ESTRO 36 Abstract Book

S500 ESTRO 36 2017 _______________________________________________________________________________________________

Agility ® collimator system included 160 MLC, minimum leaf width was 5 mm. MLC effective speed was 6.5 cm/sec and leaf travel was 15cm over the central axis. VMAT plans were generated on Monaco 5.1 ® (Crawley, Elekta) treatment planning system with Monte Carlo algorithm. All calculation parameters were grid spacing 0.3 cm, minimum segment width 1.0 cm, Max. 180 of control Points Per Arc, Fluence smoothing medium, Statistical Uncertainty 1% per plan, increment of gantry 30° and dose to medium. The VMAT-TBI tecnique consisted of three isocentres and three dual overlapping arcs from top of head to the bottom of pelvis region. The prescribed dose was 90% of target volume receiving dose of 12Gy. Mean dose to lung and kidney were restricted less than 10Gy and maximum dose to lens were restricted less than 6Gy. The plans were verified using 2D array IBA Matrixx ® and CC13 ion chamber. The comparison between calculation and measurement were made by γ-index (3%-3mm) analysis and absolute dose measurement at the isocentre. Results An average total delivery time was determined 923±34 seconds and an average monitor unit (MU)s was determined 2614±231MUs for dual arc VMAT technique. When we evaluated organ at risk(OAR)s, mean dose to lungs was 9.7±0.2Gy, mean dose to kidneys was 8.8±0.3Gy, maximum dose to lens was 5.5±0.3Gy and maximum point dose was 14.6±0.3Gy, HI of PTV was 1.13±0.2, mean dose to PTV was 12.6±0.15Gy and mean γ- index (%3-3mm) pass rate was %97.1±1.9. Absolute doses were measured by CC13 ion chamber and we determined %2.0±0.6 dose difference between measurement and treatment planning system's (TPS) calculation at the isocentre. Conclusion The results show that dose coverage of target and OAR’s doses are feasible for TBI using VMAT tecnique on the coach. A benefit could be demonstrated with regard to dose distribution and homogeneity and dose-reduction to organs at risk. Additionally,we determined highly precise dose delivery by patient QA and point dose measurement at the isocentre. Based on the dose distributions we have decided to plan TBI in our clinic with dual arc VMAT technique on the treatment coach. PO-0911 Can the therapeutic benefits of microbeam radiation therapy be achieved using a clinical linac? N. Suchowerska 1 , V. Peng 1 , L. Rogers 1 , E. Claridge- Mackonis 1 , D.R. McKenzie 2 1 Chris O'Brien Lifehouse, Radiation Oncology, Camperdown- Sydney, Australia 2 University of Sydney, School of Physics, Sydney, Australia Purpose or Objective The increasing availability of high definition multileaf collimators (HDMLCs) with 2.5mm leaves provides an opportunity for ‘grid’ therapy to more closely approach the clinical outcomes of Microbeam Radiation Therapy (MRT). However, periodic spatial modulation of the dose in the target volume runs counter to current clinical practice. To optimize the modulation, a better understanding of cell dose responses to such treatments is needed. The aim of this study is to determine if some of the therapeutic benefits of MRT can be achieved using a clinical linac with HDMLCs and if so, to develop a predictive model to optimize the benefits of such treatments. Material and Methods Varian Novalis Tx TM HD120-MLCs were used to generate grid patterns of 2.5mm and 5.0mm spacing, which were dosimetrically characterized using Gafchromic TM EBT3 film [Figure 1]. Clonogenic survival of normal (HUVEC) and cancer (lung NCI-H460, breast HCC-1954, melanoma MM576) cell lines were compared in vitro for the same

average dose, following irradiation with periodically modulated and open 6MV photon fields. Results Survival of normal cells in a 2.5mm striped field was the same as for an open field, but the survival of the cancer cells was significantly lower. However for cancer cell lines in the 5.0mm modulated fields, the response compared to an open field was no longer statistically significant. A mathematical model was developed to incorporate the dose gradients of the spatial modulation into the standard linear quadratic model. Our new extended bystander LQ model assumes spatial gradients drive the diffusion of soluble factors that influence survival through bystander effects. The model successfully predicts the experimental results that show an increased therapeutic benefit. Conclusion We have confirmed that HDMLCs can create spatially modulated fields that increase the therapeutic advantage between normal and cancer cells. Our results challenge conventional radiotherapy practice and propose that additional gain can be realized by prescribing spatially modulated treatments to harness the bystander effect. PO-0912 Short- and long term stability of the isocenter of a three-source Co60 MR guided radiotherapy device Purpose or Objective Recently a 0.35T Co 60 MRIdian system (Viewray Inc., Cleveland) is implemented at our institution. In a similar way as for other image guided radiotherapy techniques, the coincidence of the radiation therapy (RT) and imaging isocenter is of major importance. The purpose of this study is to present a method for daily QA of MR-RT isocenter coincidence and to assess its short- and long term stability using daily film-based isocenter QA. Material and Methods Two pieces of radiochromic film (GafChromic EBT3) are taped to square inlays on the top and the side of a cubic water-filled phantom. The phantom is aligned to the MRI isocenter using MR guided setup relying on three internal cylindrical markers. D. Hoffmans 1 , M.A. Palacios 1 , J.P. Cuijpers 1 1 VU University Medical Center, Radiotherapy, Amsterdam, The Netherlands

A treatment plan, consisting of an AP and a lateral square field is delivered. The direction of the lateral field is altered daily between 90 o and 270 o in order to monitor all 3 treatment heads (Head 1 and 2 for the lateral fields respectively, head 3 for the AP field). The films are digitized and the positions of the square fields with respect to the phantom are determined. These data provide a daily measurement of the coincidence of the RT- and MR- isocenters in 3 dimensions. The AP field provides