The bladder-rectal interface (BRI) position is often dependent on bladder and rectal filling in post-prostatectomy patients. Nodal irradiation is of increasing importance given recent randomized data. It is unknown how alignment to the BRI may impact nodal coverage. We hypothesized that the shifts between alignment to the BRI vs pelvic bone would fall within the standard nodal PTV expansion. CBCTs from patients receiving prostate bed RT using 6 degrees of freedom (6DOF) couch were analyzed. At time of treatment, each CBCT was approved by a prostate specialized radiation oncologist prioritizing alignment to the BRI and prostate bed. Retrospectively each CBCT was re-aligned to pelvic bone. Reference bone points were chosen to represent the superior extent of pelvic nodal target volumes {midline sacral promontory (SP) and anterior aspect of sacroiliac joints at the S1-S2 (LSI and RSI)}. Three cardinal coordinates of each reference point (x, y, and z), were transformed to the machine coordinate system. Transformation matrices were defined by the parameters: lat (x), vert (y), long (z), pitch, roll, and rotation obtained with prospective matching. Using transformation matrices, alignment to the BRI was compared to bone alignment for each reference point by differences between the two sets of coordinates (x”-x’), (y”-y’), (z”-z’). A total of 139 CBCT from 14 patients were evaluated. Differences between coordinates with both alignment techniques were calculated and evaluated across all image sets (see table below). Mean differences between spatial alignment to BRI vs bone were <1.5 mm and >95% of all offsets were ≤5 mm. Approximately 75% of all coordinate differences were ≤1 mm and 90% were ≤3 mm. Less than 4% were >5 mm. Only 2 patients (4 CBCTs) exhibited >7 mm offsets between BRI and bone points. Coordinate differences >3 mm and >5 mm were more common in y (12.5% and 6%) and z (14.6% and 2.9%) than in x dimension (1.4% and 1.4%), respectively. Differences in pitch, roll, and rotation >1° were noted in n = 8, 3, and 1 CBCT sets, respectively. These data suggest that alignment to the prostate bed and BRI did not result in large offsets at the superior aspect of the pelvis. With rare exception, the required shifts should fall within the standard nodal PTV expansion. The impact of bladder and rectal filling on nodal target alignment and coverage is the subject of ongoing investigation.Abstract 4102; TableMean (mm)Std Dev (mm)≤ 1 mm (n)>1-3 mm (n)>3-5 mm (n)>5-7 mm (n)>7 mm (n)Point 1 SPx0.33 (0-8.72)0.951307011y1.40 (0-11.18)1.938140963z0.97 (0-5.92)1.50102191440Point 2 LSIx0.37 (0-8.81)0.981307011y1.39 (0-11.17)1.938042953z0.97 (0-5.93)1.48102161830Point 3 RSIx0.36 (0-8.64)0.941307011y1.40 (0-11.17)1.938141953z1.03 (0-6.20)1.55100171750 Open table in a new tab