ESTRO 2020 Abstract book

S785 ESTRO 2020

A correlation analysis between the 2mm-2% local 4D GI and the number of MUs normalized to the maximum prescribed dose in the fraction (MU/D) was performed. Results The plot of GI versus MU/D is shown in the figure. Data are fitted by linear regression with R2 =0.52. The correlation test shows a Pearson’s coefficient r =-0.72 (p<0.0001). Considering that a 2mm-2% local GI passing rate of 90% would represent at our institution the lowest threshold for a pre-treatment quality result, equation of the line in the figure would give us an upper limit of 3.3 MU/cGy for a prostate VMAT plan of these characteristics.

Comparing the SciMoCa dose distributions to those of Monaco and Pinnacle, the average passing rates of the γ analysis (absolute term), are 94.5±5.4% and 96.5±3.5%, respectively. Comparing TPSs to ArcCHECK measurements, on average the γ analysis (absolute term) is 94.0±3.3% and 95.3±2.5% for Monaco and Pinnacle respectively. The repeatability of the simulated dose is very high, within few ‰. Considering a standard PC (CPU: Intel Core i7- 6500U, 2.59GHz; RAM:16GB) the calculation time is in the order of 30 minutes. It decreases as dose computation uncertainty increases and the same happens increasing dose grid resolution.

Conclusion An excellent agreement was found between SciMoCa, TPSs (both MC and CCC dose calculation algorithm) and measurements. This innovation software could be used as a secondary dose check verification system and it could become a fast and positive chance for the patient QA, saving a lot of time both for physics and machine. It could also be useful in adaptive RT for checking the impact of the approved RT-Plan on the daily patient anatomy modifications. PO‐1387 Correlation between gamma analysis results and monitor units in prostate VMAT M. Gil Conde 1 , M.J. Albertos Fernández 1 , M.N. Manuel 1 , F. Campos Morillo 1 , Á. Seguro Fernández 1 1 Hospital de Jerez, Radiation Physics Department, Jerez de la Frontera, Spain Purpose or Objective The introduction of inverse planning techniques has provided the possibility of achieving high quality plans in terms of isodoses and DVH curves, at the expense of an increase in the number of monitor units (MUs) and the complexity of the plan with them. Our purpose in this work is to assess quantitatively the relation between the increase of MUs and the result of the gamma analysis of the pre-treatment QA measurements when using VMAT in one of the most standard diseases in radiotherapy: the Data of 35 prostate cancer patients that had already been treated at our centre in the last two years were used for this study. The prescribed dose to all of them was 60 Gy in 20 fractions to the PTV encompassing the prostate gland and 48 Gy in 20 fractions to a second PTV encompassing the seminal vesicles. A VMAT plan with two arcs (210º- 150º) was designed for each patient using the planning system Pinnacle 9.10 and eventually delivered in a Varian Truebeam . All the plans had to accomplish the same restrictions to the organs at risk. Pre-treatment QA of each plan was performed in the linac using the OCTAVIUS 4D system ( PTW ), which allows for volumetric measurements using the Octavius Detector 1500XDR ion chamber matrix. The resulting 3D dose matrix was compared with the one exported from the planning system by a gamma analysis, obtaining for each plan a volumetric gamma index (4D GI). prostate cancer case. Material and Methods

Conclusion The upper results support the hypothesis of a correlation between the increase of monitor units and the worsening of pre-treatment QA results. They give an idea of the quantitative relation between these two concepts in the prostate case, which is one of the simplest in radiotherapy. We used gamma 2%-2mm local criteria due to the statistical significance shown by this parameter. PO‐1388 Dosimetric accuracy of MRCAT Pelvis, an MR‐ only simulation method for RT planning in the pelvic area G. Bolard 1 1 Hôpital de La Tour, Radiotherapy, Meyrin, Switzerland Purpose or Objective Philips released recently MRCAT pelvis for MR-only radiotherapy simulation purpose. Dose calculation is performed on a virtual CT series derived from a modified mDixon acquisition (MRCAT) while the target and OAR contouring are made based on a 3D T2 series. MRCAT pelvis is an extension of the previous MRCAT prostate product released in 2015 and dedicated to prostate treatments. MRCAT series exhibit now continuous HU values (1500 levels in average) rather than stratification in five densities for an easier online CBCT matching. MRCAT generation is robust and can be now used for all types of cancers in the pelvis area (prostate, rectum, bladder, anal canal), and is compatible with different patient position (HFS, HFP). Purpose of this work is to assess dose calculation accuracy on these continuous MRCAT datasets for different pelvic tumor types and geometries in comparison to standard CT-based planning. Material and Methods Validation study was conducted on the first 15 pelvic patients (8 prostate, 6 rectum, 1 bladder) who benefited from MR simulation in our institution (Ingenia 3T). Patients were scanned using the default manufacturer provided ExamCard. VMAT planning (Pinnacle 3 16.2) was performed on the MRCAT series using the manufacturer provided HU to mass density curve and the resulting beams were recalculated (Collapsed Cone Convolution) either on standard CT following the MR simulation (Brilliance BigBore) or on the CBCT acquired to the first fraction of