Nonuniform Planning Target Volume Margins for Prostate Bed on the Basis of Surgical Clips on Daily Cone Beam Computed Tomography

PURPOSE: We hypothesized that the interfraction motions of the superior and inferior prostate beds differ and therefore require different margins. In this study, we used daily cone beam computed tomography (CBCT) to evaluate the motion of postprostatectomy surgical clips (separated to superior and inferior portions) within the planning target volume (PTV) to derive data-driven PTV margins. METHODS AND MATERIALS: Our study cohort included consecutive patients with identifiable surgical clips undergoing prostate bed irradiation with daily CBCT image guidance. We identified and contoured the clips within the PTV on the planning computed tomography and CBCT scans. All CBCT scans were registered to the planning computed tomography scan on the basis of pelvic bony structures. The superior border of the pubic symphysis was used to mark the division between the superior and inferior portions. RESULTS: Eleven patients with 263 CBCT scans were included in the cohort. In the left–right direction, the global mean M, systematic error Σ, and residue error σ were 0.02, 0.03, and 0.16 cm, respectively, for superior clips, and 0.00, 0.03, and 0.03 cm, respectively, for inferior clips. In the anterior–posterior direction, the corresponding values were M = 0.01, Σ = 0.25, and σ= 0.37, respectively, for superior, and M = 0.08, Σ= 0.13, σ= 0.15, respectively, for inferior. In the superior–inferior direction, the values were M =-0.06, Σ= 0.23, and σ= 0.27, respectively, for superior, and M =-0.01, Σ= 0.21, σ= 0.20, respectively, for inferior. The results of the 2-tailed F tests showed that the anterior–posterior motion is statistically different between the superior and inferior portions in the anterior–posterior direction. There is no statistical difference in the superior–inferior and lateral directions. Therefore, we propose a set of nonuniform PTV margins (based on the formula 2.5 Σ+ 0.7σ) as 0.2 cm for all prostate beds in the left–right direction, 0.7 cm for all in superior–inferior, and 0.9 to 0.4 for superior–inferior in the anterior–posterior direction. CONCLUSIONS: The difference in motion between the superior and inferior portions of the prostate bed is statistically insignificant in the left–right and superior–inferior directions, but statistically significant in the anterior–posterior direction, which warrants a nonuniform PTV margin scheme.