ESTRO 37 Abstract book

S1195

ESTRO 37

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

Conclusion Head and neck mARC treatments yielded clinically acceptable plans, reduced treatment times and a MU mean reduction of 18.7%. Regarding the PTVs, in mARC treatments a statistically significant improvement was registered in the HI whereas a reduction in conformity was observed. D98% showed an increase of approximately 1 Gy in both PTVs. The mARC plans yielded similar or better sparing of OARs than the IMRT plans. The parotid glands showed better results concerning the mean dose and the V30, though statistical significance was not reached. Regarding the spinal cord mean dose, a statistically significant mean reduction of 8.6 Gy was observed.