ESTRO 36 Abstract Book

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S798 ESTRO 36 2017
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Figure 1 Transmission variation per °C of EBT3 film in
dependence of irradiation dose
Figure 2 Transmission variation per °C of EBT-XD film in
dependence of irradiation dose
Conclusion
The results show that all dose levels will be influenced
differently by temperature. The common practice of
recalibration of a calibration curve with 1-3 film pieces
with known dose and the same evaluation temperature is
not sufficient to remove temperature dependent readout
error. In SRT/SRS/SBRT highest possible precision in
dosimetry is not only required in high dose region, but also
in medium and low dose areas (OAR relevant) at the same
time. For highest precision we therefore suggest to work
in a temperature controlled scanner room in order to
achieve the highest possible precision in Gafchromic film
dosimetry.
EP-1507 Comparison of Pencil Beam Convolution and
Analytical Anisotropic algorithms for lung cancer
P. Gkogkou1, D. Wills1, A. Martin1, J. Phillips1, N.
Solomou1 , A. Alexandrou1, C. Eveleigh1, Z.
Tassigianopoulos1, K. Geropantas1, T. Ajithkummar2
1 Norfolk and Norwich University Hospital, Oncology
Department, Norwich, United Kingdom
2 Cambridge University Hospitals NHS Foundation Trust,
Oncology Department, Cambridge, United Kingdom
Purpose or Objective
Radical radiotherapy using 55Gy in 20 fractions over 4
weeks is an acceptable curative treatment for early-stage
medically inoperable lung cancer. The limitation of
commonly used Pencil Beam Convolution (PBC) algorithm
in terms of inaccurate dose calculation in inhomogeneous
tissues such as lung has led to the development of new
algorithms such as AAA. However, the true clinical impact
of the differences in dose calculations using PBC
and Analytical Anisotropic (AAA) algorithms in terms of
local control and survival is not known. We compared the
clinical outcome of patients with early-stage lung cancer
who received radical radiotherapy using either PBC or
AAA.
Material and Methods
18 patients were treated using PBC and 38 using AAA
during 2009-2014. All patients had PET-staged IA or IB
disease. None of the patients in this study had received
chemotherapy. Residual or recurrent diseases were
identified by follow-up imaging. Local failure was defined
as tumor recurrence or progression inside the PTV covered
by the 95% isodose. This was identified anatomically and
volumetrically as PTV-T (Planning target volume around
the clinical target volume) = CTV+0.7cm. The minimum
follow-up time was 2 years after the completion of the
treatment.
Results
The median age at diagnosis was 77 years (range 64-87)
for the PBC group and 79 years (range 64-94) for AAA. The
median follow-up period was 34 months for the AAA vs 26
months for PBC (p=0.006). The median survival was 39
months for AAA vs. 23 months for the PBC group (p=0.008).
On univariate analysis, there were no significant
prognostic factors for either relapse or overall
survival. There were 5 (27.7%) local failures in the PBC
group and 8 (21%) in the AAA. No marginal recurrences
were found. Using the cox-proportional hazards regression
analysis, there were no statistically significant difference
in local (p=0.285) or metastatic (p=0.191) recurrence
between the two groups.
Conclusion
Radical radiotherapy in our cohort study showed an
excellent tumor control and low-risk tumor recurrence in
the treatment volume. The results of this retrospective
study showed that there was no statistical difference
between the two algorithms regarding recurrences,
whereas AAA gave a significantly better median survival.
EP-1508 Quantification of skin dose and photon beam
attenuation for the iBEAM couch and Compact
accelerator
M.A. Mosleh-Shirazi1, F. Arianfard2, S. Karbasi 1, S.
Mousavi 1
1Shiraz University of Medical Sciences, Physics Unit-
Radiotherapy & Oncology Department, Shiraz, Iran
Islamic Republic of
2Shiraz University of Medical Sciences, Radiology &
Radiobiology Department- School of Paramedical
Sciences, Shiraz, Iran Islamic Republic of
Purpose or Objective
This work aims to accurately quantify the attenuation and
skin dose increase for 6 MV photon beams of an Elekta
Compact linear accelerator transmitted through the
Elekta iBEAM Standard carbon fiber couchtop and related
immobilisation devices. A study of this combination of
couchtop and linac has not been reported. Other novel
aspects of this work include the use of Monte Carlo (MC)
simulation in conjunction with thin-buildup diode
measurements for better estimation of the clinically-
relevant dose to skin basal cell layer, as well as putting
the results into context by direct comparison of PDDs in
the buildup region and further depths with a typical Co-60
treatment unit utilizing a ‘tennis racket’ type couch
without a Mylar sheet (Theratron Phoenix).
Material and Methods
Manufacturer-supplied information was used to add an MC
model of the couchtop to an existing detailed model of the
linac head. Beam attenuation by the couchtop was
simulated and measured using an ionisation chamber both
in air and in a water-equivalent cylindrical phantom at
gantry angles 125°, 135°, 150°, 165° and 180° for field
sizes 5×5 cm2, 8×8 cm2, 10×10 cm2, 15×8 cm 2 and 20×8 cm2.
Also beam attenuations of the head-and-neck (H&N)
extension and BreastSTEP boards were measured for an
8×8 cm2 field. The effect on skin dose was studied by
measurement of percentage depth dose (PDD) in the
buildup regions of 180° gantry beams of both linac and Co-
60 units, using an electron diode in a Perspex slab
phantom for 5×5 cm 2, 10×10 cm 2 and 20×20 cm2 field sizes,
as well as the corresponding linac MC simulations.
Results
The simulated and measured couchtop attenuation results
agreed to within 0.4%, which further validated the MC