ESTRO 2023 - Abstract Book

S1807

Digital Posters

ESTRO 2023

IOERT is a method for dose escalation applying high single doses during surgery. In contrast to EBRT, no 3D dose calculation based on the individual intraoperative patient´s anatomy was available so far. We developed a workflow to overcome this limitation using intraoperative CBCT imaging. Materials and Methods Commercially available non-metal-containing operation table-top inserts and IOERT applicators were used. Our IOERT suite was recently equipped with a mobile CBCT scanner (ImagingRing, medPhoton GmbH) with a large effective bore (102 cm) capable of covering a variable FOV of up to 49.1x49.1x25.4 cm. Two orthogonal images are taken initially to define the optimal ROI including tumor bed and IOERT applicator base, which is subsequently captured in the course of CBCT acquisition using the independently rotable arms and four independently moveable collimator jaws. The system features an automated scaling (heuristic object and head scatter correction as well as beam hardening correction) of Hounsfield Units, which was checked prior to clinical use by suitable standardized phantoms with inserts of differing densities. This allows the application of one density conversion table independent of imaging preset, geometry, and patient anatomy, for standard clinical cases. After transfer of the images to the TPS (Radiance, GMV), dose calculation was performed using a Monte Carlo algorithm. Results Our patient suffered from locoregional recurrence of rectal cancer. He had already received neoadjuvant full course chemoradiation during initial treatment and was treated with neoadjuvant moderately dose-reduced re-chemoradiation prior to surgery for his recurrence. After gross total resection, we placed an adequate applicator and performed CBCT imaging via remote control. The applicator position was corrected after the CBCT scan and confirmed by a second scan. A dose distribution and the adequate monitor units were calculated to cover the tumor bed with 12 Gy (90% isodose) based on the intraoperatively aquired CBCT images. The patient was moved beneath the IOERT-LINAC (Mobetron, Intraop). After automated soft-docking, IOERT was performed while the patient was video-monitored. Conclusion Image-guided IOERT with real-time intraoperative CBCT based 3D-dose calculation is feasible. For the first time, precise intraoperative image-based verification of the correct applicator position and IOERT planning procedures similar to the standards of EBRT are enabled. 1 Vejle Hospital, University Hospital of Southern Denmark, Department of Oncology, Vejle, Denmark; 2 Vejle Hospital, Danish Colorectal Cancer Centre South, Vejle, Denmark Purpose or Objective For pelvic radiotherapy the imaging standard for treatment planning is CT-based delineation supported by co-registered MR scans for soft tissue contrast. In MR-only treatment planning the CT scan is replaced by using the MR images to simulate a CT (sCT) with correct HU for dose calculation. In June 2020 we introduced MR-only treatment planning as our clinical standard for curative pelvic radiotherapy. Here we present our experiences with the MR-only workflow for scanning, delineation, treatment planning and daily IGRT based on nearly 400 treated patients with rectal, anal or prostate cancer. Materials and Methods We use the Philips MRCAT pelvis suite (Philips Healthcare) to generate the sCT. We present the experiences of radiographers, clinicians, RTTs and physicist in the department collected during daily clinical use. Results MR-only reduces the CT workload at the cost of a little more time spent per MR scan. Initially gains in staff- and scanner time were reduced by sCT reconstruction failing e.g. in case of algorithm errors or patients unfit for the MR-only workflow, requiring unplanned CT scans. Staff training and better patient selection, along with improved software, has improved the success rate to ~95%. Patients spend significantly less time being scanned, especially for patients in bladder filling protocols this improved the comfort. We currently perform delineations on sCT using MR to discriminate between tumour and surrounding soft tissue. This benefit from the minimal organ motion between sCT and MR compared to a standard CT and MR workflow. However, it causes two challenges: First, bones are not always perfectly reconstructed in the sCT, leading to risk of incorrect delineation. Second, the lack of contrast enhancement on sCT, makes it more difficult to separate bowel loops, muscle and vessels. Both problems are alleviated by using the MR images as a reference, but is a little more time consuming compared to regular, contrast enhanced CT. The sCT is a drop-in replacement for real CT for treatment planning and dose calculation. The differences in calculated dose between sCT and real CT are clinically insignificant at <0.5 Gy in high dose (60-80 Gy) regions. The deviations in bone segmentations are too small to affect photon dose calculation. For daily imaging during treatment the sCT again is a drop-in replacement for CT. Our departments Elekta linacs accept the sCT and all automated match algorithms work as well as for real CT. Only minimal training of RTTs was necessary to use the sCT for daily IGRT. The main challenge is errors in bone segmentation, which will show up strongly when compared to CBCT. This previously caused some false alarms on changed anatomy, but are now quickly resolved based on experience. PO-2035 Experiences from the clinic after 2 years of using MR-only treatment planning for pelvic cancers H. Nissen 1,2 , B.M. Havelund 1,2 , C.V. Madsen 1 , J. Pløen 1 , C. Krog 1 , L.N. Olesen 1 , S.P. McIlroy 1 , M. Berg 1

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