467 / 1096
S452
ESTRO 36
_______________________________________________________________________________________________
Conclusion
The use of DWA for APBI improved the dose distribution
compared to that of non-coplanar 3D-CRT and coplanar
VMAT; this may reduce the risk of toxicity without
prolonging treatment time.
PO-0838 Treatment planning for the MR-linac: plan
quality compared with current clinical practice
A.J.A.J. Van de Schoot
1
, C. Carbaat
1
, B. Van Triest
1
, T.M.
Janssen
1
, J.J. Sonke
1
1
The Netherlands Cancer Institute, Department of
Radiation Oncology, Amsterdam, The Netherlands
Purpose or Objective
Clinical introduction of the MR-linac (MRL) involves
treatment planning using Monaco (Elekta AB, Stockholm,
Sweden) for both initial treatment planning and online
plan adaptation. Next to the presence of a magnetic field,
also several MRL-specific beam and collimator properties
need to be taken into account that could influence plan
quality. Our aim was to investigate the influence of MRL-
specific characteristics on plan quality for rectum cancer
and benchmark MRL plans against current clinical
practice.
Material and Methods
Eight rectum cancer patients treated on a conventional
CBCT-based linac (25 x 2.0 Gy) were included in this
retrospective study. For each patient, the clinically
acquired planning CT, delineated structures and
treatment plan generated with Pinnacle
3
(dual-arc VMAT,
10MV, collimator 20°, SAD: 100.0 cm) were available. The
same CT and structure set were used to create two MRL
treatment plans with Monaco: one plan with (MRL
+
) and
one plan without (MRL
–
) the presence of a 1.5 T magnetic
field. Both MRL plans were created using a 7-beam IMRT
technique incorporating MRL-specific properties (7MV,
collimator fixed at 90°, FFF, SAD: 143.5 cm). Plan
optimization was based on a class solution and objective
values were individually optimized. Also, a quasi MRL plan
was generated with Pinnacle
3
using a 7-beam IMRT
technique and comparable MRL properties (6MV,
collimator 90°, FFF, SAD: 143.5 cm). After rescaling (PTV
V
95%
= 99.2%), plans were accepted when the clinical
acceptance criterion was fulfilled (PTV D
1%
< 107%).
Quality differences between MRL
+
, MRL
–
and quasi MRL
plans were assessed by calculating PTV D
mean
, PTV D
1%
,
bowel D
mean
and bladder D
mean
. Also, D
mean
and D
1%
to the
patient excluding PTV
2cm
(i.e. PTV + 2.0 cm) were
determined. All MRL plans were benchmarked against the
clinically delivered treatment plans and tested for
significance (Wilcoxon signed-rank test).
Results
All MRL plans were clinical acceptable after rescaling.
Figure 1 shows an example of dose distributions for the
MRL plans and the clinical plan of one patient. The 7-beam
IMRT technique used for all MRL plans resulted in a minor
decrease in plan homogeneity, indicated by an increased
PTV D
mean
(Table 1). Also, all MRL plans showed a
significant increase in D
mean
for the bladder, bowel and
body compared to clinical practice. However, the clinical
relevance of these differences is expected to be limited.
Given the similar quality of MRL
–
and quasi MRL plans,
differences between MRL
+
plans and clinical practice are
mainly induced by the MRL-specific properties. The small
difference between MRL
+
and MRL
–
plans indicated limited
influence of the magnetic field on plan quality.