Room: Exhibit Hall | Forum 5
Purpose: To evaluate the feasibility of fiducial markers as a surrogate for gross tumor volume (GTV) position in image-guided radiotherapy of rectal cancer.
Methods: We analyzed 35 fiducials in 19 rectal cancer patients who received short-course radiotherapy or long-course chemoradiotherapy (LC-CRT). Two MRI exams and daily pre- and post-irradiation CBCT scans were acquired in the first week of radiotherapy. Weekly CBCT scans were acquired thereafter for patients that received LC-CRT. Between the two MRI exams, the fiducial displacement relative to the center of gravity of the GTV (COG(GTV)) and the COG(GTV) displacement relative to bony anatomy was determined. Using the CBCT scans, inter- and intrafraction fiducial displacement relative to bony anatomy was determined.
Results: The systematic error of the fiducial displacement relative to the COG(GTV) was 2.8, 2.4 and 4.2 mm in the left-right (LR), anterior-posterior (AP) and craniocaudal (CC) direction. Large interfraction systematic errors of up to 8.0 and random errors up to 4.7 mm were found for COG(GTV) and fiducial displacements relative to bony anatomy, mostly in the AP and CC directions. For tumors located in the mid- and upper rectum these errors were 9.4 (systematic) and 5.6 mm (random) compared to 4.9 and 2.9 mm for tumors in the lower rectum. Systematic and random errors of the intrafraction fiducial displacement relative to bony anatomy were <2.1 mm in all directions.
Conclusion: Large interfraction errors of the COG(GTV) and the fiducials relative to bony anatomy were found. Therefore, despite the observed fiducial displacement relative to the COG(GTV), the use of fiducials as a surrogate for GTV position reduces the required margins in the AP and CC direction for a GTV boost using image-guided radiotherapy of rectal cancer. This reduction may be larger in patients with tumors located in the mid- and upper rectum compared to the lower rectum.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Dutch Cancer Society/Alpe dHuZes Fund (grant number UL2013-6311) and the Leiden University Fund (LUF)/Nypels van der Zee Fonds (grant 3217/28-3-13/NZ).