ESTRO 37 Abstract book

S1050

ESTRO 37

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 beams. Results

VPTV; gradient index outside the PTV - R50%=V50%/VPTV and the maximum total dose for all fractions to any point at 2 cm from PTV (D2cm) divided to Dpr. Results Depending on the volume of PTV, 5 groups are defined and the average results with standard deviation (SD) and maximum parameter values are presented in Table1.

The NHI shows a low sensitiveness to the PTV volume, and thus remains approximately within the same limits in all cases. The CI shows a clear dependence on the PTV volume and varies from 1.16 for volumes >50cc to 1.43 for volumes <1cc. Outside the PTV, the IDMHT does not show a clear dependency on volume, but it can be postulated that it should not exceed 10% for the PTV volumes >1cc.The gradient index outside the PTV decreases with increasing volume and on the basis of the results obtained, it is easy to calculate the radius of the shell to control the gradient outside the PTV for each particular case. The normalized maximum dose to any point at 2 cm from PTV increased with PTV volume from 26% for volume <1cc up to 61% and these result can also be used in prescribing subsequent stereotactic treatments. Conclusion Data base was obtained and evaluated, based on the worksheet with parameters describing the isodose distribution in stereotactic treatments. The created protocol is used to prescribe the dose in and out PTV of each new patient. This results to an increasing in optimization parameters, but it facilitates to save time and makes treatment planning evidence based. EP-1930 Mixed beam radiotherapy for sternum and lung treatments S. Mueller 1 , T. Risse 1 , M.K. Fix 1 , S. Tessarini 1 , F. Mueller 1 , K. Zaugg 1 , M.F.M. Stampanoni 2 , P. Manser 1 1 Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital Bern University Hospital and University of Bern, Bern, Switzerland 2 Institute for Biomedical Engineering, ETH Zürich and PSI, Villigen, Switzerland Purpose or Objective Current sternum and lung treatments using VMAT suffer from a large low dose bath delivered to the lungs, heart and other normal tissue. Combining photon and electron beams for mixed beam radiotherapy (MBRT) has the potential to reduce the dose delivered to normal tissue, because of the well-defined range of the electron beams without degrading the dose homogeneity in the target. This work presents an inverse treatment planning technique for MBRT and tests the given hypothesis by plan comparisons. Material and Methods An inverse treatment planning technique for photon MLC based step & shoot MBRT is developed including a novel hybrid column generation and simulated annealing direct aperture optimization (DAO) algorithm. The hybrid DAO starts with an empty aperture pool and iteratively adds photon and electron apertures using the column generation algorithm. After each aperture addition, all apertures in the pool undergo a quasi-Newton weight optimization followed by a simulated annealing based simultaneous shape and weight optimization and a second quasi-Newton weight optimization. Thus, the optimizer has full freedom about the number, shapes and weights of photon and electron apertures and simultaneously optimizes them. After optimization, the deliverable dose