ESTRO 36 Abstract Book

S521 ESTRO 36 _______________________________________________________________________________________________

Results The number of needles varied from 0 to 8 (average 3.8 needles per application). The rescaled TRAK and mean volume of the HR-CTV was 0.37 cGy m 2 (range: 0.23-0.50 cGy m 2 ) and 26.8 cc (range: 8.0-59.1 cc), respectively. In general, the TRAK value increased with volume. In figure 1a the TRAK values are plotted against the HR-CTV. The relation between these parameters can be described by a linear equation (see figure 1b). When setting an upper and lower limit of two standard deviations a 95% confidence interval can be derived and outliers can be identified. The higher TRAK value of these outliers suggest the volume that received the prescribed dose is much larger than the HR-CTV. This was true for these plans: due to excessive reduction of the HR-CTV, a higher dose in the IR-CTV was desired and planned in the direction of the uterus top.

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).

Conclusion The HR-CTV can be used to predict the TRAK value. Outliers may indicate abnormalities in treatment planning and further inspection of their dose distributions is required. In this study, the deviations in the dose distributions of the outliers were accepted, since they resulted in an improved individualized treatment plan. Using this relationship, the quality assurance of the treatment plan can be improved. PO-0942 Real time in vivo dosimetry for cervix HDR brachytherapy - feasibility study using a MOSFET J. Mason 1 , P. Bownes 1 1 Leeds Cancer Centre, Medical Physics & Engineering, Leeds, United Kingdom Purpose or Objective Implementation of in vivo dosimetry (IVD) in brachytherapy is partly limited by lack of c ommercially available devices that support IVD. In this study a modified rectal retractor and MOSFET were used to investigate the feasibility of real time IVD for cervix brachytherapy with simulated treatment plans delivered in a water phantom. Poster: Brachytherapy: Physics