SU-F-J-43: Positional Variation of Implanted Fiducial Markers Over the Course of Image Guided Radiotherapy for Pancreatic Cancer.

PURPOSE: Pancreas is a soft-tissue organ, implanted fiducials can change positions due to migration or tissue deformation. This study quantified positional variation of fiducials in IGRT for pancreatic cancer.

METHODS: 20 patients had at least 3 gold fiducials implanted in pancreas under EUS guidance. Patients had 4D-CT simulation for gated treatment. Daily gated OBI kV images (Turebeam) were used for positional alignment with fiducials for total of 25 or 28 fractions. Relative distances among 3 fiducials (d1 - 2 , d1 - 3 , d2 - 3 ) were measured from 4D-CT end-of-expiration phase bin; and from gated kV images in first, mid, and last fraction (n=180).

RESULTS: The median duration between implant and simulation was 11 (range 0-41) days. The median duration between simulation and first fraction was 17 (range 8-24) days. The median relative distance was 12 (range 4-78) mm for d1 - 2 , 24 (range 6-80) mm for d1 - 3 , and 19 (range 5-63) mm for d2 - 3 . The median deviation was 1 mm for d1 - 2 , d1 - 3 , d2 - 3 between simulation and first fraction, first and mid fraction, mid and last fraction (n=180). Two patients (10%) had deviation >= 5 mm (5, 11 mm) between simulation and first fraction. One patient (5%) had deviation >= 5 mm (11 mm) between first and mid fraction. No patient (0%) had deviation >= 5 mm between mid and last fraction. In all 3 cases with deviation >=5 mm, only one fiducial was significantly deviated. No clear evidence that deviation size was associated with time interval between implant and first fraction.

CONCLUSION: Implanted gold fiducials were quite stable over time in their relative positions in pancreas. Our data suggested at least 3 fiducials are needed. In cases that one fiducial was significantly deviated in daily kV images, this fiducial should be excluded in image guidance.