ESTRO 36 Abstract Book

S778 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective The overall objective is to develop a 3D complexity metric for VMAT treatments. The complexity scores will be presented as a distribution in a 3D volume and correlate to the fraction of penumbra dose. Regions lacking charged particle equilibrium that might cause dose calculation errors and regions sensitive to multileaf collimator (MLC) positioning errors are located in the penumbra of the MLC opening. The hypothesis is that an increased amount of dose in a voxel that originates from a penumbra region will correlate to the probability of increased difference between planned and delivered dose in that voxel. In this pilot study, 2D distributions are analyzed to validate the correlation to differences between calculated and measured dose. Material and Methods A C# software with dynamically linked MatLab® (Mathworks, Natick, MA) libraries was developed. The input to the software is the DICOM-file of the treatment plan from where the MLC positions are collected, i.e. the appearance of the beams eye view (BEV) plane. 1. The pixels of the BEV plane (pixel size 0.25 mm) is structured binary in open beam (1) or blocked beam (0). The pixels of the BEV plane is also structured binary in field edge (1) or no edge (0). 2.

3.

The binary BEV from step 1 is convolved with a Gaussian function normalized to 1. This will weight the complexity score higher in regions with higher dose and lower in the low dose region. This is called a pseudo dose (PD) distribution. The binary BEV from step 2 is convolved with a box function (1 inside box, 0 outside). This will define a region, with the width of the box, as the region of interest where the complexity metric will have a score ≠ 0. The convolution from step 3 is multiplied with the convolution from step 4. This gives a 2D distribution of complexity scores.

4.

Conclusion 2D distributions of complexity scores were successfully calculated and comparisons to 2D distributions of differences between calculated and measured dose show conformities that are promising for further development of calculations in a 3D volume. EP-1458 3D dose reconstruction on CBCT for daily monitoring of delivered patient dose K. Eilertsen 1 , F.C. Vidaurre 2 , Y. Pylypchenko 3 1 Eilertsen Karsten, Medical Physics, Lommedalen, Norway 2 Oslo University Hospital, Medical Physics, OSLO, Norway 3 Oslo University Hospital, Medical Phjysics, OSLO, Norway Purpose or Objective The ability to reconstruct the delivered 3D dose distribution using the CBCT acquired on every fraction, can help to verify that the dose to both target as well as organs at risk comply with the treatment intentions throughout the treatment course. The objective of this work has been to study the dosimetric accuracy and feasibility of daily dose monitoring using a novel system for 3D dose reconstruction onto kV CBCT from electronic portal images and machine log data acquired during The tested dose reconstruction engine is based on a collapsed-cone convolution algorithm and is an integrated part of the PerFRACTION3D system (SunNuclear). The method uses a forward projection of MLC leaf position measurements from the EPID, as well as monitor chamber dose rate and output data derived from the associated machine log files. Two different approaches were taken to test the concept: First, kV CBCT of the ArcCHECK measurement array (SunNuclear) was acquired on Varian TrueBeam and Elekta Synergy linacs. Then a number of different patient plans and were delivered to the detector array. The generated EPIs and log files were imported to PerFRACTION3D, and the dose distributions reconstructed treatment execution. Material and Methods

5.

The 2D distributions of calculated complexity scores for different box widths and Gaussian sigmas for 30 MLC openings were compared to the 2D distributions of difference (absolute values) between calculated and film measured dose at 10 cm depth in water for the same openings. Results The correlation between the ratios “mean complexity score/mean value of PD distribution” and “mean absolute difference between calculated and measured dose/calculated mean dose” for 30 MLC openings is shown in figure 1. The sigma of the Gaussian and the box width had negligible influence on the correlation. However, those parameters will have influence when the fraction of penumbra dose is evaluated for each pixel separately. They can be chosen to match the dose gradient and width of the region of relevant dose differences at a specific depth, see example of a 2D visual comparison between complexity scores and differences between calculated and measured dose at 10 cm depth in figure 2.