ESTRO 36 Abstract Book

S808 ESTRO 36 _______________________________________________________________________________________________

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.

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

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