Role of fiducial markers in the assessment of prostate bed motion in post-prostatectomy patients treated with volumetric modulated arc therapy

Aim: Accurate localisation of target position is crucial when using techniques with sharp dose fall off such as volumetric modulated arc therapy (VMAT). Gold seed fiducial markers have been used for target localisation in image-guided radiation therapy for various tumors including intact prostate cancers. However, their role for target localisation in post-prostatectomy radiotherapy is unclear. This study was undertaken to determine the feasibility and effectiveness of gold seed fiducial markers in patients undergoing prostate bed VMAT. Materials and methods: The institutional radiation oncology database was used to analyse the treatment data of 18 post-prostatectomy patients with implanted gold seed fiducial markers. The shifts of the fiducial markers were reviewed, tabulated and statistically analysed. Results: Three hundred and eighty-six orthogonal pair images for 18 patients were reviewed. Specifically, the average gold seed fiducial shifts were 0.34 cm in the superior-inferior (S/I) axis (0.31 SD), 0.31 cm (0.29 SD) in the anterior-posterior (A/P) axis and 0.28 cm (0.25 SD) in the lateral axis (R/L). As a result, the 95% probability of fiducial marker displacement was 0.96 cm in the S/I, 0.89 cm in the A/P and 0.78 cm in the R/L axes. The most frequent shifts occurred in the inferior, left and posterior directions. The percentage of shifts more than 0.5 cm were 19.74, 21.56 and 12.47% for the S/I, A/P and R/L axes, respectively. Conclusion: In the absence of fiducial markers, non-uniform planning target volume (PTV) margins of 1 cm for S/I, 9 mm for A/P and 8 mm for the lateral direction are necessary for target localisation in post-prostatectomy radiotherapy. By improving prostate bed localisation, gold seed fiducial markers can decrease PTV margins, reduce normal tissue radiation exposure and allow for dose-escalated and/or hypofractionated radiotherapy to be considered in appropriate clinical scenarios.