ESTRO 36 Abstract Book

S140 ESTRO 36 2017 _______________________________________________________________________________________________

analyzed the factors associated with the difference of the whole prostate dose between the two dosimetry. This is the first report which evaluated those factors using 3- Tesla MRI in which contouring and fusion are thought to be more accurate than in 1.5-Tesla MRI. Material and Methods The subjects were 81 consecutive patients treated with 144 Gy of brachytherapy alone using loose I-125 radioactive seeds. For postimplant analysis, CT and MRI scans were obtained at 1 month after implantation. CT and 3-Tesla T2-weighted MR images were fused and aligned on the basis of seed distribution in MRI/CT fusion- based dosimetry. Dosimetry was computed for the whole prostate and for the prostate divided into anterior and posterior sectors of the base, mid-gland, and apex (Fig. 1). The volumetric and dosimetric results were compared between MRI/CT fusion-based and CT-based dosimetry using a paired t test. Factors associated with the absolute value of the difference of D90 between the two dosimetry (|D90MRI/CT - D90CT|) were analyzed by multiple regression. P values of <0.05 were defined to be significant.

Proffered Papers: Physics treatment verification

OC-0275 Testing an MR-compatible afterloader for MR- based source tracking in MRI guided HDR brachytherapy E. Beld 1 , P.R. Seevinck 2 , J. Schuurman 3 , F. Zijlstra 2 , M.A. Viergever 2 , J.J.W. Lagendijk 1 , M.A. Moerland 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, The Netherlands 2 UMC Utrecht, Image Sciences Institute, Utrecht, The Netherlands 3 Elekta NL, Veenendaal, The Netherlands Purpose or Objective In HDR brachytherapy, image guidance is crucial for accurate and safe dose delivery. Accordingly, MR-guided HDR brachytherapy is in development at our institution. This study demonstrates the testing of a recently developed MR-compatible afterloader, while operating simultaneously with MR imaging, as well as an MR-based method for real-time source position verification. A prototype of an MR-compatible afterloader (Flexitron, Elekta) was developed. This afterloader was made MR- compatible by providing every part as well as the cover with RF shielding. The source cable was replaced by a plastic cable containing a piece of steel at its tip, serving as a dummy source. The afterloader was placed next to the MRI scanner and connected to a catheter positioned in an Agar phantom (doped with MnCl2), see Fig. 1. Afterloader management: The afterloader was programmed to send the source (I) to 10 dwell positions, with a 10 mm step size, remaining 10 s at each position, and (II) to 20 dwell positions, with a 5 mm step size, remaining 0.5 s at each position. MRI acquisition: While sending the source to its predefined dwell positions, MR imaging was carried out on a 1.5 T MR scanner (Ingenia, Philips) using a 2D gradient echo sequence (TR/TE 2.2/1.0 ms, slice thickness 10 mm, FOV 192x192 mm, acq. matrix 96x96, flip angle 30°, SENSE=2), scanning two orthogonal slices interleaved with a temporal resolution of 0.114 s per image. HDR source localization: The MR artifact induced by the magnetic susceptibility of the metallic source was exploited. The artifacts (complex data) were simulated based on the susceptibility induced B0 field disturbance [1]. The localization was executed offline in a post processing operation by phase-only cross correlation [1,2], to find the translation between the experimental image and the simulated artifact. Results The experiments demonstrated that the prototype MR- compatible afterloader and the MRI scanner fully functioned while operating simultaneously, without influencing each other. The afterloader was able to send the source to the predefined dwell positions when placed next to the MRI scanner, without being attracted to or being disturbed by the scanner. The HDR source positions could be determined by the described localization method (now accomplished offline), see Fig. 2. The average distances between the determined 3D source positions for cases (I) and (II) were 9.9±0.2 mm and 5.0±0.2 mm, respectively. The short dynamic scan time (~0.15 s) and the fast reconstruction/post processing (<0.15 s) guarantee that source localization will be possible in real time. Conclusion The MR-compatible afterloader developed in this study and a commercial 1.5 T MRI scanner were demonstrated to fully function while operating simultaneously, enabling real-time HDR source position verification for MR-guided Material and Methods Experimental set-up:

Results D90 (176.7 Gy vs 173.0 Gy; p = 0.003) and V100 (97.2% vs 96.5%; p = 0.013) were significantly higher in MRI/CT fusion-based dosimetry than in CT-based dosimetry. Prostate volume (28.5 mL vs 30.8 mL; p < 0.001) was significantly lower in MRI/CT fusion-based dosimetry than CT-based dosimetry. Sector analysis showed a decrease in MRI/CT fusion D90 at the anterior base (154.9 Gy vs 166.5 Gy; p < 0.001) and the posterior apex (169.7 Gy vs 177.6 Gy; p < 0.001), and increase in MRI/CT fusion D90 in the anterior mid-gland (195.2 Gy vs 181.7 Gy; p < 0.001), the posterior mid-gland (196.1 Gy vs 193.9 Gy; p = 0.030), and the anterior apex (198.7 Gy vs 175.0 Gy; p < 0.001). |D90MRI/CT - D90CT| was largest at the anterior apex sector among 6 sectors (27.2 Gy). On multivariate analysis, |D90MRI/CT - D90CT| of whole prostate are associated with |prostate volume (PV)MRI/CT - PVCT| (p = 0.036), |D90MRI/CT - D90CT| at the posterior base sector (p = 0.035), |D90MRI/CT - D90CT| at the anterior mid-gland sector (p = 0.011), and |D90MRI/CT - D90CT| at the anterior apex sector (p = 0.004) (Table 1).

Conclusion Several postimplant dosimetric variables were significantly different on MRI/CT fusion vs CT. The differences between the two methods of PV, D90 at the posterior base, anterior mid-gland, and anterior apex sectors may greatly influence the difference of D90 of the whole prostate.