The dosimetric impact of image guided radiation therapy by intratumoral fiducial markers.

Purpose: Pancreatic fiducials have proven superior over other isocenter localization surrogates, including anatomical landmarks and intratumoral or adjacent stents. The more clinically relevant dosimetric impact of image guided radiation therapy (IGRT) using intratumoral fiducial markers versus bony anatomy has not yet been described and is therefore the focus of the current study.

Methods And Materials: Using daily orthogonal kV or cone beam computed tomography (CBCT) images and positional and dosimetric data were analyzed for 12 consecutive patients treated with fiducial based IGRT and volumetric modulated arc therapy to the intact pancreas. The shifts from fiducial to bone (ΔFid-Bone) required to realign the daily fiducial-matched pretreatment images (kV, CBCTs) to the planning computed tomography (CT) using bony anatomic landmarks were recorded. The isocenter was then shifted by (ΔFid-Bone) for 5 evenly spaced treatments, and the dosimetric impact of ΔFid-Bone was calculated for planning target volume coverage (PTV50.4 and PTV47.9) and organs at risk (liver, kidney, and stomach/duodenum).

Results: The ΔFid-Bone were greatest in the superoinferior direction (ΔFid-Bone anteroposterior, 2.7 ± 3.0; left-right, 2.8 ± 2.8; superoinferior, 6.3 ± 7.9 mm; mean ± standard deviation; P = .03). PTV50.4 coverage was reduced by 13% (fiducial plan 95 ± 2.0 vs bone plan 82 ± 12%; P = .005; range, 5%-52%; >5% loss in all; and >10% loss in 42% of patients), and to a lesser degree for PTV47.9 (difference, -8%; range, 1%-30%; fiducial plan 100 ± 0.3% vs bone plan 92 ± 7.6%; P = .003; with reductions of >5% in 66% and >10% in 33% of patients). The dosimetric impact of ΔFid-Bone on the organs at risk was not significant. Positional shifts for kV- and CBCT-based realignments were nearly identical.

Conclusion: Compared with matching by fiducial markers, IGRT matched by bony anatomy substantially reduces the PTV50.4 and PTV47.9 coverage, supporting the use of intratumoral pancreatic markers for improved targeting in IGRT for pancreatic cancer.