1.5T MRI for cathether placement and treatment planning in high dose rate brachytherapy for prostate cancer

Abstract

MRI provides superior visualization of the prostate and surrounding anatomy making it the modality of choice for imaging the prostate gland. This study was performed to determine the feasibility and dosimetric quality achieved when placing high dose rate (HDR) prostate brachytherapy catheters under MR image guidance in a standard 1.5T scanner. In a secondary objective, catheter encroachment and dose delivered to the path of the neurovascular bundles (NVB) was determined. Patients with intermediate and high-risk localized prostate cancer received MRI-guided HDR brachytherapy boosts (1050cGy each) before and after a course of external beam radiotherapy (4600cGy). Preliminary MRI evaluations and procedures were performed in a standard 1.5T MRI scanner with a 60cm diameter bore. Patients were placed under general endotracheal anesthesia for the entire procedure. An endorectal coil was modified with immobilization hardware and rigidly affixed perpendicular to a custom designed perineal template. The coil was inserted and placed against the anterior rectal wall adjacent to the prostate gland, centered on the urethra. Upon satisfactory device positioning, a 3D-SSFP image volume which comprises the template, pubic arch, and prostate gland was acquired. These images were sent to an adjacent PC workstation with a custom written image-visualization and targeting program. Needle paths potentially encroaching on the pubic arch and urethra were identified and marked, and a peripheral brachytherapy catheter arrangement was designed with the assistance of the pre-plan. Fast-spin-echo images of the prostate gland (scan time = 28sec) were then acquired for improved tumor definition and to confirm the adequacy of the brachytherapy catheter placements. These images were projected within the MR room to allow the operator more timely feedback of catheter position. HDR brachytherapy inverse treatment planning (IPSA-PLATO, Nucletron Inc.) was performed using an axial T2-weighted FSE image corrected for spatial distortion obtained at the end of the procedure. The PTV consisted of the prostate gland as well as any visualized extracapsular extension of disease. Dose constraints were applied to the urethra and rectum. Dose-volume histograms were generated for the PTV, urethra, rectum, and path of the NVB on contrast-enhanced MRI. Catheter encroachment on the path of the NVB was documented. Fourteen MRI-guided transperineal brachytherapy and biopsy procedures have been performed in 7 patients. Prostate gland sizes ranged from 21.6 to 81.8 cc (mean 40.6cc). The median PTV V100 was 94%, with a mean of 92% (CI 90%-94%). Urethral V125 ranged from 0 to 18% (median 5%), and rectal V75 ranged from 0 to 5.8 % (median 0.3%). In all cases, lesions highly suspicious for malignancy could be visualized on the procedural MR images, and extracapsular disease was identified in two patients and included in the PTV. On average, 21% of the left and 12% of the right NVB paths received the prescription dose, with a V150 of 3% bilaterally. In 8 of 14 cases, catheters directly encroached on the path of the NVB below the apex of the gland. HDR prostate brachytherapy in a standard 1.5T MRI scanner is feasible and achieves favorable dosimetry with high-quality image guidance. This procedure may offer a therapeutic advantage for those patients with extracapsular extension of disease visualized on MR images, whereby extracapsular disease may be included in the high dose region. MRI also permits the evaluation of catheter trauma and radiation dose to the NVB. This system establishes a procedural platform ideally suited to the evaluation and integration of anatomic, functional, and molecular imaging techniques in cancer radiotherapy