ESTRO 36 Abstract Book

S785 ESTRO 36 2017 _______________________________________________________________________________________________

Dosimetric effects induced by leaf details were more pronounced for FFF modalities. Due to the leaf tongue- and-groove, abutments of dose profiles using the leaf borders led to underdosages up to 13.7% (6MV), 12.3% (10MV), 15.5% (6MV-FFF), 14.4% (10MV-FFF), with respect to the open field profile (Fig.1, only 6MV and 6FFF are shown). On the other hand, abutments using the rounded leaf tips caused a dose increment up to 8.5% (6MV), 10.6% (10MV), 9.6% (6MV-FFF), 14.0% (10MV-FFF), with respect to the open field profile (Fig.2, only 6MV and 6FFF are shown). MLC-transmission at central axis was 1.2% (6MV), 1.4% (10MV), 1.0% (6FFF), 1.2% (10FFF). Same values were found in case of leaf interdigitation.

Metric (CAM) and Edge Area Metric (EAM) to account for clinical aspects of volumetric modulated arc therapy (VMAT) treatment plans on a control point level. Material and Methods Two complexity metrics, CAM and EAM, have been developed in a previous study 1 where the metrics have been validated using static multi leaf collimator (MLC) openings simulating fix control points in VMAT treatment plans. In this study, the two metrics have been further developed to be suitable for different types of MLC and adjusted to better differentiate between score values of clinical treatment plans. The metrics are also weighted against number of monitor units (MU) for each control point. Four prostate cancer and four head & neck cancer VMAT plans with the High Definition 120 leaf MLC (Varian) and one anal cancer, three prostate cancer and three head & neck cancer VMAT plans with Millennium 120 leaf MLC (Varian) were used in this study. Results The complexity scores on a control point level is illustrated by an example of a head and neck cancer VMAT treatment plan with the High Definition MLC, figure 1.

The dotted lines in figure 1 shows unmodified versions of the metrics for all control points, which have been validated for static fields. The complex region defined in EAM have been reduced from enclosing an area of 5 to 2.5 mm on both sides of the MLC edges (dashed line, figure 1a), to better differentiate between complexity scores. CAM have been adjusted to be suitable for the High Definition MLC. Measurements were taken every 5 mm in both directions to give one measure for each MLC leaf pair 1 for the Millennium MLC. Since the central leaves are 2.5 mm for High Definition MLC the distances are now measured every 2.5 mm. The correlation between dose differences and complexity scores for the static fields 1 were remained for the improved versions of EAM and CAM. The scores were also weighted against number of MU according to an inverse exponential function to mainly lower the impact of the complexity scores for control points with no or very few MU (solid lines, figure 1). In this example the beam was turned off for beam directions coming through the shoulders and those parts should not contribute to the complexity. Larger variations in complexity scores for adjacent control points are seen for CAM compared to more consistent scores for EAM. The

Conclusion The Varian HD120 MLC was dosimetrically characterized for several beam qualities. The leaf geometric details have a strong influence on the dose distribution, especially at the location of leaf abutments. These effects need to be considered in treatment planning, especially for intensity modulated techniques. The reported measurements are propaedeutic to the modeling of the observed effects in a treatment planning system for stereotactic radiotherapy. EP-1486 Further developments of two complexity metrics to consider clinical aspects of VMAT treatment plans J. Götstedt 1 , A. Bäck 2 , A. Karlsson Hauer 2 1 University of Gothenburg/Sahlgrenska University Hospital, Radiation Physics, Gothenburg, Sweden 2 Sahlgrenska University Hospital, Therapeutic Radiation Physics, Gothenburg, Sweden Purpose or Objective The objective of this study is to further develop two aperture-based complexity metrics Converted Aperture