ESTRO 36 Abstract Book

S516 ESTRO 36 2017 _______________________________________________________________________________________________

through MRI at D and at BT. This 3DMD had been developed by the authors to provide precise reproducible topographic and quantitative information in one comprehensive overview (Fig1). Dimensions of GTV D , HR-CTV and IR-CTV for width, thickness and maximum height (GEC-ESTRO Recommendations) as assessed on MRI D/BT (SE T2 weighted sequences) was drawn at all grid levels and in all 3 dimensions as requested in the 3DMD. The cervical canal was taken as the central axis and the external os (surrogate for flange) as reference for the various dimensions. A qualitative observation was done based on the drawings of all 42 cases on individual maps. A quantitative analysis was done with SPSS v20. The dimensions (height, width and thickness) and volumes were compared and correlated (n= 42). Thereafter another quantitative analysis of the widths of GTVD, HRCTV and IRCTV at 0, 1, 2 and 3 cm and at NMD (Near Maximum distance from the central tandem/central canal) was done (n=84).

Conclusion The advanced schematic 3D mapping diagram provides precise topographic and quantitative 3D information on extent of disease and for CTV for BT, using repetitive MRI. There is a significant correlation of GTVD with HRCTV and IRCTV in regard to volumes and dimensions. This new tool may also be used for BT CTV definition based on GE and CT/US. PO-0941 Verifying the treatment planning system in individualized HDR brachytherapy of cervical cancer M. Van den Bosch 1 , B. Vanneste 1 , R. Voncken 1 , L. Lutgens 1 1 MAASTRO Clinic, Department of Radiation Oncology, Maastricht, The Netherlands Purpose or Objective In state of the art high-dose-rate (HDR) brachytherapy of the cervical cancer interstitial needles are regularly placed in addition to the standard applicators to increase the possibility for dose optimization, i.e. higher tumour coverage and/or sparing of OAR’s. The use of these needles enables more individualized treatment plans. Consequently dose distributions and dose plans have become highly individualized. As a result, the main output parameter of the planning system, the total reference air kerma (TRAK), is more difficult to verify. In this study, it is investigated whether the high risk clinical target volume (HR-CTV) can be used to predict the TRAK. Material and Methods 26 treatment plans of 10 cervical cancer patients were included in this study. In all patients the titanium Varian Fletcher applicator was inserted. The number of interstitial PEEK Varian needles was determined by the radiation oncologist at the time of the application. T2- weigthed MR scans were acquired in treatment position and used for delineation of the HR-CTV, intermediate risk- CTV (IR-CTV) and organs at risk (OARs). Contouring was done by the responsible radiation oncologist whereas a treatment plan was made by the radiation therapist using BrachyVision (algorithm: TG-43). The calculated TRAK values of each plan were rescaled to a source strength of 10 Ci and to a fraction dose of 7 Gy (prescribed to the HR- CTV). Results

Results The dimensions of the HRCTV followed closely that of GTVD, with some variations and exceptions. The IRCTV volumes were closely overlapping the HRCTV volumes. In most of the cases the HRCTV and IRCTV were encompassing the GTVD volumes. For the detailed quantitative results see table 1.