MSS12 Presentation Time: 4:40 PM: A Proposed Formalism for Postoperative Dosimetry Evaluation for Permanently Implanted Collagen Tile Embedded Cs-131 Brachytherapy in the Brain

Abstract

<h3>Purpose</h3> To propose a standardized formalism for delineating target volume contours and evaluating dose on postoperative imaging for patients receiving permanent low dose rate (LDR) brachytherapy (BT) in the brain. The BT applicator considered for this work consists of a 20 × 20 × 4 mm collagen tile embedded with four 3.5 U Cs-131 seeds with a 10 mm inter-seed spacing (GammaTile®, GT Medical Technologies, USA). Following maximum safe resection, multiple tiles are used to line the cavity and deliver a nominal dose of 60 Gy to a depth of 5 mm from the operative bed surface. The tiles are designed to limit the maximum dose to brain tissue via a 3 mm seed offset from brain surface that functions as an implanted internal compensator (Fig 1A). <h3>Materials and Methods</h3> Collagen tiles are placed by neurosurgeons under surgical visualization on resection the surfaces 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. As illustrated in Fig 1B-D, the algorithm proposed to generate a CTV is): 1) outline the brain, 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. Postoperative dose was calculated as per the AAPM TG43 3D formalism using the consensus dataset for the Cs-131 seed model utilized in the GammaTile device. Dose statistics pertaining to the CTV, including the mean isodose values covering 90% and 95% of the volume (D90% and D95%), the volume receiving at least 90% and 95% of the prescription 60 Gy (V90% and V95%), minimum dose, and maximum dose were tabulated for 11 patients using a commercial Treatment Planning System (MIM, MIM Software, USA). <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 D90% and D95% (Gy) to the CTV was 55.8 (45.5 - 73.8) and 49.4 (36.3 - 67.1), respectively. Mean V90% and V95% (% of total volume) to the CTV was 88.7 (77.6 - 99.8) and 85.5 (72.6 - 99.4), respectively. 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 <i>cavity</i> 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>Conclusions</h3> A formalism to 1) generate a CTV representing brain containing subclinical disease following gross tumor resection and collagen tile BT implant and 2) obtain dose statistics is proposed. The contouring algorithm utilizes the surgical cavity (well-seen on MRI), LDR seeds (placed with direct surgical guidance, and well-seen on CT) and Boolean operations, resulting in a straightforward and reproducible CTV. The proposed dose statistics (D90%, D95%, V90%, V95%, min, and max) proved to be useful metrics for dose review. The standardized formalism for contouring CTV and its use in dose evaluation will be further tested to determine its utility in minimizing interobserver variability. Once fully validated, this formalism will be used to generate recommendations for quantitative evaluation GammaTile implant dosimetry.