Audit of fiducial marker placement and utility in image guided prostate radiotherapy IS Bhattacharya, R Hughes Mount Vernon Cancer Centre, Northwood, United Kingdom Background Fiducial markers are inserted prior to radiotherapy to allow image guided treatment. 3 single markers are inserted except for trial patients where 2 paired markers are used within our centre. There are no guidelines on the optimum separation between markers and what the acceptable proportion of usable markers is. The aim is to identify what proportion of markers are usable, compare single and double markers and measure the shifts performed. Methods Demographic data, usability of markers, shortest distances between markers and shifts applied was collected on markers inserted between January and September 2013. Results 113 patients had single markers inserted; these were not used in 13 patients. 7 patients had double markers inserted and all were used. In the 13 patients where markers were not used; 2 had changes of radiotherapy plan and 1 required CBCT to assess bladder filling. Markers were unusable in 10/13 patients; 4 had only 2 visible markers, 5 had inappropriately positioned markers either too closely related or positioned outside the gland and 1 the markers moved during treatment. Table 1: Average distances between single markers on D1 and D19   AP distance1 AP distance 2 PA distance 1 PA distance2 DAY1 1.16(0.22-3.41) 2.14(0.63-3.62) 0.98(0.22-2.42) 1.49(0.34-2.96) DAY 19 1.15(0.33-3.22) 2.13(0.08-4.02) 0.93(0.22-2.44) 1.43(0.35-2.93) Table 2: Average distances between paired markers on D1 and D39   AP distance 1 AP distance 2 PA distance 1 PA distance 2 DAY 1 2.35(1.77-3.1) 2.75(2.38-3.51) 0.82(0.82-1.35) 0.96(0.69-1.65) DAY 39 2.25(1.77-2.96) 2.71(2.34-3.44) 0.71(0.56-1.28) 0.98(0.63-1.6) Table 3: Shifts applied using single and double fiducial markers (cm)   Average AP shift AP shift >0.5cm (n) Average S/I shift S/I shift >1cm (n) Average R/l shift R/L shift >1cm (n) DAY 1 Single markers 0.37 (0-1.58) 21 0.26 (0-1.69) 1 (0-1.01) DAY 19 0.39 (0-1.33) 19 0.33 3 0.25 (0-0.8) Double markers 0.56 (0.1-2.28) 2 0.6 (0-2.63) 0.32 (0.1-0.64) DAY 39 (0.09-0.7) 0.24 (0.09-0.34) 0.23 (0-0.5) Conclusion 10/113 (8.8%) were markers not used. Both types of marker allowed image guided radiotherapy however paired markers were more evenly distributed in the gland and maybe more representative of prostate position. The majority of shifts applied were in anterior/posterior direction. This audit established a baseline of marker usability and implant technique. A switch to paired markers for all patients may allow better prediction of prostate motion and fewer unusable markers. Reductions in shifts may potentially allow us to reduce our margins. For further information please contact: indrani.bhattacharya@nhs.net