Session Item

To compare the dosimetric differences and volumes of organs at risk (OARs) and planning target volumes (PTVs) in patients who were treated with prostate SBRT.

Patients with low and intermediate-risk prostate cancer were included. All the patients had proper rectal and bladder preparation before the simulation. The acquired planning CT images were transferred to the Precision treatment planning system (TPS). Before the contouring, MR images were fused with planning CT images. Gross tumor volumes (GTVs), PTVs, and OARs were contoured and robotic-based SBRT plans were generated using the same dose objectives for each patient. The prescribed doses were 36,25Gy or 37Gy/5F to PTV. All the patients are treated with robotic radiosurgery machine and fiducial tracking algorithm. The same rectum and bladder preparation protocol were applied before each fraction and a new CT scan (PreCT) was obtained. PreCT images were sent to Precision TPS. Original plans and structures were transferred to PreCT using PreciseRTx software and first predicted plans (FPPs) were created. Dose and volume changes in PTV and OARs were compared with the original plan and treatment was initiated. After the daily treatment, another CT image (PostCT) was taken from the patients, and second predicted plans (SPPs) were created transferring the original plans and structures to the PostCT with PreciseRTx. Dose and volume differences in OAR and PTV were examined and compared with FPPs.

Ten patients were included in the analysis. No clinically significant differences were observed between the original plans and the FPPs about PTV coverage and the maximum point doses within the PTV (95,8% vs. 96,4% and 41,3Gy vs. 41.3Gy respectively). When the FPPs and SPPs were compared, there were no clinical differences for PTVs. The bladder volumes in the original CTs were not statistically different from their volume in PreCT (p=0.08) and there was no statistically significant difference for bladder dose criteria (all p>0.05). However average bladder volumes increased from 171,4cc to 393,9cc after the treatment and this was statistically significant (p<0,001). This increase in volume caused a decrease in bladder doses between FPPs and SPPs (V50%: 25,8% vs. 24,3%; V100%: 3,02% vs. 1,7%). In terms of rectum volumes and doses, statistically significant differences were not found between the original CTs and PreCT, and between the PreCT and PostCT (p>0.05). 

As long as there is a strict preparation for OARs and accurate tracking for prostate SBRT, daily adaptation might not be feasible and necessary.