PO-0855: Flattening Filter Free VMAT for extreme hypofractionation of prostate cancer
M. Ahlström, H. Benedek, P. Nilsson, Tommy Knöös, Crister P Ceberg · Radiotherapy and Oncology · 2016
________________________________________________________________________________(PTV) was created adding a margin of 5 mm to the ITV .The dose distribution was optimized on the average CT prescribing a dose of 20 Gy per fraction delivering a total dose of 60 Gy to the PTV.The plans were calculated in a Phillips Pinnacle 9.10 planning system using conformal 3DRT and heterogeneity correction.The parameters obtained in the average CT optimized plan, were copied to the different image sets with identical monitor units to analyze the differences. Results:The average GTV volume was 1.6 ± 1.1 cc.The ITV size is twice the lesion size in most of the cases except in those with higher breathing amplitude.The ITVs outlined in the average CT were smaller than those outlined in the 4DCT ranging from 0.1 cc, where there hardly was lesion movement, to 0.6 cc.The differences between the volumes were usually found in the cranio-caudal direction due to the higher movement of the lesion in this direction.The ITVs outlined in the MIP CT were equivalent to the 4DCT except in the cases where there was a higher density organ in the vicinity of the tumor.Respect to dose distribution, the dose of the organs at risk shows no significant differences in the different image sets.The V100 of the ITV presents significant variations up to 15% due to the variation in electron densities depending on the CT mode chosen.The V100 of the GTV calculated in each phase is greater than 97%. Conclusion:We recommend using the ten phases of the 4DCT study for proper delineation of ITV.If the institution does not have the technology the CT average (low pitch CT) could be used selecting the appropriate window level and increasing margins.There is no significant difference in dose to organs at risk between the images modalities studied.Optimized planning in the average CT provides adequate coverage of GTV at different breathing phases.