Early Experience with the Use of Gold Fiducial Markers in the Image Guided Radiotherapy (IGRT) of Pancreatic Cancers

Abstract

To study daily set up variations of the target during pancreatic IMRT using implanted Gold Fiducial Markers. To determine daily variations in target position relative to skeletal anatomy and to estimate margins required around the target when IMRT is delivered without using fiducials, but using skeletal anatomy for Image Guidance. Nine patients with pancreatic cancer underwent placement of at least 3 gold fiducials in the tumor (n = 6) or residual pancreas (n = 3), by endoscopic ultrasound guidance (EUS). Patients were immobilized using alpha-cradle for RT. For RT planning, 2 gated CT scans (at end of expiration & end of inspiration performed with exact immobilization) or a 4-D CT was obtained. 54, 50 and 45 Gy in 25 fractions was planned to the GTV, GTV+1cm, and lymphatics respectively in patient with inoperable tumors. In post-op RT, 50 Gy was planned to the tumor bed and 45 Gy to the lymphatics. An appropriate gating margin was added to all volumes to account for residual respiratory movements. All patients were treated using gated IMRT on a linear accelerator. A gated on-board kilovoltage (kv) image was obtained daily in the treatment position prior to RT and fused with planning gated DRR. Treatment was delivered following alignment of fiducial positions on the kv and DRR images. For purpose of this analysis, image fusion was also performed based on skeletal anatomy, which is the current standard of care and the shifts of fiducials in the anterior-posterior (A-P), superior-inferior (S-I) and left-right (L-R) directions were recorded. A non-gated CT was performed at week 4 of RT and another at 5 weeks following RT to check for possible migration of fiducials. There was no increase in time required for gated kv imaging and image fusion using fiducials, compared to conventional IGRT using skeletal registration. Migration of fiducial markers was not seen based on CT imaging. The mean daily shifts (and range) between images based on skeletal fusion and fiducial location were 0.2 cm (0.1-1.0 cm), 0.5 cm (0.2-1.5 cm) and 0.4 cm (0.2-2.0 cm) in the A-P, S-I and L-R directions respectively. A margin of 0.4, 1.0 and 0.8 cm (2 standard deviations) would be needed in the A-P, S-I and L-R directions to account for soft-tissue set-up error when daily imaging is performed solely using skeletal alignment. The daily soft-tissue set-up variations of target in the treatment of pancreatic cancers were studied. IGRT using implanted fiducials helps to reduce the target margins compared to skeletal registration. An additional margin is needed around the target when IMRT is delivered using skeletal registration without fiducials. Our data also suggests that the margins currently used in conventional pancreatic RT may be inadequate for some patients.