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ESTRO 36 2017
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PO-0916 Feasibility and potential for treating loca lly
advanced non-sma ll cell lung cancer with a MR-linac
M.J. Menten
1
, H. Bainbridge
2
, M.F. Fast
1
, S . Nill
1
, F.
McDonald
2
, U. Oelfke
1
1
The Institute of Cancer Research and The Royal Marsden
NHS Foundation Trust, Physics, Sutton, United Kingdom
2
The Institute of Cancer Research and The Royal Marsden
NHS Foundation Trust, Lung Unit, Sutton, United
Kingdom
Purpose or Objective
Treatment plans for MRI-guided radiotherapy delivered
with an MR-linac vary from those designed for
conventional linacs due to differing technical
specifications of dose delivering systems and the presence
of a static magnetic field. This study investigated this
issue for radiotherapy of locally advanced non-small cell
lung cancer (LA NSCLC) by comparing treatment plans for
a conventional Versa HD linac (Elekta AB, Stockholm,
Sweden) and the Elekta 1.5 T MR-linac. Furthermore, the
effect of reducing planning target volume (PTV) margins
on the MR-linac was examined.
Material and Methods
Ten patients with LA NSCLC were retrospectively re-
planned six times using the Monaco treatment planning
system, research version 5.19.00. Three plans were
designed according to our institution’s protocol for
conventionally fractionated treatment (55 Gy/ 20
fractions) and three plans following guidelines for isotoxic
dose escalation up to 79.2 Gy/ 44 fractions
(NCT01836692). In each case, two plans were designed for
the MR-linac, using IMRT with nine equidistant, coplanar
beams, either with standard (7 mm) or reduced (3 mm)
PTV margins, while one plan was created for a
conventional linac using VMAT with standard margins.
Treatment plan optimization and dose calculation were
conducted under consideration of magnetic field effects.
Potential to escalate tumour dose was quantified for the
isotoxic plans, and differences in dose-volume metrics
were analysed for conventionally fractionated treatment
plans. Statistical significance was evaluated using a
paired
t-
test after confirming normal distribution and
correcting for multiple endpoints.
Results
All generated treatment plans fulfilled their respective
planning constraints and would have been clinically
acceptable. With the conventionally fractionated
schedule small differences in dose-volume metrics could
be identified with statistical significance (see table). Mean
lung doses were similar between conventional and MR-
linac plans, whereas high lung doses were reduced and low
lung doses increased on the MR-linac (graphically
illustrated in the figure). In terms of dose-escalation, the
mean achievable doses were 75.4, 74.0, and 76.9 Gy for
Versa HD, MR-linac (standard margins) and MR-linac
(reduced margins) respectively, with inferiority of the
standard margin MR-linac plans versus the Versa HD plans
(
p
=0.003).
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