ESTRO 2020 Abstract book

S1052 ESTRO 2020

Static electron fields are widely used for the treatment of breast boost or superficial tumors.Cerrobend aperture cut- outs for beam collimation are made using dedicated foam cutting machines. Our purpose is to find another way to build such electron block apertures with a low cost 3D printer and Acrylonitrile Butadiene Styrene plastic. Material and Methods In our facility electron beams are delivered using a Mevatron Primus linac (Siemens AG) and planned with Eclipse Treatment Planning System 15.6 (Varian Medical Systems Inc.). Aperture cut-outs are cast in Cerrobend blocks after surrounding customized Styrofoam molds with melted alloy. After electron beam aperture is designed in the TPS, DICOM RT Plan is exported to a file folder where it is converted to XML format using DCMTK-DICOM Toolkit software (OFFIS e.V.) and its dcm2xml.exe compilation. Thus, we can read and extract the aperture contoured points of each electron beam collimation, defined at SID=100cm plane, to a spreadsheet . The origin is located at beam central axis intersection. Coordinates are reescaled to backproject them into the electron tray plane (95 cm distance) and imported to Inventor® CAD software (Autodesk, Inc.) with its sketch Point tool. Using this program, we can also generate an inner contour to the aperture to give it at least 5 mm width. We extrude the sketch 1 cm height and export the outcome in a printable STL format. The mold design file is then opened in Ultimaker Cura 4.0 and set to be printed with ABS material using an extrusion temperature of 250 °C, heated bed temperature of 90 °C, 60 mm/s speed, 20% and concentric infill pattern. ABS material melting point is well above Cerrobend ( ≃ 70 °C) so it can withstand following cast of the aperture cut‐out. Once two 1cm height ABS molds are printed, they are aligned and joint applying a thin acetone layer between contact surfaces. Usual Cerrobend block making then takes place and a weight is placed on top of the ABS mold to prevent moving over the printed TPS electron aperture template. Thanks to cooling shrinkage, when Cerrobend block solidifies it is easy to remove the ABS mold from it. Final electron aperture cut-out is obtained. Results Electron beam aperture cut‐out is produced in an easy way. Figure 1 shows mold design within Inventor® after DICOM RT Plan processing and figure 2 the final printed mold when Cerrobend block cools. ABS mold external dimensions are 10x15 cm and 30 g weight. It takes 2 h printing time. After easily removing it from the Cerrobend block, cut-out aperture matches printed TPS electron aperture template (±0.5 mm).

Figure 2.

Conclusion We show the feasibility of producing customized Cerrobend blocks with the aid of ABS plastic mold cut-outs. Low cost 3D printers prove to be a versatile and efficient alternative to foam cutting dedicated machines in case they break down. ABS material is suitable and eligible for this purpose because of its physical properties and post- printing working capabilities. PO‐1793 BioXmark liquid fiducials to enable radiotherapy tumor boosting in rectal cancer feasibility trial M. Berbée 1 , T. Opbroek 2 , J. Paulissen 2 , R. Houben 2 , E. Van Limbergen 2 , B. Ta 2 , I. Steenbakkers 2 , F. Verhaegen 2 , R. De Ridder 3 , R. Jansen 4 , J. Melenhorst 5 , F. Bakers 6 , H. Grabsch 7 , J. Buijsen 2 , R. Canters 2 1 MAASTRO Clinic, Radiation Oncology, Maastricht, The Netherlands ; 2 Maastro, Radiation Oncology, Maastricht, The Netherlands ; 3 Maastricht University Medical Center, Gastroenterology, Maastricht, The Netherlands ; 4 Maastricht University Medical Center, Medical Oncology, Maastricht, The Netherlands ; 5 Maastricht University Medical Center, Surgery, Maastricht, The Netherlands ; 6 Maastricht University Medical Center, Radiology, Maastricht, The Netherlands ; 7 Maastricht University Medical Center, Pathology, Maastricht, The Netherlands Purpose or Objective Dose-escalation in rectal cancer may result in a higher complete response percentage and thereby omission of surgery and organ preservation. In order to introduce tumor dose escalation it is crucial to develop a technique that allows accurate image guided radiotherapy (IGRT). A prerequisite for IGRT is good visibility of the tumor on clinical imaging, or a tumor surrogate such as fiducial markers. Hypothesized advantages of the novel marker is a better visibility on different imaging modalities when compared to gold markers, an easy injection via thin needles and a better positional stability. Our goal was to determine the performance of a novel liquid fiducial marker i.e., BioXmark, in rectal cancer patients by assessing its positional stability, technical feasibility, visibility on different imaging m odalities and safety. Material and Methods This was a prospective, non-randomized, single-arm feasibility trial performed at Maastro and Maastricht University Medical Center, the Netherlands. In twenty patients referred for chemoradiation (25 x 2 Gy) for locally advanced rectal cancer four fiducial markers were injected in the submucosa around the rectal tumor during endoscopy prior to treatment (figure 1). Injection was performed using a 25G needle. Primary endpoint was positional stability of the markers, i.e., lack of marker migration, assessed by marker pair distance as a function over radiotherapy course time and measured on daily cone beam CT (CBCT). Analysis was done on individual patients basis using linear regression. Moreover, technical aspects

Figure 1.