Uppskattning av dosimetriska osäkerheter baserat på andel dos från penumbraregionen för VMAT-planer av olika komplexitet

• Absorbed dose delivered to the patient from a penumbra region have higher uncertainties compared to the dose delivered from the center of the treatment field. The magnitude of these uncertainties could be important input when evaluating absorbed dose to organs at risk (OARs) and target volumes, especially for plans of increased complexity. The aim of this work was to examine the fraction of penumbra dose in 3D for VMAT plans of different complexities. Methods and Materials: A C# software was developed in our department, able to estimate the fraction of penumbral dose in relation to the total dose, in each voxel and visualize it as a 3D distribution (uncertainty map). Twelve treatment plans of previously treated patients have been selected to represent different types of treatment geometry and different types of diagnose (prostate, head & neck, lung and gynecological cancer). The clinical plan used for the actual treatment of the patient was reoptimized in Eclipse TPS (Varian Medical Systems, Palo Alto, CA), to create one plan with reduced complexity (“simple plan”) and one of higher complexity (“complex plan”). These additional plans were generated by altering the MU constraint and the aperture shape controller (ASC) level, while retaining a similar dose distribution. The fraction of penumbra dose was calculated for both the clinical and the newly generated plans creating in total 36 uncertainty maps. Results: The generated uncertainty maps were able to highlight anatomical regions with high fraction of penumbra dose (e.g., above 50%). These regions were mostly observed outside the planning target volume (PTV). Generally, the fraction of penumbra dose, both inside and outside the PTV, was increased with increased plan complexity. For certain cases, the absorbed dose in OARs originated from penumbra regions to a considerable degree, even for the least complex version of the plan. An example is shown for the rectum in Figure 1. This observation proves the importance of evaluating dosimetric uncertainty in 3D. Conclusion: A connection was established between dose uncertainty due to fraction of penumbra dose and plan complexity. Furthermore, it is important to evaluate this uncertainty on a 3D-voxel-level.

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NATURVETENSKAP  -- Fysik -- Annan fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Other Physics Topics (hsv//eng)

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