Purpose: Evaluating the dosimetric impacts of the variations of organs during day to day delivery is a challenge. The aim of this study is to demonstrate a simple and efficient method to evaluate rectum and bladder dose volume histogram (DVH) on daily basis using MIM image registration software for prostate cancer radiotherapy.
Methods: A prostate cancer patient with 76 Gy in 38 fractions was selected for this pilot study. The rectum and bladder contours were drawn on the daily CBCT images transferred in MIM. Each daily CBCT and original CT image data set was rigid-image-registered and the dose was transferred to the CBCT in MIM. The DVHs of bladder and rectum for each CBCT and the average of all fractions of DVH with one standard deviation (SD) generated in MIM were analyzed and graphed for the bladder and rectum.
Results: Figure 1 displays the volume variations of bladder and rectum on daily CBCT. Figure 2 and 3 shows the DVH of bladder and rectum on CBCT. The variation of DVH on daily basis can be seen. Figure 4 and 5 compared the DVH of bladder and rectum between initial planning and the daily averaged DVH on CBCT. The dose parameters such as Maximum Dose and Vx are noticeable different. Table 1 lists the averaged dose parameters of maximum dose and Vx, e.g., V76, V70, V60, V50, and V40 of the bladder and rectum on CBCTs, and the corresponding dose parameters for the original planning CT.
Conclusion: This study demonstrated the dosimetric impacts due to the changes of bladder and rectum as detected on daily CBCT during the course of prostate cancer radiotherapy. The comparison of dosimetric parameters and DVH between original treatment plan and day to day delivery on CBCT shows a noticeable difference due to the organ changes.