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ESTRO 36 2017
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Conclusion
In general, plan complexity indices exhibit a weak
correlation with the fraction of small gaps in VMAT plans.
Similarly, setting limits on the number of MUs, or MU/Gy
has no clear impact on the fraction of small gaps
generated during the optimization process. A good
prediction of the fraction of small gaps can be obtained
from the median gap of the plan. Thus, tolerance levels
for the fraction of small gaps can be defined in terms of
the median gap of the plan, which can be useful to
generate more robust VMAT plans.
Electronic Poster: Physics track: Treatment planning:
applications
EP-1540 Optimal fractionation schemes for
radiosurgery of large brain metastases
J. Unkelbach
1
, H.A. Shih
2
1
University Hospital, Radiation Oncology, Zürich,
Switzerland
2
Massachusetts General Hospital, Radiation Oncology,
Boston, USA
Purpose or Objective
Stereotactic radiosurgery (SRS) is an established
treatment option for patients with brain metastases.
While small metastases are successfully treated with
single-fraction SRS, the optimal fractionation scheme for
large lesions is unclear and involves a trade-off between
the number of fractions, tumor dose, and dose to normal
brain. In this work, we demonstrate that spatiotemporal
fractionation schemes, ie. delivering distinct dose
distributions in different fractions, may improve the
therapeutic ratio for these patients.
Material and Methods
Fractionation effects are described using the biologically
effective dose (BED) model, assuming alpha-beta ratios of
10 in the tumor and 2 in normal brain. Treatment planning
is performed based on objective functions evaluated for
BED instead of physical dose. Constrained optimization
techniques are used to ensure that all treatment plans
have identical target coverage and conformity. Plans are
compared regarding integral BED to normal brain, and BED
to normal brain adjacent to the tumor.
Results
Traditional fractionation schemes deliver the same dose
distribution in all fractions. Increasing the number of
fractions reduces the BED to normal brain in the high dose
region adjacent to the tumor for a fixed tumor BED.
However, the integral BED to normal brain typically
remains approximately the same. Spatiotemporal
fractionation can lower the integral BED via the following
mechanism: Distinct treatment plans for different
fractions are designed such that each dose distribution
creates a similar dose bath in the normal brain surrounding
the tumor, ie. exploit the fractionation effect. However,
each fraction delivers a nonuniform target dose such that
a high single-fraction dose is delivered to alternating parts
of the tumor (Figure 1). Thereby, partial
hypofractionation in the tumor can be achieved along with
relatively uniform fractionation in normal brain, leading
approximately to a 10% reduction of BED in normal brain.
Figure 1: Illustration of spatiotemporal fractionation
using 4 fractions for a large brain metastasis (26cc).
Treatment planning was performed for rotation therapy
(VMAT or Tomotherapy) such that a cumulative BED
equivalent to 20 Gy single-fraction dose is delivered to
the target, while minimizing normal brain BED. It is
assumed that 1x20Gy in the tumor corresponds to 2x13Gy,
3x10Gy, and 4x8Gy. Hence, a uniformly fractionated 4-
fraction treatment increases the total physical dose from
20 Gy to 32 Gy. Spatiotemporal fractionation delivers
approximately 20 Gy in a single fraction to parts of the
tumor. Thereby, the same tumor BED is achieved with a
lower physical dose, which translates into a net BED
reduction in the normal brain where the dose is relatively
uniformly fractionated.
Conclusion
Delivering distinct dose distributions in different fractions
may improve the therapeutic ratio. For patients with brain
metastasis this may lower integral dose to normal brain
and reduce cognitive decline.
EP-1541 4D dose reconstruction using a standard TPS
in combination with a respiratory motion model
M. Ziegler
1
, J. Woelfelschneider
1
, H. Prasetio
1
, C. Bert
1
1
University Hospital Erlangen, Radiation Oncology,
Erlangen, Germany
Purpose or Objective
Dynamic tracking (
DT
) is one approach to treat intra-
fractionally moving tumors due to a conformal irradiation
while sparing of healthy tissue. Clinical tracking systems
rely on correlation models to predict the internal tumor
position based on external surrogates. However,
assessment of the actually delivered dose is still
challenging as many treatment planning systems (
TPS
) do
not have the ability to calculate dose on time-resolved
(
4D
) computed tomography images. The aim of this study
was to determine the possibility for 4D dose
reconstruction of DT patients using a common TPS and a
respiratory motion model that is based on external
surrogates.
Material and Methods
The University Hospital in Erlangen is equipped with a Vero
system (Brainlab, Feldkirchen, Germany) that is used to
treat patients with intra-fractionally moving tumors by
DT. This system further provides the extraction of the
patients’ surface as a surrogate by external infrared
markers during the treatment. These surrogates are used
as an input parameter for a 4D motion model to predict