Treatment planning for MR-guided SBRT of pancreatic tumors on a 1.5 T MR-Linac: A global consensus protocol.

Background and purpose: Treatment planning for MR-guided stereotactic body radiotherapy (SBRT) for pancreatic tumors can be challenging, leading to a wide variation of protocols and practices. This study aimed to harmonize treatment planning by developing a consensus planning protocol for MR-guided pancreas SBRT on a 1.5 T MR-Linac. Materials and methods: A consortium was founded of thirteen centers that treat pancreatic tumors on a 1.5 T MR-Linac. A phased planning exercise was conducted in which centers iteratively created treatment plans for two cases of pancreatic cancer. Each phase was followed by a meeting where the instructions for the next phase were determined. After three phases, a consensus protocol was reached. Results: In the benchmarking phase (phase I), substantial variation between the SBRT protocols became apparent (for example, the gross tumor volume (GTV) D99% ranged between 36.8 - 53.7 Gy for case 1, 22.6 - 35.5 Gy for case 2). The next phase involved planning according to the same basic dosimetric objectives, constraints, and planning margins (phase II), which led to a large degree of harmonization (GTV D99% range: 47.9-53.6 Gy for case 1, 33.9-36.6 Gy for case 2). In phase III, the final consensus protocol was formulated in a treatment planning system template and again used for treatment planning. This not only resulted in further dosimetric harmonization (GTV D99% range: 48.2-50.9 Gy for case 1, 33.5-36.0 Gy for case 2) but also in less variation of estimated treatment delivery times. Conclusion: A global consensus protocol has been developed for treatment planning for MR-guided pancreatic SBRT on a 1.5 T MR-Linac. Aside from harmonizing the large variation in the current clinical practice, this protocol can provide a starting point for centers that are planning to treat pancreatic tumors on MR-Linac systems.

SBRT focal dose intensification using an MR-Linac adaptive planning for intermediate-risk prostate cancer: An analysis of the dosimetric impact of intra-fractional organ changes.

INTRODUCTION: Using an magnetic resonance linear accelerator (MR-Linac) may improve the precision of visible tumor boosting with ultra-hypofractionation by accounting for daily positional changes in the target and organs at risk (OAR). PATIENTS AND METHODS: Fifteen patients with prostate cancer and an MR-detected dominant lesion were treated on the MR-Linac with stereotactic body radiation (SBRT) to 40 Gy in 5 fractions, boosting the gross tumor volume (GTV) to 45 Gy with daily adaptive planning. Imaging was acquired again after initial planning (verification scan), and immediately after treatment (post-treatment scan). Prior to beam-on, additional adjustments were made on the verification scan. Contours were retrospectively adjusted on verification and post-treatment scans, and the daily plan recalculated on these scans to estimate the true dose delivered. RESULTS: The median prostate D95% for plan 1, 2 and 3 was 40.3 Gy, 40.5 Gy and 40.3 Gy and DIL D95% was 45.7 Gy, 45.2 Gy and 44.6 Gy, respectively. Bladder filling was associated with reduced GTV coverage (p = 0.03, plan 1 vs 2) and prostate coverage (p = 0.03, plan 2 vs 3). The D0.035 cc constraint was exceeded on verification and post-treatment plans in 24 % and 33 % of fractions for the urethra, 31 % and 45 % for the bladder, and 35 % and 25 % for the rectum, respectively. CONCLUSION: MR-Linac guided, daily adaptive SBRT with focal boosting of the GTV yields acceptable planned and delivered dosimetry. Adaptive planning with a MR-Linac may reliably deliver the prescribed dose to the intended tumor target.

Stereotactic ablative radiation for pancreatic cancer on a 1.5 Telsa magnetic resonance-linac system.

Purpose: Ablative radiation therapy (A-RT) appears to improve outcomes in locally advanced pancreatic cancer (LAPC) yet requires solutions for respiratory and digestive motion. We report outcomes of A-RT for pancreatic cancer using 1.5 T MR-adaptive treatment delivery. Methods: Between March 2020 and July 2021, we treated 30 patients with pancreatic cancer with 50 Gy in 5 fractions (biologically effective dose [BED10] = 100 Gy10) using a novel compression belt workflow and remote planning on the Unity 1.5 T MR linac system. Cumulative incidence of progression was computed from A-RT initiation with death as a competing risk. Overall (OS) and progression-free survival (PFS) were calculated using Kaplan Meier methods. Results: Of 30 patients, most (73 %) were locally advanced, 4 (13 %) were metastatic, 2 (7 %) were medically inoperable, and 2 (7 %) were locally recurrent. Most (73 %) received FOLFIRINOX prior to A-RT. Median follow-up times from diagnosis and A-RT were 17.6 (IQR 15.8-23.1) and 11.5 months (IQR 9.7-16.1), respectively. Cumulative incidences at 1-year of local and distant progression were 19.3 % (95 %CI 6.7-36.8 %) and 47.4 % (95 %CI 26.7-65.6 %), respectively. Median OS from diagnosis and A-RT were not reached. One-year OS from diagnosis and A-RT were 96.4 % (95 %CI 77.2-99.5 %) and 80.0 % (95 %CI 57.3-91.4 %), respectively. Median and 1-year PFS were 10.1 months (95 %CI 4.4-14.4) and 39.7 % (95 %CI 20.3-58.5 %), respectively. No grade 3 + toxicities were observed. Conclusions: A-RT using the 1.5 T Unity MR Linac resulted in promising LC and OS with no severe toxicity in patients with LAPC despite radiosensitive organs adjacent to the target volumes. Longer follow-up is needed to assess long-term outcomes.

An investigation of using log-file analysis for automated patient-specific quality assurance in MRgRT.

OBJECTIVE: Adaptive radiation therapy (ART) is an integral part of MR-guided RT (MRgRT), requiring a new RT plan for each treatment fraction and resulting in a significant increase in patient-specific quality assurance (PSQA). This study investigates the possibility of using treatment log-file for automated PSQA. METHOD: All treatment plans were delivered in 1.5T Unity MR-Linac (Elekta). A Unity compatible version of LinacView (Standard Imaging) was commissioned to automatically monitor and analyze the log-files. A total of 220 fields were delivered and measured by ArcCheck® -MR (Sun Nuclear) and LinacView. Thirty incorrectly matched fields were also delivered to check for error detection sensitivity. The gamma analysis, γ, with 3%, 3 mm criteria was used in both ArcCheck® -MR and LinacView. Additionally, the gantry angle, jaws, and multileaf collimators (MLC) positions reported in the log-file were compared with plan positions using TG-142 criteria. RESULT: The γ (3%, 3 mm) for the 190 plans were found to be between the range of 72.5%-100.0% and 95.4%-100.0% for ArcCheck® -MR and LinacVeiw, respectively. All the delivered gantry angle and jaws were found to be within 0.2° and 2 mm. MLCs that were outside the guard leaves or under the diaphragms were found to have more than 1.0 mm discrepancy. This was attributed to the linac internal override for these MLCs and had no dosimetric impact. Excluding these discrepancies, all MLC positions were found to be within 1.0 mm. The γ (3%, 3 mm) for the 30 incorrectly matched fields were found to be 3.9%-84.8% and 0.1%-64.4% for ArcCheck® -MR and LinacVeiw, respectively. CONCLUSION: Significant ranked correlation demonstrates the automated log-file analysis can be used for PSQA and expedite the ART workflow. Ongoing PSQA will be compared with log-file analysis to investigate the longer term reproducibility and correlation.

Feasibility of ablative stereotactic body radiation therapy of pancreas cancer patients on a 1.5 Tesla magnetic resonance-linac system using abdominal compression.

BACKGROUND AND PURPOSE: Stereotactic body radiation therapy delivered using MR-guided radiotherapy (MRgRT) and automatic breathold gating has shown to improve overall survival for locally advanced pancreatic cancer (LAPC) patients. The goal of our study was to evaluate feasibility of treating LAPC patients using abdominal compression (AC) and impact of potential intrafraction motion on planned dose on a 1.5T MR-linac. METHODS & MATERIALS: Ten LAPC patients were treated with MRgRT to 50 Gy in 5 fractions with daily online plan adaptation and AC. Three orthogonal plane cine MRI were acquired to assess stability of AC pressure in minimizing tumor motion. Three sets of T2w MR scans, pre-treatment (MRIpre), verification (MRIver) and post-treatment (MRIpost) MRI, were acquired for every fraction. A total of 150 MRIs and doses were evaluated. Impact of intrafraction organ motion was evaluated by propagating pre-treatment plan and structures to MRIver and MRIpost, editing contours and recalculating doses. Gross tumor volume (GTV) coverage and organs-at-risk (OARs) doses were evaluated on MRIver and MRIpost. RESULTS: Median total treatment time was 75.5 (49-132) minutes. Median tumor motion in AC for all fractions was 1.7 (0.7-7), 2.1 (0.6-6.3) and 4.1 (1.4-10.0) mm in anterior-posterior, left-right and superior-inferior direction. Median GTV V50Gy was 78.7%. Median D5cm3 stomach_duodenum was 24.2 (18.4-29.3) Gy on MRIver and 24.2 (18.3-30.5) Gy on MRIpost. Median D5cm3 small bowel was 24.3 (18.2-32.8) Gy on MRIver and 24.4 (16.0-33.6) Gy on MRIpost. CONCLUSION: Dose-volume constraints for OARs were exceeded for some fractions on MRIver and MRIpost. Longer follow up is needed to see the dosimetric impact of intrafraction motion on gastrointestinal toxicity.

Impact of varying air cavity on planning dosimetry for rectum patients treated on a 1.5 T hybrid MR-linac system.

PURPOSE: To investigate the dosimetric impact of magnetic (B) field on varying air cavities in rectum patients treated on the hybrid 1.5 T MR-linac. METHODS: Artificial air cavities of varying diameters (0.0, 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 cm) were created for four rectum patients (two prone and two supine). A total of 56 plans using a 7 MV flattening filter-free beam were generated with and without B-field. Reference intensity-modulated radiation therapy treatment plans without air cavity in the presence and absence of B-field were generated to a total dose of 45/50 Gy. The reference plans were copied and recalculated for the varying air cavities. D95 (PTV45 -PTV50 ), D95 (PTV50 -aircavity), V50 (PTV50 -aircavity), Dmax (PTV50 -aircavity), and V110% (PTV50 -aircavity) were extracted for each patient. Annulus rings of 1-mm-diameter step size were generated for one of the air cavity plans (3.0 cm) for all four patients to determine Dmax (%) and V110% (cc) within each annulus. RESULTS: In the presence of B-field, hot spots at the cavity interface start to become visible at ~1 cm air cavity in both supine and prone positioning due to electron return effect (ERE). In the presence of B-field Dmax and V110% varied from 5523 ± 49 cGy and 0.09 ± 0.16 cc for 0 cm air cavity size to 6050 ± 109 cGy and 11.6 ± 6.7 cc for 5 cm air cavity size. The hot spots were located within 3 mm inside the rectal-air interface, where Dmax increased from 110.4 ± 0.5% without B-field to 119.2 ± 0.8 % with B-field. CONCLUSIONS: Air cavities inside rectum affects rectum plan dosimetry due ERE. Location and magnitude of hot spots are dependent on the size of the air cavity.