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for 16 fractions, 96 isocenters are needed). Random shifts
were sampled from a Gaussian distribution with standard
deviations σ ranging from 0, 2.5, 5, 7.5 and 10 mm. For
each standard deviation 3 simulations were performed.
Results
Figure 1 shows the results of the simulations. As expected
the PTV and CTV coverage decreases for higher σ values.
The V95% dose coverage of the PTV and the CTV decreases
below the clinically acceptable value of 95% when the σ
of the anterior–posterior motion exceeds 2.5 and 7.5 mm,
respectively.
Figure 1:
The V95% coverage (in %) for CTV (red cubes)
and PTV (blue diamonds) based on simulations for
different σ values. For σ higher than 7.5 mm the V95% of
the CTV is below the clinical acceptable value of 95%.
The difference in dose to the OARs σ ≤ 7.5 mm is
negligible.
Conclusion
Planning simulations in Pinnacle showed that the plans are
robust and the influence of intrafraction breath hold
motion on the dose becomes clinically relevant only when
σ > 7.5 mm.
Clinical data of the chest wall movement of patients in
anterior–posterior motion measured using an in-house
developed 3D camera breath hold monitoring system (ref
abstract) showed a σ of approximately 2.5 mm. This
suggests that for most patients residual intrafraction
motion during breath hold as achieved in our institute is
small enough to assure a good CTV coverage.
In this work only anterior–posterior motion is taken into
account for the simulations. Therefore, we need to
combine this with other inaccuracies before we can
clinically interpret these simulations for reducing the CTV
to PTV. Moreover, further investigation is required to
determine what threshold should be taken as warning
signal of the in-room breath hold monitoring system.
Currently we choose to set the threshold value to 7.5 mm
(maximum – minimum breath hold).
Electronic Poster: Physics track: Inter-fraction motion
management (excl. adaptive radiotherapy)
EP-1640 Dosimetric consequences of PTV margin
reduction in cervix cancer radiotherapy with VMAT
and IGRT
T. Berger
1
, M.S. Assenholt
1
, N. Jensen
1
, L. Fokdal
1
, J.
Lindegaard
1
, K. Tanderup
1
1
Aarhus University Hospital, Oncology, Aarhus C,
Denmark
Purpose or Objective
To evaluate the safety of PTV margin reduction in the
elective lymph node target (CTV-E) under condition of
Volumetric
Arc
Therapy
(VMAT)
and
daily
IGRT. Furthermore, the benefit of margin reduction for
Organs At Risk (OARs) was evaluated.
Material and Methods
Ten locally advanced cervix cancer patients treated from
December 2015 until June 2016, were analysed. The
patients were treated with 45Gy in 25 fractions with VMAT
and whole pelvic irradiation according to the Embrace II
protocol. Patients with para-aortic irradiation were not
included in this study. Daily image guidance was
performed with CBCT, bony fusion and couch correction
(translational and yaw). The ITV-45 defined the combined
elective lymph node CTV (CTV-E) and the ITV related to
the primary tumour. Four different dose-plans with ITV-45
to PTV margin of 0, 3, 5, and 8mm were evaluated. The
target constraints were: ITV-45 D99.99>42.75Gy and PTV-
45 D95%≥42.75Gy. CTV-E was assumed to move as a rigid
structure as lymph nodes are located mainly in relation to
bone and muscles. CTV-E was transferred from plan CT to
each CBCT by rigid bony registration with 6 degrees of
freedom. The propagated CTV-E was visually validated and
transferred back to the plan CT in the position of the
patient during treatment. For each of the 4 plans, the
accumulated D98 and D99.9 (average of the DVH of the 25
structures) was evaluated. For the 4 plans, V30 and V40
were extracted for bladder, bowel and rectum as well as
PTV volumes and body V43.
Results
Figure 1 shows the accumulated CTV-E D98 for all 10
patients with 0, 3, 5 and 8mm PTV margins. Generally, a
wider margin allows a better CTV-E coverage. With a 0mm
margin, CTV-E D98 is larger than 43.4Gy for 9 patients and
reduced to 42.2Gy for one patient. As for the 3mm PTV
margin, all 10 patients have an accumulated CTV-E D98
larger than 43Gy. CTV-E D99.9, with a 3mm margin,
reaches its lowest value for patient 2 with 39.3Gy. The
second lowest value is 40.6Gy for patient 7. For the eight
patients left, a 3mm PTV margin allows a minimum of
41.8Gy for CTV-E D99.9.
Table 1 shows for each of the 4 dose-plans the average
DVH parameters for the targets and OARs. An 8mm margin
(1621cm
3
) results in increased PTV volume of almost 50%
compared with a 3mm margin (1114cm
3
). V43 is reduced
by 469cm
3
when the PTV margin is reduced from 8 to 3mm,
corresponding to a relative volume reduction of about
30%. The average volume of bowel receiving more than
30Gy is decreased by 82 cm
3
when the PTV margin was
decreased
from
8
to
3mm.
Conclusion
The PTV and body V43 are significantly reduced through
margin reduction. For all patients, CTV-E D98 was larger
than 43Gy for PTV margins ≥3mm.
We notice a considerable advantage for the OARs when
decreasing the PTV margin from 8 to 5mm. The advantage
for OARs of decreasing the margin further to less than