Daily Verification of Prostate Motion Both with and Without a Rectal Balloon (RB) Over the Course of Treatment

Abstract

Purpose/Objective: To evaluate the effect of a RB on prostate motion as verified by daily imaging of fiducial markers. Other investigators have reported that a RB can be used to immobilize the prostate gland in patients undergoing external beam radiation for prostate cancer. Since prostate position depends largely on rectal filling the RB is used to standardize and thus decrease the daily variation in rectal volume. We undertook this study to compare daily prostate motion in patients with fiducial markers implanted in the prostate who were treated with and without a RB. Materials/Methods: Thirty patients were identified who were treated definitively for prostate cancer with external beam radiation using IMRT. Patients with localized prostate cancer of all stages, Gleason grades, and PSA levels were included in the analysis. Fourteen patients were treated using a RB and fifteen patients were treated without a RB. Prior to treatment planning, all patients had three gold fiducial markers placed in the prostate under ultrasound guidance. All patients underwent a treatment planning CT scan with and without a RB. The decision regarding the use of the RB was based on anatomical considerations and patient tolerance. For daily treatments, the patient was lined up to isocenter with tattoos, and orthogonal images obtained. A physician reviewed the images, and shifts were made to within a 2 mm tolerance and recorded. To assess the effectiveness of the RB as an immobilization device, we reviewed the daily shift data and compared the two patient cohorts. Additionally, we used the data to determine whether there is a change in prostate motion over the treatment course. Results: The range of prostate motion (defined as the sum of the greatest shift in the positive and negative direction) was calculated for each patient. The average range in the lateral, cranial/caudal, and anterior/posterior directions for the cohort of patients treated with the RB was 0.82 cm (min 0.2 cm, max 1.3 cm), 0.91 cm (min 0.2 cm, max 2.1 cm), and 0.96 cm (min 0.3 cm, max 1.5 cm) and for those treated without a RB 0.93 (min 0.2 cm, max 2.9 cm), 1.04 cm (min 0.2 cm, max 2.5 cm), and 1.34 cm (min 0.2cm, max 3.1 cm). We analyzed prostate motion over the treatment course by dividing the 42 fractions into trimesters of 14 fractions each. The prostate motion in the lateral and cranial/caudal dimensions did not change over the radiation treatment course. Prostate motion in the anterior/posterior dimension increased with treatment time in those patients treated without a RB but not in patients treated with the RB. We identified the maximum movement of the prostate in any dimension for each treatment. We analyzed whether this movement changed over time and unexpectedly discovered that the movement increased with each trimester to a statistically significant degree in the cohort of patients treated without a RB. The magnitude of the increase was approximately 2mm over the treatment course. This increase was not seen in patients treated with a RB. Conclusions: Use of a RB can decrease the variability in daily prostate position and the maximum shift magnitude. Prostate motion in the anterior/posterior direction actually increased over the course of treatment in patients without a balloon. This increase in prostate motion was prevented by use of the balloon. Even with the RB, the prostate position was variable and daily shifts were necessary.