Dose Computation of Keloids in Brachytherapy: Tg-43 or Model-Based-Dose-Calculation?

Abstract

Keloids are formed as a result of an overgrowth of granulation tissue at the site of healed skin after an injury. Interstitial HDR brachytherapy (BT) using plastic catheters is a common technique to reduce the risk of recurrences for keloids. In our clinic treatment planning is usually conducted using the TG-43 formalism in a virtual water phantom without 3D imaging. To assess the impact of backscatter and inhomogeneities for keloid BT treatments, we performed CT-based treatment planning and dose calculation with TG-43 and the Model-based dose calculation algorithm Acuros (Ac) for 6 keloid patients, having 8 keloids to treat. Results were analyzed. 6 patients with 8 keloids were included in our investigations. After excision of the keloids in a surgical procedure a single flexible plastic tube was implanted subcutaneous before closing the wound. Treatment delivery was carried out using a GammamedPlus afterloading machine (Varian Medical Systems, Palo Alto) with an HDR Ir-192 source. Treatment planning was done CT-based with the BrachyVision treatment planning system (TPS) applying TG-43. BT was given in three fractions with single doses of 6 Gy in 5 mm tissue depth. The skin was delineated in the TPS with 2 mm thickness. All plans were recalculated with Acuros. Dose parameters for TG-43 and the MBDCA were evaluated. The mean dose in 5mm distance from the center of the catheter was 8,9 (TG-43)/8,9 (Ac) Gy. Skin doses for D2 were 6,3 (TG-43) and 5,8 (Ac) Gy. For D0,2cc the following values were computed: 15,5 (TG-43) and 10,2 (Ac). Reference doses between TG-43 and Acuros are very similar for our small keloid patient cohort. Differences occurred in skin doses, the mean D0,2cc was 31% higher for TG-43 computations.