ESTRO 36 Abstract Book

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S470 ESTRO 36 2017
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IMC coverage was equivalent between DIBH and FB plans
(Table 1). Twenty-one patients (70%) had ≥5% reduction in
ipsilateral lung V20 with DIBH (Figure 1). A reduction in
lung metrics was observed with an absolute reduction in
mean ipsilateral lung V20 by 7.8% (range: 0.0-20.0%; Table
1). There was a mean reduction of 42.3cc (range: 0-178.9)
in the volume of liver receiving 50% of the prescription
dose. The differences in cardiac doses were statistically
significant, but unlikely clinically significant (Table 1).
Seven patients with stages IA-IIIC right-sided breast cancer
treated between April to October 2016 received a DIBH
scan as a result of a FB V20Gy ≥30%, treatment volumes
included IMCs in all cases. In this cohort a ≥5% difference
between plans was observed for all patients with a mean
FB V20 of 34.8% (range: 30.0-43.4%), which was reduced
by an absolute value of 11.8% (range: 5.4-19.1%) with
DIBH.
Conclusion
DIBH reduced mean ipsilateral lung V20 by 7.8% and mean
lung dose by 3.4Gy. For some patients, the volume of liver
receiving ≥25Gy can also be reduced with DIBH. DIBH
should be available as a treatment strategy to reduce right
lung V20 without compromising on IMC coverage for
patients with right-sided breast cancer during loco-
regional RT. This strategy can be advantageous when the
ipsilateral FB V20 ≥30%, a value that prompts many
radiation oncologists to exclude IMCs. Within a small
prospectively evaluated cohort reflective of a change in
institutional policy, we have observed an absolute
reduction in mean ipsilateral lung V20 by 11.8% with DIBH.
PO-0871 Study of the effect of heterogeneous setup
random errors in treatment margins
D. Sevillano1, A.B. Capuz Suárez 1, R. Colmenares
Fernández1, R. Morís Pablos1, M.J. Bejar Navarro1, D.
Prieto Moran 1, F. García-Vicente1
1 Hospital Ramón y Cajal, Medical Physics, Madrid, Spain
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. Josipovic 1
1Rigshospitalet, Department of Oncology- Section of
Radiotherapy, Copenhagen, Denmark
2Zealand 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