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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. Boccia
1
, L. Manti
2
, S. Clemente
3
, C. Oliviero
3
, F.
Perozziello
2
, R. Liuzzi
4
, M. Conson
1
, L. Cella
4
, R. Pacelli
1
1
Federico II University School of Medicine, Department of
Advanced Biomedical Sciences, Napoli, Italy
2
University of Naples Federico II, Department of Physics,
Napoli, Italy
3
Azienda Ospedaliera Universitaria Federico II, Radiation
Oncology Department, Napoli, Italy
4
National Council of Research CNR, Institute of
Biostructures and Bioimaging, Napoli, Italy
Purpose or Objective
The innovative radiotherapy techniques such as Intensity-
Modulated Radiation Therapy (IMRT) and Volumetric-
Modulated Arc Therapy (VMAT) allow for more conformity
of dose to the tumor target and sparing of healthy tissues.
However, these techniques require an increase of monitor
units (MUs) and therefore an increase of
treatment/delivery times for each fraction. In addition, a
higher dose outside the field caused by photons scattering
in the flattening filter (FF) is expected. Flattening Filter-
Free (FFF) photon beams can deliver higher dose rates and
reduce the treatment time by about a factor 4 compared
to conventional photon beams. Additional benefits also
include reduced head scatter, a lower peripheral dose and
neutron contamination. Purpose of the present study is to
compare the radiobiological effects of FFF versus FF
photon beams in mammary epithelial tumor cells (MCF7).
Material and Methods
MCF7 cells were irradiated with conventional and FFF 6MV
photon beams using a TrueBeamSTx (Varian Medical
Systems). Different dose rate values were considered (600
MU/min and 1400 MU/min). The cells were exposed to
0.25, 0.5, 1.0, 2.0 and 4.0 Gy doses. The number of
monitor units required to deliver the desired doses to the
cells was calculated using Pinnacle
3
(Philips) Treatment
Planning System (TPS). Irradiations were performed with
the flasks placed on 5 cm of equivalent water phantom
(RW3) slabs and gantry angle at 180° to deliver
homogeneous dose to the cell layer. A check of the actual
dose delivered to cells was done exposing 9 thermo-
luminescent dosimeters (LiF:Mg,Ti TLD-100) placed on the
bottom of one of the irradiated flask. Clonogenic cell
survival of MCF7 cells was determined. Cell survival data
were fitted to linear-quadratic model.
Results
In the investigated dose range (0-4Gy), no statistically
significant differences on breast cancer cell survival
curves was observed a) with or without flattening filter
(600 MU/min vs. 600 FFF MU/min) and b) at different dose
rates (600 FFF MU/min vs 1400 FFF MU/min). Cell survival
curves are reported in figure 1.