Pancreatic SABR using peritumoral fiducials, triggered imaging and breath-hold

Abstract

Background: We aim to present our linear accelerator-based workflow for pancreatic stereotactic ablative radiotherapy (SABR) in order to address the following issues: intrafractional organ motion management, Cone Beam CT (CBCT) image quality, residual errors with dosimetric consequences, treatment time, and clinical results. Methods: Between 2016 and 2021, 14 patients with locally advanced pancreatic cancer were treated with induction chemotherapy and SABR using volumetric modulated arc therapy (VMAT). Internal target volume (ITV) concept (5), phase-gated (4), or breath hold (5) techniques were used. Treatment was verified by CBCT before and after irradiation, while tumor motion was monitored and controlled by kV triggered imaging and beam hold using peritumoral surgical clips. Beam interruptions and treatment time were recorded. The CBCT image quality was scored and supplemented by an agreement analysis (Krippendorff's-α) of breath-hold CBCT images to determine the position of OARs relative to the planning risk volumes (PRV). Residual errors and their dosimetry impact were also calculated. Progression free (PFS) and overall survival (OS) were assessed by the Kaplan-Meier analysis with acute and late toxicity reporting (CTCAEv4). Results: On average, beams were interrupted once (range: 0-3) per treatment session on triggered imaging. The total median treatment time was 16.7 ± 10.8 min, significantly less for breath-hold vs. phase-gated sessions (18.8 ± 6.2 vs. 26.5 ± 13.4, p < 0.001). The best image quality was achieved by breath hold CBCT. The Krippendorff's-α test showed a strong agreement among five radiation therapists (mean K-α value: 0.8 (97.5%). The mean residual errors were <0.2 cm in each direction resulting in an average difference of <2% in dosimetry for OAR and target volume. Two patients received offline adaptation. The median OS/PFS after induction chemotherapy and SABR was 20/12 months and 15/8 months. No Gr. ≥2 acute/late RT-related toxicity was noted. Conclusion: Linear accelerator based pancreatic SABR with the combination of CBCT and triggered imaging + beam hold is feasible. Peritumoral fiducials improve utility while breath-hold CBCT provides the best image quality at a reasonable treatment time with offline adaptation possibilities. In well-selected cases, it can be an effective alternative in clinics where CBCT/MRI-guided online adaptive workflow is not available.

Keywords: deep inspiration breath hold; motion control; pancreatic cancer; stereotactic ablative radiotherapy; triggered imaging.

FIGURE 1

Verification workflow.

FIGURE 2

Corresponding axial CT slices on diagnostic scan (A) and treatment planning CT (B) fused by rigid registration. LAPC with abutment of gastric wall. Residual tumor (green), CTV (red). Note the implanted titanium clips (yellow) between the target volume and OARs assisting in target volume delineation.

FIGURE 3

Multiple peritumoral fiducials (yellow) provide multidimensional visualization of the tumor (red) on triggered imaging, while certain clips indirectly represent the preserved gastric wall (purple).

FIGURE 4

Corresponding axial slices on planning breath-hold-CT and breath-hold-CBCT (half fan mode): Systematic posterior displacement of the stomach (blue) and clip (arrow) was observed during the 1–4th fractions becoming closer to the high-dose area. Consequences/conclusions: 1. The position of this clip (arrow) is no longer relevant during intrafractional triggered imaging 2. This simultaneously alerts radiation therapists to the anatomical changes on CBCT.

FIGURE 5

Image quality assessment of different CBCT modes. FB, free breathing; DIBH, deep inspiration breath hold; AIP, average intensity projection; MIP, maximum intensity projection.

FIGURE 6

Kaplan Meier curves: OS (overall survival), PFS (progression free survival), LPFS (local-progression free survival). diag: from the start of induction CT, sabr: from the end of SABR.

FIGURE 7

Offline treatment plan re-optimization based on CBCT contouring of the stomach (Blue: original stomach, yellow: original PRV, green: new PRV).