ESTRO 2020 Abstract book

S774 ESTRO 2020

The new decision-tree integrating MUVscores provide help with patient-QA controls. The two LTMCS / MUV scores help rule out the false positives. Specifics models including the complexity scores and the thresholds are dependent on each TPS / Linac pair. Work in progress to reduce the 15% of false negatives. References : [1] Jong Min Park (2015). Modulation index for VMAT considering both mechanical and dose calculation uncertainties. Physics in Medicine & Biology.60 [2] Crowe (2014). Treatment plan complexity metrics for predicting IMRT pre-treatment quality assurance results. Australasian Physical & Engineering Sciences in Medicine, 37(3) [3] AAPM Task Group No. 119, IMRT Commissioning Tests, Instructions for Planning, Measurement, and Analysis [4] AAPM Task Group No. 218, Tolerance limits and methodologies for IMRT measurement-based verification QA PO‐1368 Clinical implementation of RayStation for Accuray Radixact tomotherapy platform M. Fusella 1 , A. Scaggion 1 , A. Roggio 1 , M.A. Rossato 1 , N. Pivato 1 , R. Zandona 1 , M. Paiusco 1 1 Istituto Oncologico Veneto, Medical Physics, Padova, Italy Purpose or Objective The helical tomotherapy platform (Accuray Radixact )has been recently installed at our Institute. Alternatively to Precision TPS, we acquired RaySearch TPS (RayStation) for tomotherapy planning. The commissioning and testing of the new TPS is here presented. At the time of the submission this was the first application of the RayStation TPS for tomotherapy planning. Material and Methods First step: tuning the absolute dose normalization of the model. As suggested by AAPM's TG-148, a tomotherapy plan has been optimized and calculated for two cylindrical targets and one OAR inside the Virtual Water phantom (VW) provided with the machine. A total of five plans have been generated, one for each jaws opening (static and dynamic mode). Multiple point dose measurements (up to six points for each plan) has been performed to check whether the calculated dose respects the actual output of the linac. The dose normalization of the model has been tuned by iteratively generating the machine model and recalculating the plan in VW Phantom. Second step: the model was further refined correcting to transverse profiles. Third step: analysis of the model. All the profile and PDD curves was analyzed with an in-house developed routine for 1-D gamma index calculation with 1%/1mm local criterion. The second validation step of the model consisted of a volumetric measurement of the 5 plans calculated for the VW to control the helical output constancy. As third validation step a total of 150 patient- specific QA measurements were taken with the 3D diode array ArcCHECK (Sun Nuclear Corporation), calibrated in absolute dose. Various gamma index (GI) were calculated: 3%/3mm, 3%/2mm and 2%/2mm, both local (L) and global (G) normalization, with 10% dose threshold. The disease sites of the calculated and measured plans are: prostate, Head and neck (SIB treatments), mesothelioma, pelvis, uterus and rectum. All the plans were calculated with a uniform grid of 2.5 mm. Results The mean GI for all profile and PDD curves resulted 98%(±1.2). The GI 3%G/2mm for the VW plans is 100%. Evaluating the plans with a tighter GI like 2%L/2mm, the average (±SD) results is 97% (±1)The average (±Standard Deviation) GP% values obtained for ArcCHECK measures are reported in table 1

average, 4.6%, 4% and 3.7%, for esophagus, H&N and lung, respectively) in all studied pathologies. Conclusion Dose calculations in treatments with heterogeneous regions are more accurate when calculated with Acuros XB. AAA algorithm does not account for this type of heterogeneities and overestimate PTV coverage. These differences are more acute for H&N treatments due to the large section of inhomogenity regions. PO‐1367 Patient‐QA prediction: a new approach of complexity indexes. D. Julian 1 , Z. Jazouli 1 , S. Muraro 1 , E. Beguier 1 , K. Serre 1 , P.A. Daviau 1 , O. Lauche 1 1 Clinique Clementville, CCGM Unité de Physique, Montpellier, France Purpose or Objective This study proposes a model based on new complexity indexes compilation to predict the feasibility of treatment plans in radiotherapy with intensity-modulated. Material and Methods The clinical model was created in three stages. First, development of an algorithm in python 3.7, integrating the calculation of 7 complexity scores: MCS / LTMCS [1] and MAD / MFA / CLS / CAS [2]. The scores describe parameters such as moving and opening leafs for each control point (CP). An ad hoc score has been developed to integrate the impact of Monitor Units Variability (MUV). Then, a retrospective study evaluated the correlation between the scores and the Gamma Indexes (GI) of the patient-QA from a database including 1379 VMAT beams. To validate the model, a prospective study was carried out on 451 VMAT beams. The model has been established to identify plans that are likely to fail the GI test as recommended by AAPM [3], [4]. An home made software has been developed. Results A strong correlation (R = 0.79) was found with the LTMCS / MUV combination and the GI (3% 3mm) for the normo- fractionated planes and (2% 2mm) for the stereotaxic planes. The highest found value of LTMCS is 0.57 (low complexity) and the lowest value is 0.02 (high complexity). The MUV varies between 0 (UM stable between all CPs) and 180 (Maximum variation of UM between CPs). The results showed that a LTMCS greater than 0.15 and a MUV less than 5 of a plan mean that its beams are predicted as true positives (GI> 95%). Sorting by the LTMCS and the MUV made it possible to have 80% (1104 beams out of 1379) of true positives (p-value <0.001). The scores calculated for the 20% of the remaining plans (275 beams) reflect a high complexity between each CP, and may lead to GI <95% (70 out of 275 beams). The method combining the LTMCS / MUV allowed to discriminate the risk of false positives (p- value <0.001). The prospective use of the model proved its detection efficiency of 80% of true positives (359 out of 451 beams, p-value <0.001), of the remaining 92 beams, only 25 were true negatives (GI <95%).

Conclusion