ESTRO 37 Abstract book

ESTRO 37

S558

to expand the sample of patients and extend this work to other treatment groups.

PO-1000 Modulation indexes for VMAT SBRT liver treatments: data from a multicentric evaluation. M. Esposito 1 , V. Hernandez 2 , J. Saez 3 , C. Carbonini 4 , S. Clemente 5 , D. Fedele 6 , C. Fiandra 7 , M. Fusella 8 , C. Garibaldi 4 , F.R. Giglioli 7 , V. Landoni 9 , C. Marino 10 , E. Moretti 11 , R. NIgro 12 , S. Russo 13 , A. Savini 14 , L. Strigari 9 , S. Strolin 9 , C. Talamonti 13 , E. Villaggi 15 , M. Stasi 7 , P. Mancosu 4 , I. Veronese 4 1 Esposito Marco, S.C. Fisica Sanitaria, Bagno a Ripoli, Italy 2 Hospital Universitari Sant Joan de Reus, Medical Physics, Reus, Spain 3 Hospital Clinic de Barcelona, Radiation Oncology, Barcelona, Spain 4 AIFM Working Group SBRT, AIFM Working Group SBRT, Milano, Italy 5 AIFM Working Group SBRT, AIFM Working Group SBRT, Napoli, Italy 6 AIFM Working Group SBRT, AIFM Working Group SBRT, Pistoia, Italy 7 AIFM Working Group SBRT, AIFM Working Group SBRT, Torino, Italy 8 AIFM Working Group SBRT, AIFM Working Group SBRT, Padova, Italy 9 AIFM Working Group SBRT, AIFM Working Group SBRT, Roma, Italy 10 AIFM Working Group SBRT, AIFM Working Group SBRT, Catania, Italy 11 AIFM Working Group SBRT, AIFM Working Group SBRT, Udine, Italy 12 AIFM Working Group SBRT, AIFM Working Group SBRT, Rieti, Italy 13 AIFM Working Group SBRT, AIFM Working Group SBRT, FIrenze, Italy 14 AIFM Working Group SBRT, AIFM Working Group SBRT, Meldola, Italy 15 AIFM Working Group SBRT, AIFM Working Group SBRT, Piacenza, Italy Purpose or Objective Modulation indexes (MIs), computed from the dicom rtplan files, are scores for measuring plan complexity and could predict dosimetric accuracy of intensity modulated plans. In this work, different MIs for liver SBRT were evaluated, with the aim to find correlations between MIs, dosimetric, and planning parameters. Material and Methods Four Liver SBRT plans optimized by 7 planners with VMAT were analyzed. Linac model, TPS, and MLC thickness (Leaf DIM), for the 7 centers, are reported in table 1. Dose prescription was 75Gy in 3 fractions. Major constraint for liver was: at least 700 cm 3 should receive <15Gy. Quality Index (QI) was defined as (PTV mean dose)/(liver mean dose), Homogeneity Index (HI) was computed as D2%/D98%, Dose Spillage (DS) was computed as V50%/PTVvol. MIs used in this work were: mean gantry speed variation (MGS), mean dose rate variation (MRR), two parameters quantifying the modulation of leaf speed and acceleration (MLspeed, MLacc defined in Park et al. Phys. Med. Biol. 59 (2014) 7315), and a global modulation index that considers in a single parameter gantry speed, leaf speed and acceleration (MItot defined in Park et al.). Spearman rank test was computed for assessing correlations between dosimetric results, TPS/linac and MIs.

Table1. Participants to the multicentric study: TPS version, linac and MLC characteristic are shown.

Results Data from 28 plans were analyzed. Spearman rank correlation coefficients are shown in Figure 1. In detail, MLspeed, MLacc, MGS, MDR shown significant correlations with TPS and linac used. MItot correlated with target volume (larger PTV required higher modulation), QI (better quality needed higher modulation), and mean liver dose. No correlations were obtained between MItot and linac/TPS or any planning parameters.

Fig1. Spearman correlation between linac/TPS, dosimetric parameters and modulation indexes are shown. Boxes without cross have p<0.05. Conclusion MItot is a promising universal modulation index for liver VMAT SBRT because it is independent of TPS and linac. PO-1001 Implementation and experimental verification of a faster gantry rotation in lung SBRT C. Ohlmann 1 , F. Schneider 1 , A. Arns 1 , V. Steil 1 , H. Wertz 1 , F. Wenz 1 , J. Fleckenstein 1 1 Universitätsmedizin Mannheim- Medical Faculty Mannheim- Heidelberg University, Radiation Oncology, Mannheim, Germany Purpose or Objective A radiation therapy treatment of lesions inside the lung requires advanced treatment delivery techniques. A well- established method to mitigate intrafractional movement of pulmonary targets is a treatment in deep inspiration breath-hold (DIBH). If this technique is used for stereotactic body radiation therapy (SBRT) a faster gantry rotation can potentially lead to a twofold reduction of treatment times: the time reduction during beam-on itself, and additionally, a reduction of the number of required breath-holds. This can reduce positioning errors and thus allow for smaller treatment planning margins. The aim of this study is to experimentally quantify (a) by