Abstract Book

S1048

ESTRO 37

coverage for both techniques, while maintaining comparable maximal dose values within the target. Paddick new conformity index (nCI), gradient index (pGI), and V15% were compared for both techniques. Wagner conformity gradient index (CGI), gradient index (CGIg), and conformity index (CGIc) were also used since they were correlated with brain normal tissue complication probability. Results For the same PTV coverage, VMAT plans achieved significantly better dose conformity (p<0.01) and lesser low dose spread, V15% (p<0.01) as compared with the CyberKnife plans ( table1 ). Although pGI was similar for both techniques (p=0.55), CGIg was higher using VMAT (p=0.013). CGIc and the combined CGI were both significantly higher for VMAT plans (p<0.01). Table 1: Plan metrics (median values) Metrics VMAT CyberKnife MLC p-value CGIc 101.87 87.29 0.002 CGIg 74.46 71.07 0.013 CGI 87.77 78.68 0.002 nCI 1.03 1.17 0.002 pGI 2.81 2.83 0.556 PTV coverage 98.18 98.18 n/a Conclusion For solitary intracranial targets, Novalis VMAT seems to offer higher conformity and steeper dose gradients than CyberKnife MLC non-isocentric plans, even when beam modulation using partial segments is allowed. EP-1928 VMAT-SIB WBRT hippocampal sparing in patients with large single metastasis O. Cudic 1 , S. Cilla 2 , I. Djan 1,3 , M. Baucal 1 , B. Petrovic 1,4 , B. Djuran 1 , M. Novakovic 1 , A. Morganti 5 1 Institute of Oncology Vojvodina, Radiotherapy Department, Sremska Kamenica, Serbia 2 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 3 University of Novi Sad, Faculty of Medicine, Novi Sad, Serbia 4 University of Novi Sad, Faculty of Sciences, Novi Sad, Serbia 5 Università di Bologna- Radiation Oncology Center, Department of Experimental- Diagnostic and Specialty Medicine - DIMES, Bologna, Italy Purpose or Objective Whole brain radiotherapy (WBRT) has long been considered the treatment of choice given its effectiveness in providing palliation for patients with brain metastases (BMs). Available literature data evidenced a potential detrimental effect of WBRT in neurocognitive functions (NCFs); in particular, high dose irradiation to hippocampus has shown to be critical in determining NCFs outcomes. Technological improvement, such as volumetric modulated radiotherapy (VMAT), allowed the delivery of simultaneously WBRT and a boost dose to brain metastases. These technical advancements are now able to obtain dose painting in strategic areas in brain. We evaluated the feasibility of VMAT-SIB technique for simultaneous irradiation of whole brain and single large metastases to generate hippocampal avoidance. Material and Methods Five patients underwent WBRT with a boost to a large single metastases (4 cm or more) using the VMAT-SIB technique for the purpose of hippocampal avoidance. Bilateral hippocampi were delineated on T1w MRI co- registered with the planning CT. Two plans were generated for hippocampal sparing: standard WBRT-only plans (ST-plans) and plans with a simultaneous boost to

the metastases (SIB-plans). For the latter, doses of 30 Gy and 40Gy were prescribed in 10 fractions to whole-brain and the metastases, respectively. Patients were treated with an Elekta Versa HD accelerator using two full coplanar arcs and one vertex arc with table at 270 ° . Treatment planning optimization was performed with Monaco TPS (Elekta Medical Systems Inc.) for 6MV x-ray The average metastases volume was 108.0 cm 3 (range: 39.7-151.5 cm 3 ). With regard to SIB-plans: the average mean dose on the bilateral hippocampus was 13.1 Gy (range: 12.2-14.4 Gy). The average values of maximal and minimal doses were 17.9 Gy (range: 16.7-18.7 Gy) and 9.9 Gy (range: 8.6 -11.7Gy), respectively. With regard to targets irradiation the mean doses to PTV30 and PTV40 were 33.2 Gy (range: 32.2-34.1 Gy) and 41.3 Gy (range: 40.6-42.1 Gy), respectively. The average value of conformity index was 0.85 (range: 0.81-0.88). The average values of near-minimal and near-maximal doses to metastases were 37.7 Gy and 43.1 Gy, respectively. With regard to ST-plans: the mean dose to bilateral hippocampi was 12.4 Gy (range: 11.4-13.2 Gy). The average values of maximal and minimal doses were 17.8 (range: 16.3-18.7 Gy) and 9.3 Gy (range: 9.2-9.6 Gy), respectively. The average value of conformity index was 0.74 (range: 0.68-0.81). The average values of near- minimal and near-maximal doses to whole-brain were 22.7 Gy and 32.1 Gy, respectively. Conclusion Usage of VMAT-SIB in WBRT for hippocampal dose sparing is very promising. The mean, minimal and maximal doses to hippocampi were found similar between the two optimization strategies, despite the dose boost to the large metastases. The decrease of the hippocampi mean dose to about 13 Gy can translate in lower rates of radiation-induced neurocognitive decline. EP-1929 Prescribing and recording the dose in and outside of the target volume for stereotactic treatments N. Gesheva-Atanasova 1 , A. Balabanova 1 , B. Antonov 1 , D. Stoeva 1 1 University Specialized Hospital for Active Treatment in Oncology, Clinical Dosimetry Laboratory, Sofia, Bulgaria Purpose or Objective The purpose of the study is to create a protocol for prescribing and recording the dose distribution in and outside the PTV for stereotactic treatments (ST), based on our experience and compatible with international protocols and the newly issued ICRU 91. Material and Methods For the last year, 88 patients were treated with ST on conventional linear accelerators or a tomotherapy machine. 43 patients had ST of the brain and 45 of other parts of the body. In 14 of the cases there were more than one PTVs and the maximum number in one case was 9. Treatment planning was done on Eclipse TPS, v.13.6 and on Tomo Planing. Тhe prescribed isodose surface is selected so that 95% of the target volume is covered and 99% of the PTV receives at least 90% of the prescribed dose (Dpr).A worksheet for recording and evaluating the treatment plans has been prepared. Some of parameters describing the isodose distribution in the worksheet are: conformity index – CI=(V100%/TV100%)/(TV100%/VPTV), where V100% is the cumulative volume determined by the prescribed isodose surface and TV100% is the part of the volume of the PTV receiving the prescribed dose; non homogeneity index – NHI=Dmax/Dpr, where Dmax is the maximum dose in the representative voxel size in PTV; index of dose maximum in healthy tissues (IDMHT) - cumulative volume of all tissues outside the PTV receiving a dose > 105% divided to VPTV; gradient index outside the PTV - R50%=V50%/VPTV beams. Results