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7-field 6MV IMRT for a conventional Elekta Agility linac
(Elekta AB, Stockholm, Sweden), 6MV FFF single 360° arc
VMAT using Pinnacle 9.10 (Philips Radiation Oncology
Systems, Fitchburg, WI) for a non-MRL and CyberKnife
treatment using Multiplan (Accuray inc, Sunnyvale, CA).
Plans were acceptable if the 16 dose constraints of the
PACE trial (NCT01584258) were achieved, without a major
variation to the protocol.
Results
Clinically acceptable 7-field IMRT MRL plans (see Figure 1)
were achieved in all ten patients. Clinically acceptable
plans were also achieved for all ten patients using 9-field
IMRT, non-MRL 7-field IMRT, non-MRL VMAT and
CyberKnife treatment. Clinically acceptable 5-field IMRT
MRL plans were only possible in seven patients. Table 1
summarises the number of exceeded constraints, mean
rectal doses and mean bladder V37Gy for each plan type.
Given the small patient group, exploratory ANOVA
analyses were undertaken for the number of co nstraints
missed, the rectum D1cc and the two most challenging
constraints to achieve- rectum V36Gy and bladder V37Gy.
For the MRL, 5-field IMRT MRL plans performed
significantly worse in all these analyses compared to 7-
field IMRT. 7-field IMRT MRL plans had significantly lower
rectal doses compared to CyberKnife plans. No significant
differences were seen between 9-field IMRT MRL plans and
non-MRL VMAT plans compared to 7-field IMRT.
Conclusion
MRL IMRT plans for prostate SBRT achieved the PACE trial
constraints in all patients with 9-field appearing similar to
7-field IMRT. 5-field IMRT in this set-up appears inferior
for the MRL. All platforms could produce clinically
acceptable plans. Further work is needed for dosimetric
validation and feasibility of MRL delivery.
PO-0829 Robustness of IMRT and VMAT for
interfraction motion in locoregional breast irradiation
R. Canters
1
, M. Kunze-Busch
1
, P. Van Kollenburg
1
, M.
Kusters
1
, P. Poortmans
1
, R. Monshouwer
1
1
Radboud University Medical Center, Radiation oncology,
Nijmegen, The Netherlands
Purpose or Objective
Conventional techniques for locoregional breast
irradiation using field abutment are challenging, even
more in combination with breath-hold irradiation and with
hypofractionation, since over- or underdosage may occur
more consistently in the abutment region in these
circumstances. IMRT and VMAT techniques are likely to
result in more conformal and homogenous irradiation,
though robustness for anatomical and posture variations is
possibly an issue. Compared to conventional plans, the
beams are not fully opposing and fields cannot be opened
manually outside the outer contour of the breast and the
body. Therefore, in this study we evaluated the robustness
of both an IMRT and a VMAT technique for daily variations
in patient positioning in comparison to our conventional
technique.
Material and Methods
20 Patients treated with a dose of 16x2.66 Gy using a
conventional technique to the breast and axillary lymph
nodes levels I to IV (Figure 1a) were replanned with both
an IMRT and a VMAT technique using Pinnacle
autoplanning. The IMRT technique consisted of 6 beams
with 20
o
spacing, while the VMAT technique consisted of
opposing pairs of 24
o
arcs (Figure 1). The delivered dose
was calculated using the cone beam CT (CBCT) (Elekta XVI)
images for each fraction to quantify the influence of
patient positioning, both for an online and offline
correction protocol. Contours were transferred from
planning CT to CBCT by deformable image registration
using Mirada RTx. Density overrides were applied to
account for imperfections in Hounsfield unit values on the
CBCT. IMRT and VMAT techniques were compared to the
conventional technique for the V95%, conformity index
(CI), mean lung dose and mean heart dose. The CTV-PTV
margin used is 7mm. Since the setup error is accounted for
when evaluating dose on the CBCT, we used the CTV for
the evaluation.
Results
Evaluation of the treatment plans for 20 patients showed
that V95% coverage of IMRT and VMAT plans was
comparable to conventional plans (Table 1). Conformity
was significantly higher for IMRT and VMAT. Mean lung
dose was approximately 0.7 Gy lower on average, while
mean heart dose increased by approximately 0.7 Gy using
IMRT or VMAT. Robustness evaluation of the dose on daily
CBCT’s using an online positioning protocol showed that
V95% coverage remained stable for conventional, IMRT an
VMAT. Significant conformity improvement was obtained
using both IMRT and VMAT, and small differences in mean
heart dose (+0.7 G) and mean lung dose (-0.8 Gy) were
found. Evaluation of an offline positioning protocol
showed similar results.
Conclusion
Presented IMRT and VMAT techniques show a similar
robustness for interfraction motion in locoregional breast
irradiation compared to the conventional technique, while
conformity of the target volume is increased significantly.
An offline positioning protocol would be sufficient for
clinically acceptable set-up accuracy.