2050 - Intrafractional Changes in Bladder and Rectum Volumes during Prostate Cancer Adaptive Radiotherapy: A Preliminary Quantitative Analysis.

Purpose/Objective(s): Rectal (RV) and bladder volumes (BV) may change significantly with each treatment fraction. With the implementation of an online adaptive radiotherapy (oART), the reproducibility of organ volumes is no longer a prerequisite. Patients with prostate cancer are obvious candidates for this kind of sophisticated treatment; however, the entire process is time-consuming. The aim of this study was to determine intrafraction changes in BV and RV during an oART workflow on a treatment planning system.

Materials/Methods: Data obtained from 11 previously treated oART prostate cancer patients qualified for hypofractionated radiotherapy (Df 7.25 Gy to TD 36.25 Gy) were analyzed. The bladder and rectum were contoured on each cone beam computed tomography (CBCT) scan performed for each fraction, the initial CBCT1 at the beginning of the treatment session, and the verification CBCT2 after the adaptation process. In total, 55 adaptation sessions and 110 CBCT scans were analyzed. To reduce variability, all contours were done using AI auto-countering with the use of commercially available software. All patients were instructed to drink 500 ml of water, an empty bladder, and the rectum 60 minutes before each treatment. The patients were also instructed to follow a proper diet to minimize the risk of flatulence. A paired t-test was applied to evaluate the statistical significance of differences in bladder (BV) and rectal volumes (RV).

Results: The median time required for the adaptation workflow (AT) was 25 min (SD 5 min) and ranged from 16 to 39 min. The mean BV measured in the CBCT1 images was 222 cm³ (SD=117 cm³), while the BV measured in the CBCT2 images was 277 cm³ (SD=137 cm³) (p < 0.03). The mean RV measured in the CBCT1 and CBCT2 images was 66 cm³ (SD=17 cm³) and 67 cm3 (SD = 18 cm3), respectively (p = 0.67). In 66% of the cases, intrafractional variability in BV was =60 cm³. Large changes in BV (>100 cm³) were observed in 14,5% of the cases. These cases were analyzed in detail. In one case, such a large difference in BV resulted from the change in the doctor conducting the oART and the two-fold extension of the AT to 39 min. In the remaining cases, the mean AT was comparable and did not exceed 30 min, and the attending physician did not change. Considering that the volume differences in these patients on other days of treatment were smaller, we conclude that on a given day, the patient did not follow the preparation protocol.

Conclusion: Significant variability in BV can be observed during a single oART session, whereas the RV appears to remain stable. Differences in BV can be influenced by both the duration of the adaptation process and the problem of increased diuresis. Due to the observed cases of increased diuresis during some treatment fractions, modification of the bladder filling preparation protocol is necessary to optimize BV management during oART sessions. The other crucial issue is a well-trained adaptation team to make the whole adaptation process as fast as reasonably possible.