Abstract Book

S1163

ESTRO 37

Amsterdam, The Netherlands 5 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands 6 Leiden University Medical Center, Division of Image Processing- Department of Radiology, Leiden, The Netherlands Purpose or Objective In rectal cancer patients with complete clinical response an organ-preservation strategy seems safe. Dose response analyses suggest that higher tumor doses result in higher complete response rates. Tumor dose can be increased by applying a boost with external beam radiotherapy, endorectal brachytherapy or contact therapy. With position verification using CT, CBCT or a radiograph, verification of tumor position is difficult due to limited soft tissue contrast. Fiducial markers can be used as a surrogate for tumor position, after their position relative to the tumor is established on MRI. The aim of this study was to evaluate the MRI visibility of different gold fiducial markers implanted in the tumor, rectal wall or mesorectum. Material and Methods We included 20 rectal cancer patients who received neoadjuvant (chemo)radiotherapy. Three or four markers were inserted in the tumor, rectal wall or mesorectum by sigmoidoscopy or endoscopic-ultrasonography. We tested 4 marker types (Visicoil (0.5x5 mm and 0.75x5 mm)[IBA Dosimetry, GmbH, Germany], Cook 0.64x3.4 mm [Cook Medical, Limerick, Ireland] and Gold Anchor 0.28x20 mm [Naslund Medical AB, Sweden]), each placed in 5 patients. Two radiologists and two radiation oncologists were blinded for marker type and identified marker locations on MRI in two scenarios: without (scenario A) and with (scenario B) a rigidly registered CT or CBCT with markers available to aid in identifying the marker locations on MRI. Included MRI sequences were a transverse and a sagittal T2-TSE, a T1 3D with short TE (1.6–2.5 ms), a T1 3D with long TE (5–15 ms) and a transverse B0 map. Observers labeled marker positions on the sequence on which the marker could most accurately be identified. In addition, the observers graded the visibility of each identified marker on each sequence (0=not visible, 1=poor/average, 2=good/excellent). A marker was defined to be consistently identified if at least three observers labeled that marker on the same position on MRI. Results Of the 64 inserted markers, 41 were still present at the time of MRI as determined on corresponding CT or CBCT. Table 1 summarizes the results for scenario B. The Gold Anchor marker was the most consistently identified marker (9 out of 12). In comparison, in scenario A only 4 out of 12 present Gold Anchor markers were consistently identified. The consistently identified Gold Anchor markers were best visible on the T1 3D (long TE) sequence (86% good/excellent) and 73% were labeled on that sequence. The markers were least visible on both T2-TSE sequences (43-46% good/excellent). Examples of the Gold Anchor marker on the different MRI sequences are shown in Figure 1. Conclusion The Gold Anchor marker was the best visible marker on MRI as it was the most consistently identified marker. The use of a rigidly registered CT or CBCT improves marker identification on MRI. Standard anatomical MRI sequences are not sufficient to identify markers, it is therefore recommended to include a T1 3D (long TE) sequence.

extending outside the 95% isodose. The brain was segmented into white matter, grey matter and cerebrospinal fluid. Anatomic boundaries were delineated manually. A reaction-diffusion growth model was used assuming uniform proliferation throughout the brain and difference in white and grey matter diffusion of a factor of 10 with no migration across the barriers. The volume of image driven CTV (iCTV) corresponded to the standard CTV. The volumetric comparison was performed using the shortest distance (SD) between all surface points in the recurrence volume and respective CTV and iCTV. Results Three of five recurrences were distant, located in the temporal, parietal and frontal lobe. The median CTV and recurrence volume was 236 cm3 (range: 175-240 cm3) and 89 cm3 (range: 1-112 cm3), respectively. The median SD of the recurrence volume decreased from 7.0 mm (range: 4.7-36.0mm) for CTV to 5.5 mm (range: 2.9- 28.1mm) for iCTV indicating that iCTV was closer to the recurrence volume. However, distant recurrences were not included in either CTV or iCTV.

Vol CTV (cm3)

Vol recur (cm3)

Median SD-CTV (mm)

Median SD-iCTV (mm)

Patient no.

1 (distant) 386 2 (distant) 200 3 (marginal) 240 4 (marginal) 236 5 (distant) 175

112

8.5

5.5

1

36.0

28,1

101

5.1

2.9

89

7.0

4.3

1 2.9 Table 1 : Patient specific volume data and corresponding surface distance (SD) between original clinical target volume (CTV) and the growth model driven CTV (iCTV) 4.7

Figure 1 : Patient example with recurrence (blue contour) outside the standard CTV (light yellow contour). The light green and purple contours correspond to the delineated GTV and the growth model image derived CTV (iCTV), respectively. A, B, C: Axial, coronal and sagital slice of T1w image overlaid with the compared structures and a tumor density map. The arrow indicates the location of distant recurrence for this patient. Conclusion The image driven CTV was found superior to predict marginal recurrences outside 95% isodose compared to standard CTV. However this population was limited in size and validation is warranted in a larger cohort. EP-2115 MRI visibility of gold fiducial markers for image-guided radiotherapy for rectal cancer R.P.J. Van den Ende 1 , L.S. Rigter 2 , E.M. Kerkhof 1 , E.L. Van Persijn van Meerten 3 , E.C. Rijkmans 1 , D.M.J. Lambregts 4 , B. Van Triest 5 , M.E. Van Leerdam 2 , M. Staring 6 , C.A.M. Marijnen 1 , U.A. Van der Heide 1 1 Leiden University Medical Center, Department of Radiation Oncology, Leiden, The Netherlands 2 Netherlands Cancer Institute, Department of Gastroenterology, Amsterdam, The Netherlands 3 Leiden University Medical Center, Department of Radiology, Leiden, The Netherlands 4 Netherlands Cancer Institute, Department of Radiology,