ESTRO 36 Abstract Book

S900 ESTRO 36 _______________________________________________________________________________________________

therapy as an additional dose delivery check. The transit dosimetry has the potential of testifying dose delivery, the accuracy of MLC leaf positioning and the calculation of dose to a patient or phantom. Material and Methods In total 42 patients with stereotactic plans were evaluated. Delivery was carried out on a Varian TrueBeam linac equipped with an aS1000 EPID. Continuous portal imaging was performed at each treatment fraction during the delivery of treatment for all beams. To validate the method, we executed treatment plans on a commercial respiratory motion phantom containing plastic spheres as target. Phantom CT scans were made in different phases. First phase were done by applying sinusoidal breathing cycle in different motion amplitudes (-20, -10, 0, 10, 20 mm) in superior/inferior direction and second phase was done by pre-defined breathing simulation with a short pause after exhalation in oscillation mode. Three techniques: 3D-CRT, IMRT and VMAT-SBRT were generated and on board transit dose was collected by EPID during the treatment. The daily obtained portal image were compared with the reference image using the gamma evaluation method with criterion 2% dose difference and 2 mm distance to agreement (DTA) criteria with a threshold value of 5% of maximum value. Results The area gamma passing rate per arc in most of the plans was higher than the acceptable limit but in some arcs it had lower agreement, the lowest value was 3.7%. Besides irradiating phantom in planned respiratory motion, we re- irradiated the same plans due to displacement of the target by stopping the movement or changing the breathing speed. Gamma parameters such as maximum gamma, average gamma, and percentage of the field area with a gamma value>1.0 were analyzed. For all the VMAT arcs in phantom measurements, the gamma evaluations were within the tolerance limits (γmax = 3.5, γavg = 0.5 and γ% >1 = 2%) tough in some measurement 20 mm target displacement was applied. For IMRT fields, measurements were not in good agreement in different tumor motion. 3DCRT fields showed poorest gamma agreement in portal dosimetry analysis. Conclusion This research increases the need of a tool for monitoring inter-fraction errors by confirming the tumor position within the treatment field over the course of therapy. Using daily EPID images over the course of treatment could potentially provide accurate verification of dose delivery to heterogeneous anatomical regions in patients receiving 3D-CRT and IMRT radiation therapy treatments. However, further studies are required to assess 3D IN VIVO dose verification of VMAT techniques of various treatment sites. EP-1658 The effect of weight loss in head and neck patients in the presence of a magnetic field R. Chuter 1 , P. Whitehurst 1 , M. Van Herk 2 , A. McWilliam 2 1 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering CMPE, Manchester, United Kingdom 2 University of Manchester, Manchester Academic Health Science Centre MAHSC, Manchester, United Kingdom Purpose or Objective Head and neck patients tend to experience weight loss during treatment in a predictable pattern loosing between 5-15% of their initial weight over the first two weeks. Adaptive radiotherapy for these patients focuses on an Electronic Poster: Physics track: Adaptive radiotherapy for inter-fraction motion management

offline protocol where the patient is re-scanned and re- planned two-to-three weeks through treatment. The MR- Linac (Elekta, AB, Stockholm, Sweden) will provide excellent soft tissue contrast which may be desirable for this group of patients. However the electron return effect, caused by the Lorentz force may potentially result in an increased dose to superficial tissues, for example the parotid glands. This effect can be controlled in plan optimisation, however it is unknown whether the presence of a magnetic field makes it necessary to adapt the plan at an earlier stage or more frequently during treatment. The purpose of this abstract is to assess the suitability of the current off-line adaptive radiotherapy workflow for head and neck patients in the presence of a magnetic field. Material and Methods Ten patients treated with either 66Gy or 60Gy in 30 fractions, were selected from the clinical archive that had shown significant weight loss during treatment which required a repeat CT. Both the initial planning CT (pCT) and the repeat CT (rCT) were fully contoured by an oncologist specialising in head and neck cancer. Two plans were optimised, at 0T and 1.5T using Monaco v5.09 (Elekta AB Stockholm, Sweden) which met the departmental constraints for Target and OAR doses. These plans were copied to the rCT and re-calculated with a 1% statistical uncertainty, allowing the segmentation and delivered MU to remain constant. The magnitude of the change in dose to the target and OARs due to weight loss was compared between the 0T and 1.5T plans. The difference between the dose distribution on the pCT was compared to the distribution on the rCT and how this was affected by the presence of the magnetic field. Results The percentage difference between 0T and 1.5T plans for the targets showed statistical differences for the D95% for PTV1, PTV2 and PTV3, D50% and mean dose for PTV2, and mean dose and 2cc min for PTV3. The only statistical difference for the OARs was the 2cc max dose for skin which increased by 1.1% for 1.5T plans. However differences between the 0T and 1.5T plans were on average all within 2%, so were considered clinically acceptable.