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

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.

[Linac-based stereotactic ablative radiotherapy for pancreatic cancer with intrafractional triggered imaging]. / Hasnyálmirigyrák lineáris gyorsító alapú sztereotaxiás sugárterápiája kezelés alatti tumorkövetéssel.

Our aim was to present different treatment strategies (non-gated [NG], respiratory-gated [RG] and deep inspiration breath-hold [DIBH] technique) of linac-based stereotaxic ablative radiotherapy (SABR) for pancreatic cancer in terms of use of marker, abdominal compression, image quality, and time efficiency. From October 2016 to October 2020 14 patients were treated with VMAT-based SABR (NG: 6/14, 8/14 RG RT including 3/8 DIBH SABR). Treatment verification consisted of 3D/4D CBCTs. For intrafractional tumor visualization (11/14) different type of fiducials were used. The average treatment time was the shortest with NG RT, followed by DIBH and RG RT. However, the best image quality was achieved with DIBH technique. The Krippendorff's agreement test among three independent RTTs showed that DIBH CBCT (Cone Beam CT) can produce sufficient image quality for OARs and can be used to reliably determine OARs position related to safety zone (PRV). Overall, marker-based DIBH SABR with intrafractional tumor visualization appears to be the best technique on linac at present.

Triggered Imaging With Auto Beam Hold and Pre-/Posttreatment CBCT During Prostate SABR: Analysis of Time Efficiency, Target Coverage, and Normal Volume Changes.

PURPOSE: Our purpose was to investigate time efficiency and target coverage for prostate stereotactic ablative radiation therapy (SABR) using triggered imaging (TI) and auto beam hold. METHODS AND MATERIALS: A total of 20 patients were treated with volumetric modulated arc-based SABR. Treatment verification consisted of pre- and post-radiation therapy cone beam computed tomography (CBCT) with gold marker-based TI every 3 seconds. In case of ≥3 mm (deviation limit) displacement, the treatment was interrupted and imaging-based correction was performed. Beam interruptions, intrafractional shifts, and treatment times were recorded. Prostate, rectum, and bladder were delineated on each CBCT. Target coverage was evaluated by comparing the individual prostate delineations with 98% isodose contour volumes (% of the evaluated volumes exceeding the reference). Both inter- and intrafractional changes of bladder and rectal volumes were assessed. RESULTS: The average overall treatment time (±standard deviation) was 18 ± 11 min, with a radiation delivery time of 6 ± 3 min if no intrafractional CBCT acquisitions were necessary (91% of fractions). On average, 1.2 beam interruptions per fraction were required with 0/1 correction in 71% of the fractions. The mean residual 3-dimensional shift was 1.6 mm, exceeding the deviation limit in 8%. In the case of intrafractional CBCT and/or ≥2 corrections the treatment time dramatically increased. The 98% isodose lines did not encompass the prostate in only 8/180 (4%) evaluations in 6 different patients, leading to a loss of D98 between 0.1%-6% as a worst case scenario. The bladder volumes showed significant increases during treatment (P < .01) while rectal volumes were stable. CONCLUSIONS: Time efficiency of TI + auto beam hold with 3 mm/3 sec threshold during prostate SABR is comparable with competitive techniques, resulting in minimal 3-dimensional residual errors with maintained target coverage. Technical developments are necessary to further reduce radiation delivery time. Use of CBCT allowed full control of rectal volumes, while bladder volumes showed significant increases over time.

[Multimodal imaging during lung and abdominal stereotactic ablative radiotherapy: from cine MRI through 3D/4D CBCT to intrafractional kV verification]. / Multimodális képalkotás tüdõ- és hasi sztereotaxiás ablatív radioterápia során: a cine MRI-mérésektõl a 3D/4D CBCT-n át a kezelés alatti kV-os képi verifikációig.

Our aim was to present our treatment and verification protocols of linear accelerator-based lung and abdominal stereotactic ablative radiotherapy (SABR). During our treatments both the volumetric imaging (3D/4D CBCT/CT) and triggered kV intrafractional tumor motion control could be combined allowing a full control on the whole workflow. The most optimal kV directions from which the tumor is well detectable were defined. Tumor movements measured on cine MRI in treatment position correlated well with the ones on 4D CBCT, thus cine MRI is considered an excellent device to pre-select the appropriate image/treatment verification SABR protocol. In abdominal targets implanted markers and cine MRI are preferred due to limited image quality of CBCT with the current version. In selected lung SABR cases (≥8mm motion) the dose delivery of organs at risk (lungs - GTV, chest wall) could be reduced compared to free breathing conditions, however, the treatment time is at least two-folds higher.