ESTRO 36 Abstract Book

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S475 ESTRO 36
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Purpose or Objective
To analyze the effects of considering the real distribution
of random errors (s) within our patient population in the
outcome of setup correction protocols. Results are
compared to those predicted by Van Herk`s margin
formula (VHMF) considering constant random errors.
Material and Methods
Displacement data from 31 prostate and 31 head and neck
(HN) treatments were employed in this study, based on
640 and 540 CBCT images respectively. Values of σ at each
direction were calculated by obtaining the standard
deviation of the corrections during the treatment of each
patient. The proposed distribution for the modelling of
heterogeneous σ2 is an IG distribution (eq. 1). This kind of
distribution has been demonstrated to be suitable for
modelling random errors (Herschtal et al, Phys Med Biol
2012:57:2743-2755). Parameters a and b of the IG
distribution can be obtained from the mean value and
standard deviation of the measured σ2 distribution.
Treatment margins proposed by VHMF for a No Action
Level using the first 5 fractions for setup correction (NAL
5) protocol were obtained by considering a constant σ for
all the patients.
Given the margins proposed, the patient coverage for the
real σ distribution was obtained by weighting the dose
coverages for each combination of σ values at each
direction with the probability that a patient has those
values of σ, based on the fitted IG distributions.
Results
Results are shown in Table 1. It can be seen that, if
heterogeneities in random error distribution are taken into
account, the coverage probability yields values smaller
than those predicted by VHMF when homogenous σ is
considered. After this results were obtained, calculations
for different sets of margin were done. It was found that
in the HN case, margins had to be increased 1 mm at each
direction to obtain coverages of a 92 %, while in the
prostate case, margins had to be increased 1.4 mm in all
directions in order to achieve a coverage of the 90%. These
results suggest that the effects of heterogeneous random
errors depend on the characteristics of the random error
distribution of the patient population.
Conclusion
The effect of heterogeneous random errors should be
taken into account when applying treatment margins, its
effects depend on the characteristics of the patient
population and should be analyzed for each treatment
location at each institution.
PO-0872 Respiration motion management strategy for
sparing of risk organs in esophagus cancer
radiotherapy
S.B.N. Biancardo1, J.C. Costa1, K.F. Hofland1,2, T.S.
Johansen 1, M. Josipovic1
1 Rigshospitalet, Department of Oncology- Section of
Radiotherapy, Copenhagen, Denmark
2 Zealand University Hosptial, Department of Oncology-
Section of Radiotherapy, Naestved, Denmark
Purpose or Objective
Esophagus and the organs at risk (OAR) nearby move with
respiration. The purpose of this study was to determine if
respiratory gating or deep inspiration breath hold (DIBH)
facilitate dose reduction to OAR.
Material and Methods
CT image sets from ten patients were analysed. Esophagus
and OAR were delineated on end expiration (EE) and end
inspiration (EI) phases of the 4DCT and on DIBH CT. 5 cm
long mock GTVs were delineated in the proximal (P),
medial (M) and distal (D) part of the esophagus. CTVs were
defined by expanding the GTVs according to our clinical
practice. CTV to PTV margin was 7 mm. Relative position
of OARs and target were evaluated with cumulative
distance volume histograms (DiVHs) [Wu et al. Med Phys
2009], calculated for the part of the OAR located in the
beam path. The most and least optimal phase for
treatment was selected by comparing the percent of the
OAR volume located within the distance intervals A (below
2.5 cm), B (2.5-5.0 cm) and C (5.0-7.5 cm) from the PTV.
The organ sparing achieved or lost, by changing treatment
from FB to a specific breathing phase, was estimated by
assuming that FB can be simulated with 50% EE and 50%
EI.
Results
Esophagus elongation during 4DCT was median 11mm
(range 2-20mm) and from EE to DIBH 23mm (10-42mm).
Lung volume increased 13.3% (6.9-24.9%) from EE to EI and
63.5% (34.1-120.8%) from EE to DIBH. Absolute volume of
lung in the beam path either increased or remained largely
constant upon inspiration in all patients. In seven P, four
M and four D targets, the absolute volume of lung located
within 5 cm of the PTV increased; however, increase in
the total lung volume still resulted in either a reduction or
a largely unchanged percent lung volume located within 5
cm of the PTV. Results extracted from DiVHs are presented
in Table 1.
DIBH was the optimal treatment phase for all P and M
targets and 8/10 D targets. For all targets EE was the least
optimal phase.
Heart displacement was ≤12mm on 4DCT and ≤26mm from
EE to DIBH. Relative heart volume DiVH’s are shown in
Figure 1 for 3 patients. The same respiration phase is
clearly not optimal for all patients, neither for M nor for
D targets. EE was most optimal for heart sparing in two M
and three D, EI in four M and three D and DIBH in four M
and four D targets. EE was least optimal in four M and two
D, EI in two M and three D and DIBH in four M and five D
targets.