ESTRO 2020 Abstract book

S866 ESTRO 2020

collisions, incorrect isocenter shifts and beam mix-up due to predicted absolute table coordinates, which are recorded to the R+V system with the corresponding beams. PO-1513 Overdose areas in whole brain irradiation with simultaneous integrated boost and hippocampal sparing A. Pierelli 1 , M.G. Giri 1 , P.M. Polloniato 1 , N.L.V. Cernusco 2 , R. Micera 2 , A. Muraglia 2 , R. Mazzarotto 2 , C. Cavedon 1 1 Azienda Ospedaliera Universitaria Integrata Verona, Medical Physics, Verona, Italy ; 2 Azienda Ospedaliera Universitaria Integrata Verona, Radiotherapy, Verona, Italy Purpose or Objective To evaluate and compare overdose areas in whole brain irradiation with Hippocampal Sparing (HS) and Simultaneous Integrated Boost (SIB) on multiple metastases (HS-WBRT-SIB) for three intensity modulated techniques: non-coplanar IMRT, non-coplanar VMAT and Tomotherapy. Whole brain volume without metastasis and without the Hippocampal Avoidance Zone (HAZ) was defined (WB*). Differences in terms of dose homogeneity, usually neglected, and in terms of global absolute amount of overdoses in WB* volume were analyzed. Material and Methods The inner clinical protocol used for HS-WBRT-SIB treatment provides a prescription of 30 Gy and 40 Gy in 10 fractions to the whole brain and metastases, respectively, and a simultaneous hippocampus sparing with “near maximum dose” D2% ≤ 16Gy. Ten patients, all with brain metastases, were selected for this study and planned after a review of all the anatomical structures mainly defined using CT and MRI images. The dosimetric data collected from each plan and each modulation technique were first used to determine the WB* coverage, the dose homogeneity index (HI) and the near maximum dose as described by ICRU 83, together with the absolute volume irradiated at high dose levels; the dose to the organs at risk was also considered. To evaluate the significance of the differences between the three modulation techniques, a statistical analysis was performed using the Freidman test for paired samples. Results As regards the organs at risk, the difference for HAZ D2% is statistically significant between Tomotherapy and IMRT and between VMAT and IMRT but not between Tomotherapy and VMAT. The dose reduction is also statistically significant for mean eyes dose and maximum lens dose between Tomotherapy and the other two techniques. WB* is well covered in each technique: we found D95% (mean value ± STD): (28.9±0.4)Gy, (28.9±0.5)Gy, (29.1±0.3)Gy respectively for IMRT, VMAT and Tomotherapy. About the WB* D2%, the results obtained were respectively: 114%, 110% and 107% of WB* dose prescription. Values of HI were 0.18±0.02, 0.13±0.03 and 0.09±0.03, respectively; the absolute volumes (in cc) exceeding 107% of WB* dose prescription were 125±38, 59±27 and 31±15, respectively. The overall treated metastases volume was 14±10 cc.

Results The first 20 patients had a median age of 47 years, and received TBI before bone marrow transplantation for acute myeloid leukaemia. Most patients (13/20) received a TBI dose of 4 Gy in 2 fractions, twice daily. The mean number of applied monitor units (MU) was 6476 MU using a multi- arcs and multi-isocenter VMAT-TBI technique. The tabletop has been successfully used in daily clinical practice and helped to keep the treatment times at an acceptable level. During the first treatment fraction, the mean overall treatment time (OTT) was 57 minutes. Since no additional image guidance was used in fraction 2 of the same day, the OTT could be significantly reduced to 38 minutes in mean.

Conclusion The easy and reproducible rotation of the patient on the treatment couch using the rotatable tabletop, is time- efficient and overcomes the need of repositioning the patient after turning from a HFS to a FFS position during VMAT TBI. Furthermore, it prevents couch-gantry