ESTRO 38 Abstract book

S951 ESTRO 38

RTSafe company (RTsafe P.C, Athens, Greece). In addition, two phantoms of same size and shape as above but with ionization chamber and film inserts at location of QA target were created. A VMAT treatment plan with 5 non-coplanar VMAT arcs (lateral, vertex and 45°) was devised using Monaco. Using this plan, all three phantoms were irradiated at VersaHD linac. Forty-eight hours after the irradiation, the gel phantom was scanned on a 1.5 T MRI unit in order to obtain 3D dose distribution. In addition, absolute point dose was measured with PTW PinPoint ion chamber and 2D dose distribution at the QA target was measured with film dosimetry. Results The difference between absolute point dose measured with ion chamber and the corresponding dose calculated with Monaco was 0.9%. The gamma passing rate in the film plane using 3D global gamma analysis was 98.2% (3%/2mm). For all clinical targets, the 3D gamma passing rate measured with gel dosimetry was more than 94.5%, with passing rate 97.1% averaged for all targets (3%/2mm). Comparison between planned and measured relative dose distributions in terms of DVHs for all clinical targets is presented in Figure1 (all dose distributions were normalized to the corresponding D50% metric of each target).

Results Table 1 shows the measured and theoretical range shifts, and also the range shifts calculated in Eclipse for CT series with and without iMAR. The calculations based on both the uncorrected and the iMAR images showed higher ΔR 50 values compared to the measured deviations. Also the theoretical ΔR 50 using equation (1) were higher the the measured values.

In Fig. 1 the result of the film measurement for the solid water setup with and without the craniofix can be seen. It can clearly be seen that the center of the craniofix caused a reduction in dose in the area where the titanium rod connects the two plates of the implant.

Conclusion The cranial implants investigated were shown to reduce proton ranges by up to 0.9 mm. Calculated ranges in treatment plans showed an overestimation of the deviations due to metal artifacts. References: [1] Moskvin V, Cheng C, Fanelli L, Zhao L, Das I. A semi- empirical model for the therapeutic range shift estimation caused by inhomogeneities in proton beam therapy. JACMP. 2012;13(2):3-12. EP-1761 Single isocenter multiple brain mets SRS with Elekta VersaHD and Monaco: end-to-end accuracy study A. Nevelsky 1 , E. Borzov 1 , S. Daniel 1 , R. Bar Deroma 1 1 Rambam Health Care Campus, Oncology, Haifa, Israel Purpose or Objective The advantages of stereotactic radiosurgery (SRS) to multiple intracranial targets with a single isocenter have been well described. However, the efficiency of this approach strongly depends on the dosimetric and geometric accuracy at all steps of the workflow. In this work we report on the results of the end-to-end accuracy study of SRS treatment of multiple brain mets using Elekta VersaHD linac and Monaco TPS. Material and Methods A CT simulation scan from a previously treated patient was used as the data set for definition of six clinical targets (ranging in diameter from 6mm to 25 mm) and one QA target in the brain. Based on this set, pseudo-patient phantom filled with dosimetric gel was constructed by

Conclusion The end-to-end accuracy of SRS to multiple intracranial targets with a single isocenter at our center has been successfully validated using patient-like phantoms and 3D gel dosimetry. EP-1762 in vivo skin dosimetry correction factors for IMRT P. Carrasco de Fez 1 , M.A. Duch 2 , N. Jornet 1 , P. Delgado- Tapia 1 , M. Lizondo 1 , C. Cases 1 , A. Latorre-Musoll 1 , A. Ruiz 1 , M. Ribas 1 1 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain ; 2 Universitat Politècnica de Catalunya, Institut de Tècniques Energètiques, Barcelona, Spain