ESTRO 36 Abstract Book

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S532 ESTRO 36 2017
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Conclusion
Our results show that a prompt increase in γH2AX foci at
24 hours post-implant relative to baseline may be a useful
measure to assess elevated risk of late RT related
toxicities for PPB patients. A subsequent investigation
recruiting a larger cohort of patients is warranted to verify
our findings.
Poster: Radiobiology track: Radiobiology of breast
cancer
PO-0971 Estimating second malignancy risk in IMRT and
VMAT in radiotherapy for carcinoma of left breast
J. Selvaraj1, V. Sakthivel2
1 The Canberra Hospital, Medical Physics and Radiation
Engineering, Canberra, Australia
2 Advanced Medical Physics, Medical Physics, Houston-
Texas, USA
Purpose or Objective
IMRT and VMAT produce dose distributions with superior
target dose uniformity and normal tissue sparing.
However, this increases amount of volume receiving very
low doses substantially compared to conventional
techniques. This increases the risk of radiation-induced
second malignancy (SCR) as reported in the literature. The
aim of this study is to use a mechanistic radiobiological
model which is more accurate in predicting the dose-
response at low as well as high dose levels to estimate
SCR. Studies have shown patient age at exposure is
important in estimating SCR, thus patients’ age is also
accounted for in the SCR estimation. Moreover, the
mechanistic model also takes cell proliferation and dose
fractionation into account.
Material and Methods
Fifty IMRT and VMAT plans with similar dose-volume
objectives were selected for the study. The prescription
dose was 50 Gy in 25 fractions to the PTV. Monte Carlo
based dose calculation engine was the preferred choice as
it is more accurate at low dose levels which is more
relevant for estimating SCR. Appropriate model
parameters were taken from the literature for the
mechanistic model to calculate excess absolute risk (EAR),
lifetime attributable risk (LAR), integral dose and relative
risk (RR) for both lungs, contralateral breast, heart and
spinal cord.
Results
The mean MU in IMRT and VMAT plans were 751.1±133.3
and 1004.8±180 respectively for IMRT and VMAT. The mean
EAR values per 10,000 person years (PY) estimated for
IMRT and VMAT treatments including gender-specific
correction with and without age correction factor are
shown in figure 3. The mean EAR values with one standard
deviation without age correction were 42.4±11.3,
10.6±6.0, 12.3±6.7, 1.9±0.7 and 0.6±0.3 for left lung, right
lung, contralateral breast, heart and spinal cord
respectively for the IMRT plans. These values were
51.9±19.7, 28.7±11.4, 31.9±13.4, 2.3±0.8 and 1.5±0.8 for
the VMAT plans. However the values were reduced with
age correction, especially for the contralateral breast.
The values obtained with age correction were 44.6±11.9,
11.2±6.4, 5.4±4.0, 1.4±0.5 and 0.3±0.2 for left lung, right
lung, contralateral breast, heart and spinal cord
respectively for the IMRT treatments and 54.6±20.6,
30.2±12.0, 13.8±8.6, 1.6±0.6 and 0.9±0.5 for the VMAT
treatments.
Conclusion
Results showed VMAT plans had a higher risk of developing
second malignancy in lung, contralateral breast, heart and
cord compared to IMRT plans. However, the increase in
risk was found to be marginal. The increase in risk was
greater in both IMRT and VMAT for left lung and
contralateral breast compared to other organs included in
the study. Incorporating the age correction factor
decreased the risk of contralateral breast SCR. No strong
correlation was found between EAR and MU.
PO-0972 Breast cancer cell survival using flattening
filter-free beam compared to a standard flattened beam
M. Boccia1, L. Manti2, S. Clemente3, C. Oliviero3, F.
Perozziello2, R. Liuzzi4, M. Conson1, L. Cella4, R. Pacelli1
1Federico II University School of Medicine, Department of
Advanced Biomedical Sciences, Napoli, Italy