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ESTRO 36 2017
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Adding a single aperture per iteration yielded the lowest
cost function value per aperture included in the treatment
plan. However, adding one aperture per modality every
iteration resulted in a plan of comparable quality in only
120 iterations of the column generation loop instead of
580 for the single-aperture scheme.
Conclusion
MBRT planning produced a clinically realistic chest wall
plan combining the advantages of photon and electron
radiotherapy. The final plan was robust to initial
conditions despite the iterative nature of column
generation. This work opens the door to robust multi-
modality planning and delivery.
EP-1546 MR-Linac based single fraction ablative
radiotherapy for early-stage breast cancer: a planning
study
T. Van 't Westeinde
1
, K.R. Charaghvandi
1
, J.K. Horton
2
, S.
Yoo
2
, V. Scholten
1
, B. Van Asselen
1
, H.J.G.D. Van den
Bongard
1
1
UMC Utrecht, Radiotherapy, Utrecht, The Netherlands
2
Duke University Medical Center, Radiotherapy, Durham,
USA
Purpose or Objective
Our department is currently working on the
implementation of an MRI-linear accelerator (MR-linac) for
several tumor sites. Dose distribution in the presence of a
magnetic field can be affected by the electron return
effect (ERE), which can occur at tissue boundaries like skin
and lung. Other MRL settings such as the fixed collimator
and isocenter position may also influence the RT plan. We
investigated the dosimetric feasibility of single fraction
ablative radiotherapy in the prone and supine position for
early-stage breast cancer using an MR-Linac approach.
Material and Methods
Preoperative contrast-enhanced (CE) CT and MRI scans
were used from 10 cT1-2N0(sn) breast cancer patients
included in an ongoing clinical trial on preoperative
ablative radiotherapy. The gross tumor volume (GTV) was
delineated on matched CE MRI- & CT-scans in the supine
position. The clinical target volume (CTV) was created by
expanding the GTV with 2 cm, thereby excluding skin and
chest wall. The planning target volumes PTV
GTV
and PTV
CTV
were created by expanding both GTV and CTV 3 mm,
excluding the skin. Prescribed doses were 20 Gy for PTV
GTV
and 15 Gy for PTV
CTV
. Rationale for dose prescription and
organs at risk (OAR) constraints for a single fraction
ablative RT were previously defined (1). Adequate target
coverage was defined as 99% of the PTV should receive
≥95% of the prescribed dose. Intensity modulated
radiation therapy (IMRT) plans were made in the presence
of a 1.5T magnetic field, using Monaco Research version
5.19.01 planning system. 7 beams with individually chosen
beam angles were used for each plan. Dosimetry was
evaluated in all simulated plans.
Results
For supine positioning the median volume that received at
least 95% of the prescribed dose was ≥99% for PTV
GTV
and
PTV
CTV
. The median GTV volume was 1.1 cc, the median
CTV volume 72.9 cc, the median PTV
GTV
volume 5.3 cc and
the median PTV
CTV
volume was 104.9 cc. The median ratio
PTV
CTV
to ipsilateral breast was 11.6%. The predefined OAR
constraints were achieved in all plans (table 1).
Conclusion
Single fraction ablative radiotherapy in supine position on
the MR-Linac is dosimetrically feasible. The feasibility of
prone MR-linac treatment will be available at the 36
th
ESTRO conference.
(1) Charaghvandi RK, den Hartogh MD, van Ommen et al.
MRI-guided single fraction ablative radiotherapy for early-
stage breast cancer: a brachytherapy versus volumetric
modulated arc therapy dosimetry study. Radiother Oncol
2015
Dec;117(3):477-482.
EP-1547 Optimal treatment planning for H&N:
evaluation of a predict parotid glands sparing tool
N. Delaby (France), S. Martin, O. Henry, E. Chajon, C.
Lafond
3
1
Centre Eugène Marquis, Radiotherapy, Rennes CEDEX,
France
2
INSERM, U1099, Rennes, France
3
University Rennes 1, LTSI, Rennes, France
Purpose or Objective
The complexity of the clinical objectives in IMRT yields a
variability in treatment planning, especially between
operators. A major difficulty is currently to appreciate the
optimality of treatment plans. A previous published model