(OA43) Delineation of a Cardiac PRV Using Real-Time Magnetic Resonance Imaging for Cardiac Protection in Thoracic and Breast Radiation Therapy

Abstract

Historically, increased cardiac radiation dose has been associated with late cardiotoxicity events in patients with thoracic and breast malignancies. Recently, a prospective trial also revealed correlation between cardiac dose and early toxicity, including reduced overall survival in patients receiving curative-intent treatment. While breath-hold techniques reduce motion, they cannot account for cardiac cycle motion. We conducted a prospective cine magnetic resonance imaging (cMRI) study and applied an established scale-invariant feature transform (SIFT)-based algorithm to track and evaluate heart motion. We hypothesized that cMRI could be used to define a cardiac planning organ-at-risk volume (PRV) for routine application in radiation planning to improve cardiac protection. Imaging was performed on a 1.5 Tesla MR scanner (Philips Ingenia 1.5T MR System). 16 acquisitions were obtained per subject using a balanced-steady-state gradient echo sequence. Single-plane cMRI was performed in four sequential sagittal and four sequential coronal planes, spanning the heart in equidistant left-right (LR) and anterior-posterior (AP) intervals. Free-breathing (FB) and deep-inspiratory-breath-hold (DIBH) acquisitions were performed at each plane. In-plane cardiac motion was assessed on all image sets for each subject. The reference image and each image within the corresponding acquisition image set were registered via a previously published technique that describes pixels by high-dimensional descriptors, incorporating nearby intensity gradients rather than pixel intensity alone. Each descriptor, defined by the scale-invariant feature transform (SIFT) algorithm, is a set of histograms of intensity gradients within sub-regions surrounding a given pixel. Pixel trajectories were then identified by minimizing an objective function to locate best-matching descriptors between the reference image and each image within an acquisition. Subject-specific pixel motion ranges were derived by averaging slice-by-slice motion of each pixel to yield mean values for AP, LR, and superior-inferior (SI) directions. For each subject, average maximum range of pixel displacement and average pixel-based interquartile range (IQR) of displacement were calculated. These values were then utilized and averaged across the cohort to calculate planning organ-at-risk volume (PRV) expansions at FB and DIBH. 20 subjects were enrolled with median age 34.5 years (range 20-78), BMI 23.75 (range 17.7-33.3), and height 64 inches (range 57-69). A total of 3,120 image frames were collected per patient in coronal and sagittal planes, at DIBH and FB, with 62,400 frames analyzed across the cohort. SIFT methodology was successfully implemented in all patients to track motion. The cohort average of maximum motion ranges comprised PRV margin expansions of 13.7±1.8mm SI, 6.9±1.4mm AP, and 7.7±1.2mm LR plane at FB; expansions for DIBH were 7.1±1.5mm SI, 5.1±1.4mm AP, and 5.6±1.3mm LR. IQR-based PRVs, calculated to encompass the most frequent range of cardiac displacement, comprised 3.7±0.7mm SI, 1.2±0.4mm AP, and 1.4±0.3mm LR at FB and 1.8±0.6mm SI, 1.0±0.3mm AP, and 1.3±0.4mm LR at DIBH. IQR-based PRV expansions were then successfully applied for both locally-advanced lung cancer and breast cancer treatment planning. We successfully utilized SIFT-based motion-tracking for analysis of 62,400 prospectively gathered cine MR image frames to identify and apply a cardiac PRV for treatment planning to improve cardiac protection in breast and thoracic radiation.