137. Monitor Units constraints for SBRT plans and prostate cancer N. Cavalli a , E. Bonanno a , G. Stella b , G. Pisasale a , N. Ricottone a , A. D’Agostino a , A. Girlando a , C. Marino a,b a HUMANITAS-Centro Catanese di Oncologia, Department of Medical Physics, Catania, Italy b School of Medical Physics, University of Catania, Italy Purpose. In this work the correlation between total monitor units (MUs) constraints, dosimetric findings and pre-treatment verifica- tions has been studied for prostate Stereotactic Body Radiation Ther- apy (SBRT) VMAT plans. Methods. A study on 5 SBRT prostate treatments has been performed. Plans were been accomplished with Varian Eclipse TPS on True- Beam TM 2.5. The so called MU Objective tool implemented in VMAT optimization engine was used. This tool allows setting the range and the relative priority to the minimum and maximum number of MU employed for plan optimization. Dose prescription was 35 Gy in 5 fractions, using 10MV-FFF pho- ton beams and plans were calculated using Acuros XB algorithm. All plans were calculated with and without constraints on MUs. For each case the 2 plans were compared in terms of DVH, MUs, modulation complexity coefficient (MU/Gy) and pre-treatment veri- fications performed with EPID and Epiqa software. Results. Average MUs were 973 with a modulation coefficient of 277.5 MU/Gy and 1120 with 320 MU/Gy, respectively for plans with and without forced MUs. Differences in DVHs have been found. In fact, for 3 of the 5 cases, use of ‘‘MU objective” involve a better PTV coverage and a reduced dose to OARs. Pre-treatments verifications showed a better agree- ment between calculated and delivered dose distribution for all SBRT plans obtained forcing MUs (see Table 1). All plans were acceptable with a GAI > 95%. Conclusions. Important discrepancies have been found in terms of total MUs and modulation complexity coefficient, that are consider- ably reduced for plans obtained forcing the MUs. SBRT prostate plans obtained using forced MUs reveals an increasing of GAI value associated with MUs reduction. In our opinion, for SBRT treatments further studies are neces- saries on a wider sample of cases to understand if minimizing mod- ulation (i.e. decreasing total MUs) could improve SBRT plans quality. https://doi.org/10.1016/j.ejmp.2018.04.148 138. Toward adaptive radiotherapy: Morphological and setup variability in head and neck TomoTherapy treatments L. Redapi a , A. Gagliano a , L. Marrazzo b , S. Calusi a , C. Talamonti a,b , C. Arilli b , P. Bonomo c , S. Pallotta a,b a Department of Biomedical, Experimental and Clinical Sciences ‘‘Mario Serio”, University of Florence, Florence, Italy b Medical Physics Unit AOU Careggi, Florence, Italy c Radiotherapy Unit AOU Careggi, Florence, Italy Purpose. In head and neck (H&N) radiotherapy, an essential requirement is the reproducibility of the relative position of head and neck during the whole treatment. Moreover, adaptive radiother- apy for H&N cancer would benefit from indicators capable of easily identifying morphological alterations. Aim of this study was to test whether the immobilization device guarantees a reproducible head and neck position and to identify, on set-up verification images, some indicators capable of describing morphological alterations. Methods and materials. A retrospective analysis on 15 patients immobilized using s-shaped head-neck-shoulders masks and treated with TomoTherapy (50–70 Gy/25-33fr) was conducted. For each treatment session two MVCT-CT registrations were performed alter- natively optimizing matching on the head or the neck. For each patient mean differences between the two registrations parameters (Dx, Dy, Dz, Dpitch, Droll, Dyaw) over all treatment sessions were evaluated. Morphological alterations were studied (once a week for the whole treatment duration) by measuring on MVCT images the skin- to-skin distance at 3 different antero-posterior levels: at the center of parotid glands (line1) and 2 cm (line 2) and 4 cm (line 3) anteriorly. Mean percentage variations respect to the first week were evaluated for all patients. A Student’s t-test was carried out (p < 0,01). Results. Mean differences between head and neck registration parameters are reported in Table 1. Except for two patients, mean differences were smaller than 1.7 mm and 1.2°. Mean percentage variations of lines’ length are reported in Fig. 1. For all, a decreasing trend was observed. Variations are statistically significant starting from the 4th week for lines 1 and 2, and only from the 6th week for line 3. Gamma Analysis Index (GAI) 2%-1 mm SBRT Case N° Unforced MUs Forced MUs 1 95.3% 96.8% 2 95.5% 97.2% 3 95.6% 97.8% 4 95.2% 98.1% 5 96.4% 98.7% Paz# Dx½mm Dy½mm Dz[mm] Dpitch½   Droll½   Dyaw½   1 0,9 ± 1,3 0,0 ± 0,8 0,7 ± 1,3 0,0 ± 0,1 0,4 ± 0,8 0,2 ± 0,6 2 0,8 ± 1,0 0,6 ± 0,9 1,0 ± 1,4 0,4 ± 0,8 1,6 ± 1,1 0,1 ± 0,7 3 0,00 ± 0,7 0,1 ± 0,4 0,5 ± 1,3 0,0 ± 0,2 0,2 ± 1,1 0,0 ± 0,1 4 1,0 ± 1,0 0,1 ± 0,4 0,9 ± 1,2 0,1 ± 0,3 0,1 ± 0,8 0,0 ± 0,1 5 0,3 ± 1,0 0,1 ± 0,4 0,3 ± 1,5 0,1 ± 0,4 0,7 ± 1,1 0,0 ± 0,2 6 0,6 ± 1,2 0,0 ± 0,2 0,5 ± 0,9 0,1 ± 0,6 0,0 ± 1,0 0,0 ± 0,1 7 0,7 ± 1,5 0,0 ± 0,4 0,3 ± 1,2 0,0 ± 0,2 0,6 ± 0,9 0,1 ± 0,4 8 4,7 ± 1,4 0,0 ± 0,9 0,6 ± 1,9 0,5 ± 0,7 3,3 ± 1,1 0,0 ± 0,3 9 0,1 ± 1,1 0,1 ± 0,5 0,5 ± 0,8 0,1 ± 0,4 0,7 ± 0,8 0,0 ± 0,3 10 0,9 ± 1,2 0,0 ± 0,5 1,7 ± 1,3 0,2 ± 0,5 0,7 ± 0,8 0,1 ± 0,4 11 0,4 ± 1,1 0,7 ± 0,9 0,1 ± 1,4 0,5 ± 1,0 1,2 ± 1,1 0,0 ± 0,5 12 0,3 ± 1,1 0,1 ± 0,6 0,2 ± 1,1 0,0 ± 0,3 1,0 ± 1,1 0,1 ± 0,4 13 0,9 ± 1,9 0,2 ± 0,7 1,2 ± 2,0 0,1 ± 0,5 0,7 ± 1,1 0,1 ± 0,3 14 1,0 ± 1,9 0,0 ± 0,4 1,6 ± 0,9 0,4 ± 0,6 0,5 ± 0,8 0,4 ± 0,6 15 1,0 ± 1,1 0,1 ± 0,6 2,7 ± 1,9 0,1 ± 0,6 0,2 ± 0,6 0,0 ± 0,3 Abstracts / Physica Medica 56 (2018) 133–278 149