Prostate-Centric vs. Bony-Centric Registration in the Definitive Treatment of Node-Positive Prostate Cancer With Simultaneous Integrated Boost: A Dosimetric Comparison.

PURPOSE/OBJECTIVE(S) To determine the impact of daily shifts based on rigid registration to intraprostatic markers (IPMs) on coverage of boost doses delivered to gross nodal disease for prostate cancer. MATERIALS/METHODS Seventy-five cone-beam CTs (CBCTs) from 15 patients treated with definitive radiation for clinically node-positive prostate cancer underwent fiducial-based and pelvic bony-based registration to the initial planning scans. Gross tumor volumes of nodal boost targets were contoured directly on each CBCT registration. The nodal displacement (3-dimensional translation from the node centroid on planning CT to node centroid on registered CBCT) and dose coverage (minimum dose [Dmin], mean dose [Dmean], dose delivered to 95% of the GTV [D95]) were calculated for each registration on all nodal targets. All doses for each node were normalized to its prescription dose (dose covering 95% of a 3 mm PTV expansion). RESULTS Forty-one gross nodal targets were analyzed. Most boosted nodes (80.5%, 33/41) were treated with volumetric-arc radiation therapy (VMAT), while 19.5% (8/41) underwent stereotactic body radiation therapy (SBRT). Dmin, Dmean, and D95 were all significantly lower with fiducial-based registration compared to bony-based registration (P < 0.0001). Nodal displacement was significantly higher for fiducial-based registrations (P < 0.0001). The 3-dimensional translation between the fiducial-based and bony-based registrations (bony-to-fiducial vector) was the most significant predictor of nodal displacement (P < 0.0001). On fiducial-based registrations, a 3-5 mm gross nodal PTV margin is sufficient in most directions; however, superior and posterior margins of 8-9 mm are required as a result of asymmetric prostatic motion. CONCLUSION Large and anisotropic PTV margins are likely needed to adequately dose gross nodal targets when patient setup is based on rigid registration to IPMs. Alternative approaches such as adaptive replanning may be required to overcome these limitations.