SU-F-T-40: Can CBCT Images Be Used for Volume Studies of Prostate Seed Implants for Boost Treatment?

PURPOSE: In our clinic, the planning CT is used for definitive and boost low-dose-rate (LDR) brachytherapy treatments to determine the ultrasound volume in the operating room (OR) at the time of the implant. While the CT overestimation of OR volume is known, a larger estimation discrepancy has been observed for boost treatments. A possible reason is the prostate size reduction during EBRT for boost patients. Since cone-beam CT (CBCT) is often used as routine imaging guidance of EBRT, this prostate volume change may be captured. This study investigates if CBCT taken during EBRT includes the volume change information and therefore beats CT in estimating the prostate OR volumes.

METHODS: 9 prostate patients treated with EBRT (45Gy in 1.8Gy per fractions to the whole pelvis) and I-125 seed implants (108Gy) were involved in this study. During EBRT, CBCT image guidance was performed on a weekly basis. For each patient, the prostate volumes on the first and the last available CBCT images were manually contoured by a physician. These volumes were then compared to each other and with the contoured volumes from the planning CT and from the ultrasound images in the OR.

RESULTS: The first and the last CBCT images did not show significant prostate volume change. Their average +/- standard deviation of prostate volumes were 24.4cc+/-14.6cc and 29.9cc+/-16.1cc, respectively (T-test p=0.68). The ratio of the OR volume to the last CBCT (0.71+/-0.21) was not significantly different from the ratio of OR volumes to the planning CT (0.61+/-0.13) (p=0.25).

CONCLUSION: In this study, CBCT does not show significant prostate volume changes during EBRT. CBCT and CT volumes are quite consistent and no improvement of volume estimation using CBCT is observed. The advantage of CBCT as a replacement of CT for volume study of boost LDR brachytherapy is limited.