Abstract Book

S1192

ESTRO 37

Purpose or Objective The Elekta Unity MR Linac will provide on-treatment MR imaging allowing for daily online adaptive radiotherapy. This may range from a small adaptation of the reference plan for geometric shifts or to a full re-optimisation. Physical verification is impossible with the patient on the bed, therefore software based verification becomes critically important. However, the presence of the magnetic field causes perturbations of the dose to superficial tissues and at interfaces which cannot be modelled fully without Monte Carlo calculations. This study aims to establish the magnitude of the difference between the Monte Carlo dose calculation performed by the Monaco Treatment Planning System (TPS) and a collapsed cone calculation (without magnetic field) performed by Mobius 3D. Material and Methods Twenty-nine patients from four tumour sites (Head and Neck (7), Sinus (10), Sarcoma (3) and Lung (9)) were planned using Monaco (v5.19.07, Elekta AB Stockholm, Sweden) using an Agility beam model and step and shoot IMRT to match the available beam model in Mobius 3D. All of the plans met the departmental constraints for Target and OAR doses. Plans were optimised with the 1.5 T magnetic field present and recalculated without it. The DICOM for both plans was exported to Mobius 3D which performed an independent dose calculation. A comparison was performed of the 3%/3mm gamma- analysis pass rate and the doses for the overall plan and to the reportable targets and OARs. Results The average change to the gamma-analysis pass rate, between with and without magnetic field, was -6.0 ± 2.2% (1SD). Table 1 shows the results for the gamma- analysis pass rate and dose change for the target volumes and OARs for each site. The mean change in dose for all of the target and OAR volumes was less than 1% between the cases with and without the 1.5T magnetic field. This is shown for the PTV dose in Figure 1. The maximum dose change for target structures was less than 1% and for OAR structures was less than 3%, within the typical tolerance for an MU check.

Conclusion Percentage dose changes to individual clinically relevant structures such as the PTV and OARS are all well within tolerances typically used for a secondary MU check program. This was performed for a wide variety of different tumour sites, for 29 different patients. This demonstrates that using dose to individual structures for the MU check, then Mobius 3D would provide an acceptable workflow. EP-2159 mARC head and neck treatments: statistical analysis of preliminary results and comparison with IMRT R. Bermúdez Luna 1 , A. López Fernández 1 , C. Rodríguez Rodríguez 1 , E. Meilán Bermejo 1 , B. Ludeña Martínez 2 , B. Caballero Perea 2 , M. De Torres Olombrada 2 1 Hospital Universitario de Fuenlabrada, Medical Physics, Fuenlabrada, Spain 2 Hospital Universitario de Fuenlabrada, Radiation Oncology, Fuenlabrada, Spain Purpose or Objective mARC represents the Siemens solution for volumetric modulated arc therapy. This technique was implemented in our centre during 2016. Volumetric modulated arc therapy treatments are characterised by reduced times per fraction as the treatment is delivered during continuous gantry rotation and usually at a high dose rate. This can improve the patient’s comfort and reduce the probability of intra- fraction motion. These treatments also result in a lower number of monitor units (MU) than conventional IMRT treatments and have yielded a number of dosimetric advantages in the planning target volumes (PTV) and the organs at risk (OAR). The aim of this work has been to perform a statistical analysis of dosimetric parameters corresponding to the PTVs and OARs of the first 20 head and neck cancer patients treated with mARC and compare the results with those corresponding to IMRT treatments previously The dose-volume histograms (DVH) of the first 20 head and neck cancer patients receiving mARC treatment have been analysed. The results have been compared to those corresponding to a randomly selected sample of 20 patients that received 7 field step and shoot IMRT. The prescription doses to the PTVs were 70 Gy and 58.1 Gy in 35 fractions. The plans were optimised so that at least the 95% of the PTV received the 95% of the prescription dose, with the OARs being irradiated with the lowest dose as possible. The following data have been studied for each technique: MU, homogeneity index (HI), conformity index (CI), near- minimum dose (D98%), median dose (D50%), near- maximum dose (D2%) and several dosimetric parameters corresponding to the OARs. The unpaired t-test or the Mann-Whitney test have been performed to assess statistically significant differences among the data corresponding to both techniques, with a 5% significance level. Results Every mARC plan achieved the PTVs’ planning objectives. In 19 of the 20 plans 2 arcs were needed to fulfil the dosimetric requirements in PTVs and OARs. The mean delivery times were reduced from approximately 12 minutes with IMRT to a mean of 6:30 minutes with mARC. The table lists the results of the statistical analysis (mean values ± standard deviations; n.s.: not significant). The figure shows the mean DVH of the OARs. delivered in our centre. Material and Methods