A Proposed Formalism for Target Volume Delineation for Permanently Implanted Collagen Tile Embedded Cs-131 Brachytherapy in the Brain

Abstract

<h3>Purpose/Objective(s)</h3> To propose and establish the utility of a standardized formalism for delineating target volume contours on postoperative imaging to evaluate dosimetry for patients receiving permanent low dose rate (LDR) brachytherapy (BT) in the brain. The BT applicator considered for this work consists of 4 mm thick collagen tiles embedded with evenly spaced 3.5 U Cs-131 seeds designed to deliver a nominal dose of 60 Gy to a depth of 5 mm from the operative bed surface. The tiles feature a designed seed offset from brain surface of 3 mm, introduced with the intention of reducing maximum dose to brain tissue. <h3>Materials/Methods</h3> Collagen tiles are placed by neurosurgeons under surgical visualization on the resection surfaces known or suspected to harbor subclinical disease; in this setting the Clinical Target Volume (CTV) is defined as the region of parenchyma adjacent to tile placement to a depth of 5 mm. The algorithm proposed to generate a CTV is: 1) outline the brain tissue, 2) outline all seeds using a Hounsfield Unit-based threshold tool, 3) expand the seed contour by 3 mm to approximate the tile/brain interface, 4) outline the cavity visualized on a registered MRI using the seeds+3 mm contour as a guide, 5) use a Boolean operation to obtain the intersection of the cavity contour expanded by 5 mm, the seed contour expanded by 0.94 mm, and the brain contour, and 5) subtract the cavity. Manual adjustments to ensure only areas with suspected subclinical disease may be required. For 11 patients, a commercial Treatment Planning System was used to generate CTVs, calculate implant dose distributions, and obtain CTV dose statistics. <h3>Results</h3> Volumes (cc) of cavity contours across all patients ranged from 3.3 to 49.9 and the CTVs ranged from 6.9 to 27.1. Mean isodose value (Gy) covering 90% of the CTV (D90%) was 55.8 (45.5 – 73.8) and 95% of the CTV (D95%) was 49.4 (36.3 – 67.1). Mean minimum and maximum doses (Gy) to the CTV were 36.3 (19. – 64.7) and 426.3 Gy (265.6 – 573.9), respectively. Note, the maximum dose to the cavity is not accurately calculated by the TPS but is estimated to be in the range of 800 – 1500 Gy, indicating the tiles reduce maximum dose to brain by ∼28-53%. <h3>Conclusion</h3> A formalism to generate a CTV representing brain containing subclinical disease following gross tumor resection and collagen tile BT implant is proposed. The contouring algorithm utilizes the surgical cavity (well-seen on MRI), LDR seeds (well-seen on CT) and simple Boolean operations, resulting in a straightforward and reproducible CTV structure. The corresponding dose statistics (D90%, D95%, min, and max) proved to be useful metrics for dose review. The proposed standardized formalism for contouring CTV and its use in dose evaluation will be further tested to determine its utility in minimizing interobserver variability to evaluate the dosimetry of collagen tile BT implants in brain.