ESTRO 36 Abstract Book

S744 ESTRO 36 _______________________________________________________________________________________________

G2 thereafter. The mean follow-up was 12 months (range 1-50 months). The Local control rate was: 87%, 72%, 65% and 53%, 63% and 75% at 1 , 3 , 6 , 12, 24, 36 months respectively. The time to local progression was ranged between 1 and 12 months (mean: 6 months). The detailed results are reported in Table 1. Five patients are dead (4 for disease and one for vascular accident). Univariate analysis showed that Tmean, Tmax, Tmin, T90 parameters were not associated with local control rate

In both CT and MRI the GTV volumes, conformity index (CI) and distance between the centers of mass (dCOM) were compared. Statistical differences in volumes between CT and MRI were tested with Wilcoxon rank sum test. Results Nine patients with 11 RCC bone metastases were evaluated. The volumes of the lesions on MRI were larger compared to the CT, for all but one lesion (Table 1). This lesion was comparable in size on MRI and CT. Two visual examples of the difference in delineation are shown in Figure 1. The median GTV volume on MRI was 33.39mL (range 0.2mL – 247.6mL), compared to 14.87mL on CT (range 0.2mL – 179.4mL). The difference in volume as delineated on CT and MRI was statistically significant (p=0.005). The CI in the different lesions varied between 0.08 and 0.75. The dCOM varied between 0.78 and 13.34 mm.

follow-up (months) CR (%) PR (%) SD (%) PD (%) 1 20 48 19 13 3 22 28 22 28 6 26 4 35 35 12 20 6 27 47 24 37.5 - 25 37.5 36 25 - 50 25

TABLE 1: response rate in the time (months) Conclusion

RT-HT is useful combined treatment with a good local control rate and patient compliance. The clinical outcome and the time duration of the follow-up is affected by the advanced stage of diseases. A larger pool and a more detailed patient stratification are needed to evaluate the outcome data in the time Acknowledgments This work was supported by “5 per Mille 2009 Ministero della Salute-FPRC Onlus”. EP-1390 Superior target delineation of renal cell carcinoma bone metastases on MRI vs CT F.M. Prins 1 , J.M. Van der Velden 1 , A.S. Gerlich 1 , A.N.T.J. Kotte 1 , W.S.C. Eppinga 1 , N. Kasperts 1 , L.G.W. Kerkmeijer 1 1 UMC Utrecht, Radiation oncology, Utrecht, The Netherlands Purpose or Objective In metastatic RCC (mRCC) there has been a treatment shift towards targeted therapy, which has resulted in a 50% increase in overall survival. Therefore, there is a need for better local control of the tumor and its metastases. Image-guided SBRT in bone metastases provides improved symptom palliation and local control. After SBRT for mRCC, local control rates have been improved from 50% to 85% when compared to conventional fractionation schemes. With the use of SBRT there is also a need for accurate target delineation. The hypothesis is that MRI allows for better visualization of the extend of bone metastases in mRCC for contouring in the context of stereotactic treatment planning. Material and Methods From 2013 to 2016, nine consecutive patients who underwent SBRT for RCC bone metastases at our center were included. A planning CT and MRI were performed in radiotherapy position according to our clinical protocol. CT images were performed at 1 mm slice thickness on a large bore CT scanner (Philips, The Netherlands). In addition, all patients underwent a 1.5 Tesla MRI scan (Philips Ingenia, The Netherlands) at 1.1 – 4 mm slice thickness. For every patient, T1-weighted images were acquired in transversal and sagittal direction, including a transversal mDIXON scan, as well as T2-weighted images in transversal and sagittal direction, and diffusion weighted images (DWI) according to our clinical MRI protocol. Gross tumor volumes (GTV) in both CT and MRI were delineated. Contouring was performed by a specialized radiation oncologist, based on local consensus contouring guidelines (T1 images were used for target delineation aided by the information derived from the T2 and DWI sequences).

Conclusion Contouring of RCC bone metastases on MRI resulted in both clinically and statistically significant larger lesions compared with CT. MRI seems to represent the extend of the GTV in RCC bone metastases more accurately, possibly due to improved visualization of bone marrow infiltration. Contouring based on CT-only could result in an underestimation of the actual tumor volume, which may cause an under dosage of the GTV in SBRT treatment plans.