Comparison of Nodal CTV Coverage with Alignment to Bladder-Rectal Interface vs. Pelvic Bone in Post-Prostatectomy Radiotherapy

Abstract

<h3>Purpose/Objective(s)</h3> In patients receiving post-prostatectomy radiation therapy (RT), daily image guidance is used to align to the bladder-rectal interface (BRI) which varies depending on bladder and rectal filling. Alignment to pelvic bone is often used for nodal irradiation targets. The impact of alignment prioritized to the BRI on coverage of concurrent nodal targets is unknown. We hypothesized that the nodal clinical target volume (CTVn) would remain within nodal planning target volume (PTV) expansions with alignment to BRI versus bone. <h3>Materials/Methods</h3> Cone-beam CTs (CBCT) from patients receiving prostate bed RT using a 6 degrees of freedom couch were analyzed using transformation matrices to calculate differences in offsets. The CBCTs with translational offsets of ≥5mm were further evaluated for impact on nodal coverage. The CTVn was contoured according to consensus guidelines and two PTVs generated, using 5 and 7mm symmetric expansions (PTV5=5mm and PTV7=7mm) on the treatment planning CT. Each CBCT of interest was extended superiorly to evaluate the full nodal volumes. Two separate registrations of each CBCT to the treatment planning CT were performed, one with alignment prioritized to the BRI and the other to pelvic bone. The CTVn and PTVs were copied to each CBCT registration (BRI and bone) for analysis (e.g., PTVn_BRI is where the planned nodal dose is delivered when the treatment is matched to BRI). The inclusion of the CTVn within each PTV was evaluated. Boolean functions were used to compare the overlap and intersections of the nodal volumes between alignment to BRI versus bone. <h3>Results</h3> A set of 139 CBCTs from 14 patients were initially evaluated, of which 13 CBCTs had maximal translational offsets of ≥5mm. The mean CTVn was 324.3cc, PTV5 was 655.2cc, and PTV7 was 801.4cc. When alignment to the BRI was prioritized, the CTVn was completely included within the PTV7 in 9 of 13 CBCTs. At least 95% of the CTVn was within the PTV5 or PTV7 in all but one CBCT in which 92% of the CTVn was within the PTV5. The volume of the CTVn outside the PTV5 ranged from 2.7-28.4cc (mean 2.6%, range 0.6-8%) and outside the PTV7 from 0-15.4cc (mean 0.7%, range 0-4.3%). The most common location for incomplete inclusion of the CTVn within the PTV was in the most caudal and posterior nodal regions (between the prostate bed and the presacral space), primarily occurring in the event of pitch related mismatch between BRI and bone alignment. When the intersection between PTV_(bone) and PTV_(BRI) was evaluated, the overlap ranged between 80-98% and did not differ between comparisons for 5 and 7mm expansions. <h3>Conclusion</h3> These data from CBCTs with translational offsets of ≥5mm suggest that even with larger disagreement between alignment to BRI versus bone, the vast majority of the CTVn remains encompassed within the PTV, particularly when a 7mm expansion is used. The predominant site of incomplete inclusion raises concern about use of smaller posterior nodal PTV margins. These results are informative for concurrent prostate bed and nodal irradiation.