ESTRO 36 Abstract Book

S799 ESTRO 36 2017 _______________________________________________________________________________________________

model. The highest couchtop attenuation (7.6%) was measured at 135° gantry and 5×5 cm 2 field size. The attenuation values of the H&N extension and breast boards at 180° gantry angle were 6.9% and 6.7%, respectively. MC results showed that the couchtop increased dose at various depths of basal cell layer (0.1-0.4 mm) by 55.3%- 63.2%. The measured dose increase at 0.4 mm depth ranged between 60.6% and 74.6% with field sizes 20×20 cm 2 to 5×5 cm 2 , the corresponding Co-60 unit increase for a 10×10 cm 2 field being 18.1%. To directly compare two prescribed treatment beams, when the PDDs were normalized at 10 cm depth for a 10×10 cm 2 field, although dose to subcutaneous tissues was always higher with the Co-60 unit, it produced an at least 49.7% lower skin basal The beam attenuation values should be applied in treatment planning. The obtained skin dose results support and explain the higher observed skin effects in patients treated on the Compact unit compared to those previously treated on the Co-60 unit with similar 180° gantry angle beams. Modifying the treatment techniques to reduce the fraction of the dose delivered through the couchtop and/or the use of a ‘tennis racket’ type carbon fiber couchtop should be considered. EP-1509 Small fields defined by jaw or MLC: evaluation of MU estimation by AAA and Acuros algorithms F. Lobefalo 1 , A. Fogliata 1 , G. Reggiori 1 , A. Stravato 1 , S. Tomatis 1 , M. Scorsetti 2 , L. Cozzi 2 1 Humanitas Research Hospital and Cancer Center, Radiation Oncology, Milan-Rozzano, Italy 2 Humanitas Cancer Center and Humanitas University, Radiation Oncology, Milan-Rozzano, Italy Purpose or Objective The small field output factor measurements are studied in literature, covering the aspects of lack of charged particle equilibrium, the partial occlusion of the finite source, and the detector’s volume and response. However, the related accuracy of the MU calculation from dose calculation algorithms has not been investigated with similar intensity. Aim of the present work is the evaluation of the MU calculation accuracy for small fields generated by jaw or MLC for two photon dose calculation algorithms in the Eclipse system (Varian): AAA and Acuros. Simple static beam geometries were chosen in order to better estimate the accuracy with no additional biases. Flattening filter free beams (6 and 10 MV) and and flattened 6MV were evaluated. Material and Methods Single point output factor measurement were acquired with a PTW microDiamond detector for 6MV, 6 and 10MV unflattened beams generated by a Varian TrueBeamSTx equipped with HD-MLC. Since the greatest indetermination of the measurement accuracy resides in the detector sensitivity correction factors for detector, different corrections, field size dependent, were applied according to different publications on the used detector. Fields defined by jaw or MLC apertures were set; jaw- defined: 0.6x0.6, 0.8x0.8, 1x1, 2x2, 3x3, 4x4, 5x5 and 10x10 cm 2 ; MLC-defined: 0.5x0.5 cm 2 to the maximum field defined by the jaw, with 0.5 cm stepping, and jaws set to: 2x2, 3x3, 4x4, 5x5 and 10x10 cm 2 . MU calculation was obtained with 1 mm grid in a virtual waterphantom for the same fields, for AAA and Acuros algorithms implemented in the Varian Eclipse treatment planning system (version 13.6). Configuration parameters as the effective spot size (ESS) and the dosimetric leaf gap (DLG) were varied to find the best parameter setting. Differences between calculated and measured doses were analyzed. Results Agreement better than 0.5% was found for field sizes equal to or larger than 2x2 cm 2 . In the following the results are layer dose. Conclusion

given for the two extreme detector sensitivity correction factors, with the second value in brackets. A dose overestimation was present for smaller jaw-defined fields, with the best agreement, over all the energies, of 1.6 (0.5)% and 4.6 (3.5)% for a 1x1 cm 2 field calculated by AAA and Acuros, respectively, for a configuration with EES=1 mm for X, Y directions for AAA, and EES=1.5, 0 mm for X, Y direction for Acuros. Conversely, a calculated dose underestimation was found for small MLC-defined fields, with the best agreement averaged over all the energies, of -3.9 (-4.9)% and 0.2 (-0.8)% for a 1x1 cm 2 field calculated by AAA and Acuros, respectively, for a configuration with EES=0 mm for both directions, both For optimal setting applied in the algorithm configuration phase, the agreement of Acuros calculations with measurements could achieve the 3 (6)% for MLC-defined fields as small as 0.5x0.5cm 2 . Similar agreement was found for AAA for fields as small as 1x1 cm 2 . EP-1510 Dosimetric characterisation of stereotactic cones by means of MC simulations A. Nevelsky 1 , E. Borzov 1 , S. Daniel 1 , R. Bar-Deroma 1 1 Rambam Medical Center, Oncology, Haifa, Israel Purpose or Objective The objective of this work was to employ an MC model of 6MV FFF beam from the ELEKTA VersaHD linac to perform dosimetric investigation of the new ELEKTA stereotactic cones. Material and Methods The BEAMnrc code was used to create detailed model of the linac head and stereotactic cones for the 6MV FFF beam based on the manufacturer data supplied by Elekta. MC simulation with the BEAMnrc code generated the phase-space file which was used in the DOSXYZnrc code to calculate PDDs, lateral profiles and output factors in a water phantom for stereotactic cones with 5, 7.5, 10, 12.5 and 15 mm nominal diameter. Results from the simulations were compared against measurements performed in water phantom with PTW PinPoint ion chamber and Scanditronix stereotactic diode. Actual cone diameter was found by the best match between the calculated and measured lateral profiles. Sensitivity of output factor to cone diameter variations was investigated. For this purpose, nominal cone diameter was changed by +/- 0.3 mm (which is twice the manufacturer stated uncertainty of 0.15mm). Results Lateral profiles agreed within 2%/0.5mm for all cone sizes. Actual cone diameters were found to be 5.30, 7.70, 10.15, 12.65 and 15.15 mm. For the actual cone diameter, output factors agreed within 2% for all cones except for cone 5 mm where the difference was 4%. Cone diameter uncertainty of 0.3 mm lead to up to 11% variation in the output factor compared to output factor value calculated for the nominal diameter. Conclusion The MC model of the VersaHD linac was employed for investigation and characterization of stereotactic cones. Measured data were verified by the MC calculations. Differences between nominal and actual cone diameter were observed. Given the level of manufacturing accuracy and sensitivity of dosimetric parameters to the cone diameter variation, accurate commissioning of stereotactic cones must be performed and comparison with the data from other centers may be misleading. EP-1511 Radiation Dose from Megavoltage Cone Beam Computed Tomography for IGRT E. Kara 1 , B. Dirican 2 , A. Yazici 1 , A. HICSONMEZ 1 1 Onko Ankara oncology center, Oncology Department, Ankara, Turkey algorithms. Conclusion