Investigating the impact of patient arm position in an MR‐linac on liver SBRT treatment plans

Abstract

Purpose The superior soft‐tissue contrast offered by the integrated magnetic resonance imaging (MRI) of the Unity MR‐linac compared to the x‐ray‐based image guidance on conventional linacs potentially allows for liver stereotactic body radiation therapy (SBRT) without the need for implanted markers or other surrogates. On conventional linacs, liver SBRT patients are typically positioned with their arms above their heads (arms‐up) to minimize exposure to healthy tissue. However, the spatial confinement of the MRI‐bore and increased treatment delivery times can make the arms‐up position straining. Therefore, we assessed the plan quality for MR‐linac treatment plans with the patient in the arms‐up and in the arms‐down position. Additionally, we compared the MR‐linac plans with clinically used arms‐up treatment plans made for a conventional linac. Methods Fifteen consecutively treated patients with oligometastatic liver disease were included in this retrospective study. For each patient, a planning computed tomography (CT) with delineations, a diagnostic MRI, and a 3 × 20 Gy dual‐arc volumetric modulated arc therapy (VMAT) plan, which was used to treat the patient in an arms‐up position on the conventional linac, were available. For the MR‐linac, 15‐beam step‐and‐shoot intensity‐modulated radiation therapy (IMRT) plans were created for four patient positioning scenarios: arms‐up, mimicking current clinical practice; arms‐down, with treatment beams avoiding the arms on the entrance side; arms‐through, arms are down but not avoided, and right‐arm‐up; only the right arm is up and the left arm is avoided on the entrance side. Resulting treatment plans were compared. Bonferroni‐corrected two‐sided Wilcoxon signed‐ranks tests were used to assess statistical significance (P < 0.05). Results No significant differences were found in gross tumour volume (GTV) coverage (D2%, D50%, and D98%) or liver sparing (liver‐GTV V<15normalGy) between the clinical plans and any of the MR‐linac plans. The median target conformity [exterior V40%/planning target volume (PTV)] was significantly better in the clinical plans (5.8) than in the MR‐linac scenarios (arms‐down: 6.6, arms‐up/right‐arm‐up: 6.2, arms‐through: 6.3). No MR‐linac plan violated any additional organ‐at‐risk (OAR) constraint that was not already violated in the clinical plans. In the arms‐down scenario a significantly increased median spinal cord D1% (14.5 Gy) was detected compared to the clinical setup (7.2 Gy). For the arms‐down (arms‐through) scenario, the median left arm D1% was 1.5 (2.7) Gy, the median right arm D1% was 5.8 (22.7) Gy, and the median right arm V20Gy was 0.0 (14.7) cc. These differences were statistically significant. For the right‐arm‐up scenario, the median left arm D1% (2.3 Gy) and V5Gy (0.0) were not significantly different compared to the arms‐down scenario. Conclusions Mimicking the current clinical practice by treating patients in the arms‐up/right‐arm‐up position on the MR‐linac leads to plans which are dosimetrically very similar to the convention...

ICRU REPORT 97: MRI-Guided Radiation Therapy Using MRI-Linear Accelerators

“…As a result, some groups have integrated performing simulation with the patient arms positioned down and eliminating beam geometries that traverse the arms. For an oligometastatic liver disease cohort of 15 patients, comparable GTV coverage was found between planning with an MRI-linac (15 beam IMRT) with the arms positioned down and treatment beams avoiding the arms, and dual-arc volumetric modulated arc therapy using a conventional treatment plan with the arms up (van den Wollenberg et al , 2019). However, absorbed-dose conformality was reduced with arms down and OAR doses were slightly compromised.…”

Evaluation of Hybrid&nbsp;PET/MRI&nbsp;for Gross Tumor Volume (GTV) Delineation in Liver Cancer Radiotherapy

“…Future studies on tumor volume delineation based on PET/MRI may bene t from more advanced technology such as MR-linac, novel radioactive tracers and 4D-PET. For example, studies are ongoing to investigate the impact of MR-linac on liver SBRT planning [31,32,33]. On conventional linacs, based on X-ray-based image guidance, liver lesions are often poor or invisible in this pattern of images; thus, the injection of contrast material, or the implantation of markers is needed address this issue.…”

Towards mid-position based Stereotactic Body Radiation Therapy using online magnetic resonance imaging guidance for central lung tumours

“…Two plans were created for each patient: one optimized for the ITV on the averaged 3D-CT, the other plan optimized for the GTV on the midP-CT. Treatment planning was performed using 13–15 intensity-modulated beams, which were angled to avoid entering through uninvolved lung or the patients arms, simulated as positioned along the body ( [19] ). Further planning details are given in Supp.…”