Predictors of Intra-Fraction Prostate Motion during Stereotactic Body Radiation Therapy (SBRT)

Abstract

The ultimate success and safety of prostate stereotactic body radiation therapy (SBRT) relies on adequate organ motion management. The hypothesis of this study is that there are patient clinical and anatomical variables that predict for prostate motion during treatment. Data were obtained on 52 prostate cancer patients treated with 5 or 6 fraction VMAT SBRT using simultaneous MV/kV imaging for intra-fraction fiducial marker monitoring. Patients were given strict instructions to have a full bladder and empty rectum for simulation and each treatment. 260 cone beam CTs (CBCTs) were used to measure pre-treatment bladder, rectal and gas volumes. Prostate motion end points included number of “beam-offs” and shifts from the position of the day. “Beam offs” occurred when therapists interrupted treatment due to fiducial makers tracking off by >1.5 mm from their reference for 12-15 seconds. Shifts in the lateral, ventral-dorsal and cranial-caudal directions, in addition to the three-dimensional vector from position of the day were measured at 35 MV/kV imaging control points during 2 minutes of treatment and averaged over each fraction. For each patient, urinary symptom score (IPSS), age and body mass index (BMI) were collected as well as the presence of rectal spacer and use of urinary dysfunction medications. Prostate motion data were evaluated per patient and per fraction. Statistical analysis included Spearman’s rank-order correlation, Wilcoxon Rank-Sum test and a Linear mixed model fit by restricted maximum likelihood. The average patient age, BMI and pre-treatment IPSS were 70 years old (range: 56-84), 30 kg/m2 (range:19-55), and 8 (range: 0-21), respectively. Most patients (48/52) had rectal spacers placed prior to SBRT. Data analysis for predictors of intrafraction prostate motion revealed that larger pre-treatment bladder and rectal volumes were associated with more beam offs per fraction (p = 0.018 and p = 0.014, respectively). In addition, there was a trend for larger bladder volumes to predict greater shifts in the ventral-dorsal direction (p= 0.052) and magnitude of the 3D vector (p = 0.051). Patients using urinary medications had fewer number of beam offs during their treatment course (p = 0.038). Patient age, BMI, baseline IPSS and pre-treatment gas volume did not correlate with prostate motion during treatment. Our results suggest that patient bladder and rectal volumes at the time of treatment can influence prostate motion during SBRT. We can conclude that rectal emptying prior to treatment is critical for rectal dose avoidance and prostate motion mitigation, however future studies will be needed to determine the optimal bladder volume during prostate SBRT.