523 / 1096
S508
ESTRO 36
_______________________________________________________________________________________________
Conclusion
It is feasible to generate conventionally fractionated
treatment plans for LA NSCLC patients on a 1.5 T MR-linac
with minor differences in dose-volume metrics, which are
unlikely to be clinically meaningful. When using standard
PTV margins, isotoxic dose escalation was limited on the
MR-linac. However, reducing margins alleviates these
observed effects. This study only represents an early
indicator of the treatment implications of MRI-guided
radiotherapy. It is conceivable that the availability of MRI-
guidance will result in further benefits through inter- and
intrafractional treatment adaptation.
PO-0917 Nationwide audit of small fields output
calculations in Poland
W. Bulski
1
, K. Chelminski
1
1
The Maria Sklodowska-Curie Memorial Cancer Center,
Medical Physics Department, Warsaw, Poland
Purpose or Objective
The delivery of accurate intensity-modulated radiation
therapy (IMRT) or stereotactic radiotherapy depends on a
multitude of steps in the treatment delivery process.
Within the treatment planning system’s (TPS) dose
calculation algorithm, various unique small field
dosimetry parameters are essential, such as multileaf
collimator modeling and field size dependence of the
output. One of the most considerable challenges in this
process is to determine accurate small field size output
factors. Modern radiotherapy routinely involve s the use
of small radiation fields as components of IMRT. Because
of the difficulties in commissioning small field data, a set
of field size dependent output factors could prove to be
an invaluable tool to confirm the validity of an individual
institution’s dosimetry parameters. Such a set of data has
been prepared by the Radiological Physics Center (RPC),
M. D. Anderson Cancer Center, Houston. The RPC has
gathered multiple small field size output factor datasets
for X-ray beam qualities, ranging from 6 to 18 MV, from
Varian, Siemens and Elekta linear accelerators. These
datasets were measured at 10 cm depth and ranged from
10×10 cm
2
to 2×2 cm
2
. Within the framework of the IAEA
CRP E2.40.16 project "Development of Quality Audits for
Radiotherapy Dosimetry for Complex Treatment
Techniques, a methodology of the audit of small field
output performance was established.
Material and Methods
The participants had to calculate t he output factors for
the beams formed by the multi-leaf collim ator (MLC).
The results of their calculations were compared with the
reference RPC data. 32 Polish radiotherapy departments
took part in the audit. In total, 65 beams were audited.
The participants of the audit were asked to calculate the
number of monitor units (MU) for the delivery of a
prescribed dose to water with square fields of different
sizes. A dose of 10 Gy was prescribed to a reference point
at 10 cm depth on the central axis, at 100 cm source-to-
phantom distance (SFD). The output factors for five field
sizes, 10×10, 6×6, 4×4, 3×3 and 2×2 cm
2
, shaped by a
multileaf collimator (MLC), were calculated.
Results
For Elekta accelerators, all the calculation results show a
deviation from the reference values lower than 3%. For
Siemens and Varian accelerators, the resulting
calculations for fields larger than 2×2 cm
2
differ less than
4%. For 2×2 cm
2
large fields formed by Siemens and Varian
MLC, the differences between the calculated and
measured output factors often exceed 5%, but still are
below 10%.
Conclusion
The RPC measured values provide a consistent dataset for
small field output factors that can be used as a redundant
QA check of a treatment planning system dosimetry data
for small-field treatments. The results of the audit are
very useful for the participants who should carefully
investigate any detected discrepancies between the
standard dataset and calculated values, with attention to
the specific beam model.
PO-0918 Radiotherapy and Her2 targeting agents:
synergism and antagonism in clonogenic and confluence
assays
N. Suchowerska
1
, J. Toohey
2
, S. Carroll
2
, L. Rogers
2
, G.
Lyons
3
, J. Beith
4
, A. Dos Santos Esteves
2
, D.R. McKenzie
5
1
Chris O'Brien Lifehouse, Radiation Oncology,
Camperdown- Sydney, Australia
2
Chris O'Brien Lifehouse, Radiation Oncology,
Camperdown Sydney, Australia
3
Royal Prince Alfred Hospital, Dermatology Cancer,
Sydney, Australia
4
Chris O'Brien Lifehouse, Medical Oncology, Camperdown
Sydney, Australia
5
University of Sydney, School of Physics, Sydney,
Australia
Purpose or Objective
Her2 amplified cancers, comprising 15-20% of patients
presenting with breast cancer, are now routinely
prescribed Trastuzumab (Herceptin), a monoclonal
antibody targeting Her2 receptors, leading to a significant
improvement in outcomes in this previously high risk
breast cancer subtype. Such targeting agents are rapidly
being introduced into the clinic, based on trials showing a
survival advantage. Now combination therapies with drug
conjugates have emerged. The biological interactions of
combined targeting agents, when given concurrently with
radiation, are not well described. Our aim is to identify
whether there is a synergistic or antagonistic interaction
between targeting agents and ionising radiation for two
distinct Her2+ subtypes.
Material and Methods
Two molecular subtypes of HER2+ breast cancer cell lines
were used: HCC-1954, which is ER and PR hormone
negative and BT-474, a luminal B which is ER negative and
PR positive. Both cell lines were treated to Her2 targeting
agents (Trastuzumab and T-DM1) and radiation (6MV
photons, 0 to 4Gy), individually and in combination to
identify whether the response was synergistic, additive or
antagonistic. The alpha/beta ratio was experimentally
determined for each cell line. Synergy (
S
) is defined as the
fractional difference between the observed (
S
o
) and the
predicted survival for each treatment given alone (
S
1
x
S
2
):
The observed response was determined using two assays:
the clonogenic assay and the confluence assay.
Results
The alpha/beta ratio for HCC-1954 (ER-/PR-/Her2+) and
BT474 (Luminal B ER-/PR+/Her2+) are found to be 35 Gy
and 5 Gy respectively, highlighting a heterogeneous
treatment response. The survival of HCC-1954 was not
affected by Trastuzumab alone, but when combined with
radiation, a synergistic interaction was observed. BT-474