<h3>Purpose/Objective(s)</h3> Interfraction anatomical changes during intensity-modulated radiotherapy for prostate cancer may lead to suboptimal patients' treatment. This study assessed the dosimetric effects of anatomical changes on pretreatment kilovoltage computed tomography (kVCT) scans of a novel dual on-board kVCT/positiron emission tomography (PET) imaging radiotherapy system. <h3>Materials/Methods</h3> This retrospective study analyzed daily kVCT scans acquired on days 1, 5, 13, 17, and 22 of a post-prostatectomy patient treated with 44 Gy/22 fractions to the prostate bed and lymph nodes. The prostate bed CTV, pelvic lymph nodes, rectum and bladder were re-contoured on the daily kVCT, and treatment plans were subsequently created. The treatment doses were calculated using the HU density table of kVCT. Delineation and treatment plans created on the daily kVCT scans were compared with those created on the planning CT. Interfraction changes of bladder and rectum were analyzed based on median dose (Dmedian), mean dose (Dmean), maximum dose (Dmax), volume receiving 10 Gy (V10), volume receiving 30 Gy (V30), and overall volume. For bladder and rectum, the correlation coefficient between volume and Dmean was analyzed. <h3>Results</h3> For the kVCT days investigated in this study, mean bladder and rectum volume changes were 10.69 cc ± 10.08 cc and 1.31 cc ± 0.64 cc, respectively. Consequentially, the average changes in Dmedian, Dmean, Dmax for the bladder were 0.51 Gy ±0.32 Gy, 0.30 Gy ±0.22 Gy, and 0.75 Gy ±0.44 Gy, respectively. For the rectum, the average changes in Dmedian, Dmean, Dmax were 0.62 Gy ±0.36 Gy, 0.59 Gy ±0.49 Gy, and 0.76 Gy ±0.57 Gy, respectively. The average changes in V10, and V30 were 0.48 % ±0.56 %, and 1.1 %± 0.66 % for bladder and 1.48 % ±1.27 %, and 1.83 %± 1.34 % for rectum. The correlation coefficient (r2) for bladder-Dmean and Rectum-Dmean were 0.765 and 0.002, respectively. <h3>Conclusion</h3> This study showed that with adaptive planning, the interfraction dose variation was minimal because adaptive plans were optimized based on daily kVCT images (this is one primary benefit of adaptive planning). Even with adaptive planning, there is still a correlation between bladder dose and bladder volume. The high-quality kVCT images produced could potentially be used for target and OAR delineation and consequentially for treatment planning. Treatment delivery could further be improved by kVCT adaptive planning, which will account for possible interfraction variations.