ESTRO 36 Abstract Book

S809 ESTRO 36 _______________________________________________________________________________________________

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 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 layer dose. Conclusion

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 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 algorithms. Conclusion