Comparison of Cone Beam CT and kV/kV Imaging Using Fiducial Marker for PTV margin Definition in IGRT of Localized Prostate Cancer

Purpose/Objective(s): Image-guided radiation therapy (IGRT) using fiducial markers (FM) or cone beam CT (CBCT) is the preferred method in delivering RT for prostate cancer (PC) enabling precise targeting and dose escalation. These techniques should enable reduced CTV to PTV margins resulting in lower toxicity. We applied the Van Herk formula to calculate the margin for PTV of prostate using CBCT or FM. Materials/Methods: Twenty patients with low, intermediate or high risk PC received IGRT (VMAT) to a dose of 74Gy/37f, 60Gy/20f or 57Gy/ 19f. Three gold FM were implanted prior to treatment. Patients were planned and treated with rectal balloon (RB). CBCT was performed on day 1 to 3, then once every week. KV-image pairs (kV/kV) were used on all other days. For 10 patients CBCT was performed after each fraction to assess intra-fractional motion. Contouring of the prostate (P) for low risk and of the prostate/seminal vesicles (PSV) for intermediate/high risk PC was performed independently by 3 clinicians and mean differences in vertical, lateral and longitudinal directions between the intersections and the unions of the three contours were assessed. The Van Herk formula M Z 2.5S + 0.7s was used for PTV margin calculation, where S represents the systematic and s the random error. We calculated the systematic errors for intra-fractional motion, contouring as well as setup using CBCT, kV/kV and FM referring to prostate position in the CBCT and random errors for intra-fractional motion and patient setup. Results: Calculated PTV margins were 6.1 mm vertically, 7.1 mm longitudinally and 4.4 mm laterally for contouring and intra-fractional but not setup errors. Systematic error for contouring was more influential than intra-fractional motion (0.9-1.4mm) and more pronounced in intermediate/high risk (2.5-3.2mm) than in low risk PC (1.3-2.1mm). Including setup errors, enlargement was least for CBCT and kV/kVimaging using FM: +0.4-0.6mm (P) / +2.0-2.3mm (PSV) vertically, +0.1-0.7mm (P) / +2.5-2.9mm (PSV) longitudinally and +0.3mm (P) / +3.0-3.1mm (PSV) laterally. Higher increases were noticed for bone matching: +1.8-2.0mm (P) / +2.2-2.4mm (PSV) vertically, +1.4-1.5mm (P) / +3.6mm (PSV) longitudinally and +0.4-0.7mm (P) / +3.0-3.3mm (PSV) laterally. Largest increases were found for rectal balloon matching: +1.3-1.6mm (P) / +2.8-3.0mm (PSV) vertically, +0.9-4.5mm (P) / +3.1-6.2mm (PSV) longitudinally and +0.5-2.0mm (P) / +3.1-4.1mm (PSV) laterally. Conclusion: Calculated PTV margins are comparable using either CBCT or kV/kV-imaging FM matching. The systematic error for contouring was more influential than intra-fractional motion. Margins for intermediate/ high risk PC are notably larger than those for low risk PC due to seminal vesicles. Author Disclosure: C. Oehler: None. S. Lang: None. P. Dimmerling: None. C. Bolesch: None. A. Tini: None. C. Glanzmann: None. U.M. Luetolf: None. Y. Najafi: None. G. Studer: None. D.R. Zwahlen: None.