Abstract Book

ESTRO 37

S522

Carus- Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany 4 German Cancer Consortium DKTK, partner site Dresden, Dresden, Germany 5 University Hospital Carl Gustav Carus, Department of Internal Medicine I, Dresden, Germany 6 National Center for Tumor Diseases NCT, partner site Dresden, Dresden, Germany Purpose or Objective The success of light ion radiotherapy is crucially sensitive to daily setup variation and anatomic changes. Irradiation of the esophagus, in particular, suffers from significant target mobility. The proton facility at our institute uses daily kV imaging for patient setup based on bony anatomy, supplemented with in-room dual-energy CT scans as needed. We assessed target positioning with the aid of kV-visible fiducial markers placed around esophageal tumors by tracking their displacement from the planned location throughout treatment. Material and Methods Five patients with esophageal cancer scheduled for neoadjuvant (4) or definite (1) radiochemotherapy using protons (all male; age 52-64 years; tumor of the middle or lower third of the esophagus; mean GTV: 24 ml) underwent endoscopic ultrasound-guided trans- esophageal placement of gold fiducial markers (VisiCoil TM , IBA Dosimetry, Belgium; diameter 0.35 mm; length 5 or 10 mm) demarcating the proximal and distal tumor borders. Pairs of orthogonal kV images taken in treatment position were retrieved for 104/110 fractions. Markers were manually located in the images and their position in 3D determined by triangulation [1]. Vertebral landmarks were similarly reconstructed and used to register kV imaging to the planning CTs. Triangulation errors were estimated per marker from the geometrical compatibility of image point pairs with single 3D points. All calculations were carried out in custom software based on SciPy. Results Marker implantation proceeded without complications and placement of the distal marker was hindered by an obstructing tumor in one case only. All but one marker remained stable throughout treatment, and their visibility in kV images was generally limited, but sufficient. Marker-induced dose perturbations were not clinically relevant. Figure 1 shows example triangulation results for one patient. Overall the standard deviation [inter-quartile range] of the distributions of marker excursions was 2.6 [2.1], 1.8 [2.0], and 3.1 [3.6] mm in the lateral, sagittal, and longitudinal directions, respectively. A Friedman test failed to reveal significant differences between the directions (p>0.7). Inter-marker distances varied by a few mm during treatment with no discernible trend. Triangulation errors ranged from 0.9 to 7.1 mm. One notable instance of marker-indicated target displacement by 26 mm was observed (Fig. 2). This prompted the acquisition of a control CT in treatment position, which revealed an overlap of the planned and actual GTVs of merely 4% (Jaccard index 0.02). The dosimetric impact would have been a reduction of the mean and near-min (D98%) doses by 6% and 47%, respectively. Conclusion Exclusive reliance on bony anatomy for positioning of esophageal cancer patients cannot prevent occasional target misalignments. Fiducial markers (e.g., VisiCoil TM ) and daily kV imaging are valuable tools for target-centric

setup verification. References [1] Alg. 12.1 in Hartley & Zisserman, Multiple View Geometry in Computer Vision, CUP, 2004

PO-0955 Assessment of internal motion in rectal cancer radiotherapy using rigid and non-rigid approximation I. White 1 , D. McQuaid 1 , A. Dunlop 1 , S. Court 2 , H. McNair 1 , S. Bhide 1 1 The Institute of Cancer Research and Royal Marsden Hospital NHS Trust, Academic Radiotherapy, Sutton, United Kingdom 2 St George's Hospital NHS Trust, Physics, London, United Kingdom