Feasibility of MR-Informed Adaptive Marker-less SBRT in Patients With Bilateral Hip Prostheses Undergoing SBRT for Localized Prostate Cancer

Hip replacements and prostate cancer are independently and both together increasing prevalent in the aging population. In the United States, the prevalence of hip prostheses among men was 1.1 million in 20101, with approximately 500,000 hip prostheses performed annually. Bilateral total hip replacement is recommended for patients with osteoarthritis affecting both hip joints. Approximately 10-25% of total hip replacement patients undergo bilateral procedures. Stereotactic body radiotherapy (SBRT) has been widely adopted as an effective treatment for localized prostate cancer, offering high precision and reduced treatment time due to conventionally fractionated radiotherapy without increasing toxicity or comprising oncological outcomes. Therefore, the use of SBRT for treatment of localized prostate cancer has significantly increased. A cohort study using data from 302 035 patients in the National Cancer Database showed that use of SBRT for prostate cancer increased from 0.2% in 2004 to 12.4% in 2020 in the United States. However, both CT and MR-guided SBRT present unique challenges for patients with prostate cancer and bilateral hip prostheses. Metallic implants, such as hip prostheses, produce artifacts and magnetic susceptibility issues that can obscure MR imaging near the prosthetic sites, potentially complicating target contouring and planning. Given the close proximity of the prostate gland to the pelvis, which houses the hip joints, patients with bilateral hip prostheses may have significant imaging artifacts that could impair MR-guided radiotherapy quality. CT-based SBRT is less affected by metallic artifacts, but it lacks the soft-tissue resolution needed for precise prostate and OAR delineation, a gap MR imaging is uniquely positioned to fill. Low-field MRI systems are less prone to susceptibility artifacts, including those originating from metallic implants. As a result, signal loss and distortion are expected to occur only in the immediate vicinity of the implant, making these systems particularly advantageous for imaging patients with hip implants. A prospective study quantitatively and qualitatively compared the magnitude of metal total hip arthroplasty-induced imaging artifacts in vivo between 1.5T (i.e. high-field) and 0.55T (low-field) MRI in 15 patients. Qualitative artifact magnitude was on average rated as moderate to small on 0.55T and as large to moderate on 1.5T by 2 fellowship-trained musculoskeletal radiologists. In addition, metal artifacts' areas and diameters were smaller on 0.55T when compared with 1.5T MRI for all sequences (each p\>0.016). Given the limitations of current imaging systems, the investigators propose a more flexible solution: a two-room system comprising a low-field MRI scanner for optimal image quality and a C-arm linear accelerator for rapid treatment delivery. Patient transfer between the MR device and the linear accelerator will be performed with a shuttle system which uses an air-bearing patient platform for both procedures. This setup would allow for high-quality imaging with reduced artifacts while ensuring efficient treatment times, addressing the current gap in SBRT for patients with bilateral hip prostheses. This shuttle system is MR-compatible and uses an air-bearing technology that allows the patient to be effortlessly moved by a transfer sled from MR scanner couch to linac without any movement on their part as it utilises the same patient platform with treatment using supports, immobilization devices and stereotactic tools for both procedures. This means that the patient is scanned and treated in the same position, minimising the risk of translational and/or rotational positional changes during transfer between both devices. Thus, maximum use of image-based planning data is possible. By integrating a low-field MRI scanner with a dedicated treatment delivery system, the investigators can overcome existing limitations and improve treatment precision for this growing patient population.