ESTRO 36 Abstract Book

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S884 ESTRO 36 2017
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inter-session translation and rotation was shown in Fig.1.
No significant differences were observed in translation in
LR and SI direction, while AP translation was significantly
smaller. This could be partially explained by the fixed AP
level of the MR-sim couch. The yaw rotation was
significantly larger than roll and pitch rotation, which may
result from the relatively inadequate immobilization
performance in this direction, but yet to be further
investigated.
Conclusion
Small head and neck setup uncertainty and thus high
positional repeatability could be achieved on a 1.5T MR-
sim, suggesting the potentials of MRI for precise HN-RT.
EP-1651 Dosimetric impact of rotations correction in
Stereotactic RT. How much a 6DoF couch is useful?
S. Chiesa 1, S. Menna2, A.R. Alitto 1, L. Azario2, G.C.
Mattiucci 1, M. Ferro3, M. Massaccesi1, A. Re1, A.
Piermattei2, V. Valentini1, M. Balducci1
1 Università Cattolica del Sacro Cuore -Fondazione
Policlinico Universitario A. Gemelli, Radiation Oncology
Department- Gemelli-ART, Rome, Italy
2 Università Cattolica del Sacro Cuore - Fondazione
Policlinico Universitario A. Gemelli, Medical Physics
Department, Rome, Italy
3 Università Cattolica del Sacro Cuore -Fondazione di
Ricerca e Cura “Giovanni Paolo II”, Radiation Oncology
Department, Campobasso, Italy
Purpose or Objective
Setup accuracy and organ motion control are essential in
stereotactic radiation therapy (SRT) due to the use of
sharp dose gradients and tight margins around the target
volume. PRO-ART Project was designed to evaluate set up
errors and dosimetric impact of rotational patient
positioning correction using a 6-Degree of Freedom (6DoF)
robotic couch.
Material and Methods
Patients with lung, brain and abdominal lesions were
enrolled and immobilized with Uni-frame or trUpoint Arch
for brain, Breast board or Body Pro-Lok for chest and
abdomen (CIVCO support system) lesions. Eclipse™
Treatment Planning Systems (Varian Medical
System®,Palo Alto,CA) was used for dose calculations of
VMAT plans. A daily KV-Cone Beam Computed Tomography
(CBCT) was performed before each treatment fraction and
translational and rotational shifts were identified,
recorded and applied on the Protura TM Robotic couch to
correct the position. Using MIM 5.5.2 software, the
simulation CT was rigidly registered with CBCT,
considered as primary CT, for each fraction. After
registration, translational errors were applied to correct
the CT position, obtaining a tCT, i.e. CT with only
translational errors correction. Then, rotational errors
were corrected too, obtaining roto-translated CT (rtCT).
Reference treatment plan was copied to translated tTP
and to roto-transalted rtTP and dose distributions were re-
calculated, obtaining two treatment plans for each
fraction. DVHs dosimetric parameters were compared.
Results
In this study 179 CBCT were performed on 47 patients (14
with primary tumours and 33 with metastatic lesions) and
358 treatment plans were calculated (179 tTP and 179
rtTP). Geometric and dosimetric analysis are reported in
Table 1. There was no correlation between translational
and rotational shifts. Rotational shifts resulted greater
than 1° in 40% of cases. Average variations in PTV and CTV
V95% were negligible, but we observed variations of PTV
V95% >2% in 30%, 8% and 12% of cases (CBCTs) respectively
for brain, thorax and abdomen patients. OARs proximity
to the target caused variations >2% and rotations around
each axis could determine important changes depending
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. Gottlieb1, E.L. Lorenzen1, J.D. Jensen2, M.H.
Nielsen2, M. Ewertz2
1Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
2Odense 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.