Image-guided radiotherapy using surgical clips as fiducial markers after prostatectomy: A report of total setup error, required PTV expansion, and dosimetric implications

Abstract Purpose To determine the total setup error and the required planning target volume (PTV) margin for prostate bed without image guided radiotherapy (IGRT), and to demonstrate the feasibility and dosimetric benefit of IGRT post prostatectomy using surgical clips. Materials and methods Seventeen patients were treated with intensity modulated radiotherapy (IMRT) to the prostate bed with a 1 cm PTV margin. Three-dimensional shifts of the surgical clips inside the prostate bed were measured with respect to the isocenter from 364 orthogonal kV image pairs, and the total setup error was calculated to determine the required PTV margin. Alternative IMRT plans using 5 mm or 1 cm PTV expansion were generated and compared for rectal and bladder sparing. Results Surgical clips were reproducibly and reliably identified. The mean (standard deviation) shifts in the left–right (LR), superior–inferior (SI), and anterior–posterior (AP), axes were: −0.1 mm (1.7 mm), 0.6 mm (2.4 mm), and −2.1 mm (2.6 mm), respectively. The required PTV margins were calculated to be 6, 8, and 9 mm in the LR, AP, and SI axis, respectively. A PTV expansion of 5 mm, compared to 1 cm, significantly reduced V65 Gy to the rectum by 10%. Conclusions In the absence of IGRT, a non-uniform PTV margin of 6 mm LR, 8 mm AP, and 9 mm SI should be considered. Use of clips as fiducial markers can decrease the total setup error, enable a smaller PTV margin, and improve rectal sparing.