ESTRO 36 Abstract Book

S424 ESTRO 36 _______________________________________________________________________________________________

recalculation of delivered dose to the planning CT.

PO-0796 Optimisation of plan robustness to sinus filling in a magnetic field. A. Pollitt 1 , R. ChuteR 1 , P. Whitehurst 1 , R. MacKay 1 , M. Van Herk 2 , A. McWilliam 2 1 Christie Hospital NHS, Radiotherapy, Manchester, United Kingdom 2 University of Manchester, Division of Molecular and Clinical Cancer Science, Manchester, United Kingdom Purpose or Objective The MR Linac (Elekta AB, Stockholm, Sweden) will provide on-treatment MR imaging allowing for excellent soft tissue imaging. Such a machine will become an integral part of the drive towards daily online adaptive radiotherapy. However, the presence of the magnetic field results in the Lorentz force and will cause an increase or decrease in dose to superficial tissues (Raaijmakers et al. 2007). This is particularly pertinent for sinus cancers, of which 60% are squamous cell carcinoma’s and primarily on the surface layer of the nasal cavity. Recent studies (Bol et al 2015, Uilkema et al. 2015) have been performed to determine the effect of the Lorentz force on low density cavities in the body. This abstract aims to investigate the effect of the magnetic field on plan quality and optimisation for varying sinus filling and emptying states. Material and Methods Ten patients with PTV’s overlapping the sinus cavity were selected from the clinical archive. For each patient four plans were optimised at 60Gy in 30 fractions, 2 with no B- field and 2 with the 1.5T B-field present. For each, 1 plan assumed full sinuses with the volume overridden to 1gcm - 3 and the other assumed empty sinuses with the volume overridden to 0gcm -3 . All plans were created using Monaco (v5.19.07, Elekta AB Stockholm, Sweden) and met the departmental constraints for Target and OAR doses. To investigate the effect of a change in sinus filling, plans were recalculated on their opposite filling state, i.e plan optimised on a full sinus was recalculated on an empty sinus. The difference in dose between the two plans for target coverage and OARs was calculated. This comparison will determine the magnitude of the effects from sinus filling in each scenario. Investigating the range of dose differences will provide information on how to optimise these plans to minimise the effect of the Lorentz force. Results The change in dose to the Target for the different filling and magnetic field combinations can be seen in Figure 1. Several of the dose differences for plans optimized on an empty cavity, for both with and without B-field show a shift of the mean of the distribution which is greater than 2% (considered potentially clinically significant). i.e. mean Dose = 2.36%; V 50% = 2.26%; V 5% = 3.12%; V 2% = 3.21%. An OAR which also saw a difference greater than 2% was the Brainstem PRV 1cc max = 2.16 %.

Conclusion This abstract shows greater dosimetric differences due to sinus filling in a 1.5T magnetic field for plans optimised with an empty cavity. Without a B-field plans optimised on full and empty cavities show similar results. The dose to the PTV is also less conformal optimising on an empty cavity due to hotspots caused by the ERE close to the surface shown by a higher effect for the max dose to 2% of the Target. The results indicate that optimising with a full sinus cavity makes the plan more robust to the Lorentz force and therefore to changes in filling. PO-0797 Studies on optical fiber dosimeters for in-vivo dosimetry in HDR brachytherapy L. Moutinho 1 , H. Freitas 1 , J. Melo 1 , J.F.C.A. Veloso 1 , P.J. Rachinhas 2 , P.C.P.S. Simões 2 , J.A.M. Santos 3 , A. Pereira 3 , J. Silva 3 , S. Pinto 3 1 University of Aveiro, Physics Department, Aveiro, Portugal 2 CHUC, Radiology, Coimbra, Portugal 3 IPO-Porto, Radiotherapy, Porto, Portugal Purpose or Objective Dose verification and quality assurance in radiotherapy should be assessed in order to provide the best treatment possible and minimize risks for patient. Notwithstanding, due technical constraints in certain treatments there’s no such tools capable to perform real-time dose measurement. An ideal dosimeter for prostate brachytherapy should provide real-time and in-vivo dose measurement, present high sensitivity and no dependencies on energy, dose and dose rate and temperature. Also should be detectable in the anatomic volume to check its position, easy to use and calibrate and not expensive/disposable use of its implantable part. Material and Methods We developed a fiber optic dosimeter suitable for real- time dose monitoring in breast and prostate brachytherapy, thus opening the possibility for real-time dose correction. The dosimeter comprehends a sensitive optical fiber probe of 1mm or 0.5 mm diameter where a 5 mm length scintillating optical fiber is coupled. The clear optical fiber providing scintillation light guidance into the photodetectors.