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Dosimetric Impact of MR-Linac Treatment On SBRT of Lymph Node Oligometastases Poorly Visible On CBCT

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D Winkel

D Winkel*, P Kroon , A Werensteijn-Honingh , J Hes , G Bol , I Jurgenliemk-Schulz , B Raaymakers , University Medical Center Utrecht, Utrecht, The Netherlands


SU-H1-GePD-J(A)-4 (Sunday, July 30, 2017) 3:00 PM - 3:30 PM Room: Joint Imaging-Therapy ePoster Lounge - A

Purpose: To quantify the dosimetric impact of reducing the CTV-PTV margin for Stereotactic Body Radiation Therapy (SBRT) of pelvic and para-aortic lymph node metastases, poorly visible on CBCT linacs, for treatments on the upcoming MR-linac.

Methods: A planning simulation study was performed in which two plans were generated for each individual pathological lymph node (n=33) with a prescription dose of 5x7Gy to 95% of the PTV. One plan was generated for VMAT delivery using a 8mm CTV-PTV margin to represent the current clinical practice for SBRT of lymph nodes with poor visibility on CBCT. A second plan was generated for IMRT delivery on the MR-linac with a 1.5T magnetic field, using a 3mm CTV-PTV margin. All plans were dosimetrically evaluated using organ at risk (OAR) contours as delineated on MRI, overlaid on CT for electron density information to simulate current clinical practice.

Results: For 26 (79%) of the 33 conventional plans sufficient PTV coverage could be obtained without violating hard dose constraints for the OARs. This was increased to 31 of 33 plans (94%) for simulation of MR-linac treatment. Furthermore, a reduction of dose delivered to the OARs was obtained: the D2cc to the bladder was reduced by 34% from 3.8±9.1 to 2.5±6.7Gy (p=0.020). The D2cc for bowel was reduced by 17% from 20.4±8.2 to 17.0±11.5Gy (p=0.091). The D2cc for the rectum was reduced by 34% from 4.7±6.7 to 3.1±5.6Gy (p=0.007) The D2cc to the sigmoid was reduced by 37% from 9.9±8.1 to 6.2±5.2Gy (p<0.001).

Conclusion: MR-linac SBRT treatment of lymph node oligometastases, poorly visible on CBCT linacs, improves dose volume parameters for a prescribed dose of 5x7Gy. The prescribed target dose can be applied in more cases while the dose to the surrounding OAR is reduced. MRI guidance paves the way for further dose escalation and hypofractionation.

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