SU‐GG‐J‐59: Variation of Target Position Determined by Gold Fiducial Markers for Radiotherapy of Pancreatic Cancers

Abstract

Purpose: To determine daily positional variations of the target relative to skeleton during pancreatic IMRT. To assess the dosimetric impact of these variation to target and critical organ. Methods: Eight patients with pancreatic cancer underwent placement of at least 3 gold fiducials by endoscopic ultrasound guidance. Patients were immobilized using alpha‐cradle for gated CT simulation and gated IMRT. Gated CT images were acquired with either prospective gating or retrospective gating (4D‐CT). IMRT was planned and treated at end of expiration. A gated on‐board kilovoltage (kv) image was acquired daily in the treatment position prior to IMRT. The gated kV image was registered with a DRR from the gated CT based on skeletal anatomy. The deviation in fiducial position in anterior‐posterior, superior‐inferior and left‐right directions were recorded. IMRT plans were calculated to estimate change of DVH when patient was shifted with mean daily deviation for each patient. Results: The mean (and range) daily (n=25 fractions) deviation for 8 patients 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 anterior‐posterior, superior‐inferior and left‐right directions. The dose coverage at 95% target volume would decrease by 3.7% (median, ranged 1.5–6.2%; n=8) when patient daily position was not adjusted. The cord max dose would increase by 5.8% (median, range 0.1–26.6%, n=8) when patient daily position was adjusted. The cord dose would not change (median 0% range −1.2–5.1%, n=8) when patient daily position was adjusted only in the S‐I direction. Conclusions: The daily soft‐tissue set‐up variations of target in the treatment of pancreatic cancer were measured. An additional margin is needed around the target when IMRT is delivered using skeletal registration without fiducials. Without real‐time re‐optimization, position adjustment using daily image‐guidance might need compromise between tumor and critical organs.