ESTRO 36 Abstract Book

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S808 ESTRO 36
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gradient distributions consisting of many small beam
apertures. Accurate verification of such complex
treatment fields is still challenging and Gafchromic EBT3
and EBT-XD films play a key role as dosimeter with the
highest spatial resolution. Purpose of this work is to
evaluate whether well-known temperature dependences
of former Gafchromic film media are present with EBT3
and EBT-XD film. The observed systematic patterns of
temperature dependence are characterized with respect
to relevance in the pre-treatment verification.
Material and Methods
An Epson V750 pro flatbed scanner was used to perform
scan studies with 125 consecutive scans to purposely warm
up the scanner bed. During all scans two temperatures
probes were used to measure an average scanner bed
temperature. Square film pieces with irradiation dose
form 0 Gy to 64 Gy of 1 cm size were placed in the central
axis of the scanner bed. Evaluation was performed with
the software 'Image J” in all three colour channels in 8100
measurements in total.
Results
Temperature dependent relative transmission (%T)
readout differences known from former type Gafchromic
film media are found to still be present with EBT3 and EBT-
XD film type. Higher temperature results in most cases in
darker film readout. Interestingly, EBT3 red colour
channel changes temperature dependence direction
around 16 Gy irradiation dose, meaning that a higher
temperature results in less dose readout. Figure 1 and
Figure 2 illustrate the relation between the temperature
dependent transmission error and irradiation dose for
EBT3 and EBT-XD respectively.
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
1Norfolk and Norwich University Hospital, Oncology
Department, Norwich, United Kingdom
2Cambridge 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