ESTRO 38 Abstract book

S202 ESTRO 38

Purpose or Objective In our institute, we used handmade applicators and needle templates for brachytherapy of head and neck tumors. The needle entry points were defined on the template according to a clinical drawing of anatomy and clinical target volume (CTV). The drawback of this method is the difficulty of finding the optimal needle configuration. The introduction of 3D printing based on CT and/or MR imaging gives the opportunity to create more conformal applicators and templates. The aim of this work is: - to compare simulated dose distributions based on 3D printed applicators with clinically achieved dose distributions - to demonstrate the clinical workflow and to share our first clinical experiences with 3D printed applicators. Material and Methods Before introduction of 3D printed applicators and templates, an in-silico treatment plan study was performed using the Oncentra Brachy (OCB) treatment planning system (Elekta). In 4 patients treated for cancer of the vestibulum nasi, a plan based on a 3D printed template was compared to the clinical plan (Figure 1). Subsequently, a clinical workflow for the use of 3D printed applicators and templates was developed, which consists of the following steps (Figure 2): 1) A CT and/or MRI is made for defining the CTV and the body contours; 2) The virtual placement of catheters is simulated in the TPS and optimized until an adequate dose distribution is achievable; 3) In a 3D design program (Autodesk, Fusion 360) the preplanned catheters and the lock inserts (Elekta) to fix the needles are subtracted from the applicator; 4) The applicator is fitted on the patient and a CT scan is made for reconstruction and dose planning; 5) The applicator is sterilized and used for treatment; 6) The treatment plan is made; 7) The treatment plan is executed; 8) During treatment a position verification CT is made; 9) The applicator is removed. Between November 2017 and September 2018 17 patients (10 lip, 4 vestibulum nasi, 3 other) have been treated using 3D printed applicators).

Results The in-silico study showed a reduction of the dose volumes V100, V150 and V200 when using 3D printed applicators without compromising CTV coverage (Figure 1). 17 patients were treated according to the new workflow (Figure 2). The applicators fitted well in and on the patients and the needles were placed according to the preplanned positions resulting in adequate CTV dose coverage. On average it took about 3 hours to create a 3D model for printing, whereas handcrafting a template may take up to 6 hours. Conclusion The possibility to optimize needle positions based on CT and or MR imaging in a single implant gives the advantage to adapt to a wide range of tumor shapes and anatomy constraints. This translates into a reduction of dose volumes when using 3D printed applicators without compromising CTV coverage. The clinical workflow using 3D printed applicators for brachytherapy was successfully implemented and resulted in a reduction of production and processing time. OC-0399 Comparison of high-dose interstitial brachytherapy vs. stereotactic treatment in patients with HCC F. Walter 1 , S. Gerum 1 , J. Well 1 , R. Shpani 1 , N. Lukas 1 , M. Cornelius 1 , F. Streitparth 2 , J. Ricke 2 , C. Belka 1 , S. Corradini 1 1 Radiation Oncology, Department of Radiation Oncology LMU, Munich, Germany; 2 Radiology, Department of Radiology, Munich, Germany Purpose or Objective The role of brachytherapy (BT) in hepatocellular carcinoma is under investigation. Objective of the study was to evaluate and compare normal liver tissue exposure of CT-guided high-dose interstitial BT to SBRT. Material and Methods The treatment plans of 11 patients (m:f 9:2) with a median age of 65 years (47-82) who received high-dose BT for hepatocellular carcinoma between 07/17 and 06/18 were retrospectively analysed. Lesions with a maximal diameter of <6cm were included. In all cases a prescription dose of 15Gy in single fraction was aimed. SBRT plans prescribing 37.5Gy in 3 fractions to the 65%-Isodoseline were retrospectively planned using the BT planning CT images. Regarding liver exposure, the V5 and V10 Gy of the single BT treatment was compared to the V15, V15.9 and V20Gy of SBRT in 3 fractions (EQD2; α/β = 3Gy). Results A total of 13 lesions were treated with high-dose CT- guided interstitial BT using 1 catheter in 7 patients (pts), two in 5pts and three in one case. GTV had a median diameter of 2.73cm (1.17-6.53cm), and a median volume of 5.03ccm (1.39-66.43ccm). The total liver volume ranged from 832.55-2194.46ccm (median 1461.88ccm). A medium dose of 15.1Gy was achieved in D100 (11.4-