SU-E-T-556: Dosimetric Comparison of the Bladder and Rectum Based On Daily CBCT for Prostate IMRT

Purpose: With the use of CBCT, delivered dose can be calculated by transferring the planned beams onto the CBCT. Bladder and rectum volumetric doses were calculated and correlated to the daily bladder and rectum fullness. Methods: Patients for this study underwent hypofractionated prostate IMRT to 70 Gy in 28 fractions. Daily CBCT was utilized for image guidance. A clinically acceptable plan was created using a CTV to PTV margin of 5mm. Image fusion was performed to transfer the bladder and rectum contours onto each CBCT. Contours were then edited to match the anatomy of each CBCT. Using the daily treatment isocenter, the planned beams were transferred onto the CBCT and daily and cumulative DVHs calculated. Results: At the time of planning the bladder volume was 470.66cc. The mean, minimum and maximum bladder volume from the 28 CBCTs was 230.57cc, 76.16 cc and 380.8 cc, respectively. Planned bladder V70Gy was 0.98%. Actual bladder V70Gy, as calculated from the CBCTs was 2.46±1.36%. Similarly, planned V60Gy, V50Gy and V40Gy were 2.55%, 4.15% and 6.35% of the total bladder volume. The observed volumes receiving 60Gy, 50Gy and 40Gy were 5.18±2.53%, 4 to 8.21±3.76% and 6 to 12.93±5.60%, respectively. On the planning CT the rectummore » volume was 41.5cc. The observed mean, minimum and maximum rectum volumes were 58.09cc, 93.52 and 47.50cc. The planned rectum V70Gy was 6.02% while the observed cumulative rectum V70Gy was 13.68±3.39%. Rectum V60Gy, V50Gy and V40Gy changed from 13.61% to 22.5±3.86%, 19.15% to 28.68±4.45 and 25.56% to 34.75±5.15%. Conclusion: Bladder and rectum volumes changes during treatment have an affect on the cumulative dose received by these organs. It was observed that volumetric dose received by bladder decreases as volume of the bladder increases. There is no particular trend observed between volumetric dose and rectal volume.« less