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To evaluate the image quality, dosimetric properties, setup reproducibility, and planar cine motion detection of a high‐resolution brain coil and integrated stereotactic brain immobilization system that constitute a new brain treatment package (BTP) on a low‐field magnetic resonance imaging (MRI) linear accelerator (MR‐linac). Image quality of the high‐resolution brain coil was evaluated with the 17 cm diameter spherical phantom and the American College of Radiology (ACR) Large MRI Phantom. Patient imaging studies approved by the institutional review board (IRB) assisted in selecting image acquisition parameters. Radiographic and dosimetric evaluation of the high‐resolution brain coil and the associated immobilization devices was performed using dose calculations and ion chamber measurements. End‐to‐end testing was performed simulating a cranial lesion in a phantom. Inter‐fraction setup variability and motion detection tests were evaluated on four healthy volunteers. Inter‐fraction variability was assessed based on three repeat setups for each volunteer. Motion detection was evaluated using three‐plane (axial, coronal, and sagittal) MR‐cine imaging sessions, where volunteers were asked to perform a set of specific motions. The images were post‐processed and evaluated using an in‐house program. Contrast resolution of the high‐resolution brain coil is superior to the head/neck and torso coils. The BTP receiver coils have an average HU value of 525 HU. The most significant radiation attenuation (3.14%) of the BTP, occurs through the lateral portion of the overlay board where the high‐precision lateral‐profile mask clips attach to the overlay. The greatest inter‐fraction setup variability occurred in the pitch (average 1.08 degree) and translationally in the superior/inferior direction (average 4.88 mm). Three plane cine imaging with the BTP was able to detect large and small motions. Small voluntary motions, sub‐millimeter in magnitude (maximum 0.9 mm), from motion of external limbs were detected. Imaging tests, inter‐fraction setup variability, attenuation, and end‐to‐end measurements were quantified and performed for the BTP. Results demonstrate better contrast resolution and low contrast detectability that allows for better visualization of soft tissue anatomical changes relative to head/neck and torso coil systems. [ABSTRACT FROM AUTHOR]

Introduction: Temporary breast tissue expanders contain a metal port that varies in position throughout the course of radiation treatments. The purpose of this study was to quantify the robustness of the three most common external beam treatment techniques (tangential three‐dimensional conformal radiation therapy [3DCRT], volumetric modulated arc therapy [VMAT], and helical tomotherapy) against our measured inter‐fractional positional variations of the port. Methods: For eight breast cases, a clinical plan was created for each of the three techniques. The dosimetric effect of our previously measured inter‐fractional port errors was evaluated for two classes of error: internal port errors (IPEs) and patient registration errors (PREs). For both classes of error, daily variable and systematic errors were modeled, and their cumulative effects were compared against the originally planned doses. Results: For systematic IPE, the 1%–99% range in point dose differences inside a 5‐mm target abutting the implant was the highest for tangential 3DCRT, and it was within 6% and 9% when calculated with Monte Carlo and collapsed cone calculation engines, respectively. Daily variable PRE resulted in mean changes of −3.0% and −3.5% to V100%Rx of the target for VMAT and tomotherapy, respectively. For nearby organs, daily variable PRE resulted in changes to V20Gy of the ipsilateral lung of less than 2% in all three techniques, while V5Gy of the heart increased by as much as 6% in VMAT and 10% in tomotherapy. Conclusions: When IPEs were modeled, dose variability was the largest in tangential 3DCRT, leading to areas of underdosage in the shadow of the port. When PREs were modeled, the target coverage and nearby organs were affected the most in VMAT and helical tomotherapy. In reality, port positional errors result from a combination of IPE and PRE, suggesting that VMAT and tomotherapy are more robust when patient registration errors are minimized, despite the presence of IPE. [ABSTRACT FROM AUTHOR]