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dose-high gradient region (P2). In our QA program for ion
chamber verification, we have established a 3% of
deviation in P1 points, and 5% deviation in P2 points.
Results
We have recalculated the dose using DIAMOND in P1 and
P2 points over the definitive patient plan, and also over
the definitive plan simulated in the phantom. The results
were compared with the TPS values. Furthermore, we
have compared this deviation with the deviation between
the measurements and the dose calculated in the TPS. The
following table shows some results:
To establish new tolerances, we have looked into the 95%
confidence intervals for dose deviation, and also we have
done a ROC analysis between the new method (DIAMOND)
and the old (ion chamber). Even though, we show in our
results that 95% confidence intervals are asymmetric, we
have chosen our tolerances in a symmetric interval. We
believe that this decision will make the analysis more clear
and will avoid errors in the future.
As we show in figure 1, differences may appear between
phantom and patient results for P2 points in VMAT plans.
These differences could be associated with differents
algorithms used in the TPS (Monte Carlo) and the Diamond
(Clarkson), and their differences in a heterogeneity
medium. Although differences exist, we can correlate the
results between the new and the old method, over the
phantom’s plan and also over the patient’s plan. Even
though, we have obtained good results in the global plan
analysis, we have seen that it’s possible to obtain big
differences in a field. This can be explained because the
calculated point may be in a penumbra region, where the
uncertainties in the calculation are bigger than the
established tolerances.
Conclusion
We have checked the DIAMOND’s viability to verify IMRT-
VMAT plans, also we have calculated tolerances to apply
in clinical use. With this new method, we will decrease
the time in the verification and also decrease the time
between the moment that the plan is calculated and the
beginning of the treatment.
EP-1485 Dosimetric characterization of an high
definition MLC for stereotactic radiotherapy
treatments.
F. Rosica
1
, F. Bartolucci
1
, C. Fidanza
1
, A. Savini
1
, D.
Ciuffetelli
1
, A. Rastelli
1
, G. Orlandi
1
1
USL di Teramo Azienda ASL 4, Medical Physics, Teramo
TE, Italy
Purpose or Objective
High definition multi leaf collimators (MLCs) with reduced
leaf width are beneficial for treating small lesions in
modern stereotactic radiotherapy. In general, leaves have
special design details that may have a strong impact on
the delivered dose. The aim of this study was to
characterize the dosimetric impact of such details in the
Varian HD120 MLC for several beam qualities including
flattening-filter-free (FFF) modalities.
Material and Methods
A set of MLC-collimated fields was irradiated using a
Varian TrueBeam STx linear accelerator equipped with the
HD120 MLC (beam qualities: 6MV, 10MV, 6MV-FFF, 10MV-
FFF). These fields were designed using several abutment
configurations (e.g. picket fence) in order to enhance the
dosimetric impact of the MLC design details such as
tongue-and-groove and rounded leaf tip. Dose profile
scans were measured in a motorized water phantom using
small detectors (IBA-Razor stereotactic diode and PTW-
microDiamond 60019). Dose profiles of the abutted fields
were summed and compared with the dose profiles of the
corresponding open fields. In addition, average MLC
transmission was measured using a Farmer ion chamber
(IBA-FC 65-G).
Results
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.