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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.
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
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