. Van Sornsen de Koste 1 , M.A. Palacios 2 , A.M.E. Bruynzeel 1 , B.J. Slotman 1 , S. Senan 1 , F.J. Lagerwaard 1