ESTRO 36 Abstract Book

S798 ESTRO 36 2017 _______________________________________________________________________________________________

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

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