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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. Gkogkou

1

, D. Wills

1

, A. Martin

1

, J. Phillips

1

, N.

Solomou

1

, A. Alexandrou

1

, C. Eveleigh

1

, Z.

Tassigianopoulos

1

, K. Geropantas

1

, T. Ajithkummar

2

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-Shirazi

1

, F. Arianfard

2

, S. Karbasi

1

, S.

Mousavi

1

1

Shiraz University of Medical Sciences, Physics Unit-

Radiotherapy & Oncology Department, Shiraz, Iran

Islamic Republic of

2

Shiraz 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 cm

2

, 8×8 cm

2

, 10×10 cm

2

, 15×8 cm

2

and 20×8 cm

2

.

Also beam attenuations of the head-and-neck (H&N)

extension and BreastSTEP boards were measured for an

8×8 cm

2

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 cm

2

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