Abstract Book

S1191

ESTRO 37

EP-2157 A simple way to improve quality of treatment for locally advanced lung cancer M. Yaneva 1 , I. Neycheva 1 1 Complex Oncological Center, Radiotherapy, Plovdiv, Bulgaria Purpose or Objective The main goal was to evaluate two different VMAT approaches – pure VMAT and hybrid VMAT, for their efficacy of quality of treatment for locally advanced lung cancer. Material and Methods The study was based on comparative analysis for 15 clinical cases previously treated for locally advanced lung cancer with or without hilar/mediastinal limph nodes. The mean PTV volume was 323.7cc±289.8cc. For each case treatment planning was performed in two ways: using double arc-VMAT and using hybrid VMAT by adding 3D-CRT component. All plans were clinically acceptable where 95% of prescribed dose covered 95% or more of PTV. All of them were calculated by one physicist to avoid inter-observer variations. Hybrid plans were made as simple as possible: monoisocentric, coplanar, distributing delivered dose between two static fields (6/18 MV) and two complete arcs. The ratio between static and VMAT-contribution was 65% and 35% of prescribed dose, respectively. Conformity index (CI) and homogeneity index (HI) were calculated for each case and each aproach to assess quality of treatment together with clinicaly relevant doses for lung and other OARs. Homogeneity index was defined as ratio between difference of near Dmax (D2%) and near Dmin (D98%) to Dmedian (D50%). Also, we analized the difference between absolute Dmax and near Dmax (D2%) called “dose tail”. Conformity index was defined as ratio between absolute volume TV received 95% of prescribed dose to PTV received 95% of prescribed dose. MDL and V20 were estimated for healthy lung (whole lung minus PTV). Paired sample t-test and describtive analizes were used for statistical purposes. Results The basic finding was better PTV coverage for Hybrid RapidArc with better homogeneity index (p<0.001). The mean HI for pure VMAT was 0.12±0.02 vs. 0.08±0.02 for hybrid VMAT. In hybrid plans we found lower “dose tail” with mean value of 1.36±0.38 Gy. The differences for CI and MDL were not statistically significant. The mean CI was 1.23±0.20 and 1.29±0.23 for VMAT and hybrid approach, respectively. The doses were lower in a hybrid approach for V20 (p = 0.01) and CL-5 (p = 0.002). In clinical manner the mean difference of 11.55%, (SD: 11.57%) was remarkable result for sparing contralateral lung and demonstrates benefit of hybrid technique together with better dose homogeneity of PTV. The time needed to prepare a hybrid plan was at least three times shorter and in most cases could be realized without additional structures for PTV density heterogeneity. Because of small VMAT contribution, (only 35%), inaccuracies in optimizer, which does not model lateral electron transport, had very low influence for final dose calculation and could be neglected. Conclusion In our opinion the hybrid RapidArc techique could be a simple way to improve quality of treatment in localy advanced lung cases. EP-2158 Feasibility of Mobius 3D as an independent MU checker for the adaptive work flow on the MR-Linac A. Pollitt 1 , G. Budgell 1 , A. Pooler 1 , J. Wood 1 , R. Chuter 1 , A. McWilliam 1 1 Christie Hospital NHS, Radiotherapy, Manchester, United Kingdom

Table I were registered. All patients should meet the desirable constraints, but we accept as tolerable the acceptable variation in cases where an OAR is near or overlaps with the PTV. Patients must fulfill this variation for all the OAR. If a patient does not fulfill at least all the acceptable constraints, the fractionation scheme is then changed to 70Gy in 28 fractions (2.5Gy/fx) to avoid acute toxicity and late side effects in the normal tissue.

Results The dosimetric results for the PTV and organs at risk are shown in Table II. For the PTV, the mean of MU was 461.29 [range: 387.08-683.98]; the mean number of segments was 84.90 [range: 50-121] and the mean treatment time was 66.23 seconds [range: 61.29-127.14]. In addition, approximately 72% of the patients meet all the desirable constraints. A 28% do not meet one or more parameters of the desirable constraints but always meet all the acceptable variation. In these cases a considerable portion of the bladder and/or rectum was overlapping with the PTV. Furthermore, no acute toxicity for any patient was reported during the course of the treatment.

Conclusion According to the results we can conclude that our constraints are acceptable for this fractionation scheme, but a further analysis with more data and a longer time of follow-up to evaluate late toxicity should be done.