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on the symmetry of the organ.
Conclusion
Rotational errors have to be corrected regardless of
translations magnitude.
Although rotations don’t impact
on CTV V95%, 6DoF corrections allow better PTV’s
coverage. Rotational errors could cause considerable
dosimetric changes in organs at risk and must be carefully
corrected in SBRT to avoid normal tissue complications.
An ongoing analysis on setup systems and margin
reductions has been planned.
EP-1652 A new position verification protocol for breast
cancer with integrated boost
K.L. Gottlieb
1
, E.L. Lorenzen
1
, J.D. Jensen
2
, M.H.
Nielsen
2
, M. Ewertz
2
1
Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
2
Odense University Hospital, Department of Oncology,
Odense, Denmark
Purpose or Objective
The use of integrated boost compared to sequential boost
allows shortening of the overall treatment time while
maintaining the same biologically equivalent dose to the
boost region. However when the target is large as in breast
cancer treatments there can be challenges in IGRT
registrations between the boost volume and total target
volume. The present study proposes and evaluates a
protocol for daily IGRT using CBCT of breast cancer
patients with integrated boost. A threshold is set for the
allowed difference between the whole target match and
the boost specific match. If the two matches differ by less
than the threshold the boost volume and the total volume
is treated in the same setup. If the two matches differ by
more than the threshold, the total target volume is
matched and treated and then secondly an additional
CBCT is performed and matched on the boost volume
which is then treated.
Material and Methods
In order to evaluate the match protocol all patients
receiving RT during the period 1/1-2016 and 1/7 2016
after breast conserving surgery and with lymph node
involvement were retrospectively included in the study.
The number of patients eligible for inclusion was 18 (3 was
rejected because of missing (2) or faint (1) clips). All
patients received daily IGRT with CBCT using XVI from
Elekta. CBCT’s for the first ten fractions were included
resulting in 180 CBCT. Two matches were performed for
each patient: First, a chest wall-match were performed,
where CBCT images were registered automatically to the
planning CT using a grey value translational match of the
thorax wall. Secondly, a boost specific match was
performed, where the surgical clips were manually
registered to their position on the planning CT.
Results
The systematic (Σ) and random errors (σ) between the
thorax wall and the clips based boost matches are seen in
the table. The CTV-PTV margins were calculated based on
the systematic and random errors using the Van Herk
margin recipe. These numbers are found both using no re-
matching and a re-matching if there were more than 5 mm
differences between the two matches. In the figure the
percentage of rematches needed for a given threshold of
allowed difference between the thorax wall and boost
match is shown in a). The boost CTV to PTV margin
required to account for the difference between chest wall
and boost match for a given threshold is shown in b). The
grey shaded area shows the 90 percent confidence interval
obtained by bootstrapping.
Conclusion
The presented protocol can reduce the required CTV to
PTV margin for the boost region by re-scanning and re-
matching the boost region only for patients where the two
regions differ by more than a set threshold. Results are
presented that can be used for selecting a threshold with
the corresponding required CTV-PTV margin. If e.g. a
threshold of 5 mm is used, the required CTV-PTV margin
can be reduced from approximately 8mm to 5mm and re-
scanning and re-matching will be required in only 5% of
the fractions.
EP-1653 PolymarkTM fiducial markers migration in
Prostate Image Guided Radiation Therapy using CBCT
images
C. Camacho
1
, I. Valduvieco
1
, J. Sáez
1
, A. Herreros
1
, J.
García-Miguel
1
, E. Agustí
1
, C. Castro
1
1
Hospital Clinic i Provincial, Radiotherapy, Barcelona,
Spain
Purpose or Objective
Polymer-based fiducial markers, FM (Polymark
TM
) location
was analyzed to test the idea that there is no intraseed
migration within the prostate, which is fundamental for
patient set-up good quality overthe entire course of
radiotherapy treatment (RT).