Purpose: To evaluate image-guided daily cone-beam computed tomography (CBCT)-based rectum variability and its effects on delivered vs. planned dose as a potential indicator for radiation proctitis (RP) in prostate cancer patients.
Methods: Three patients with RP and four patients without RP were included, receiving 70 Gy to prostate or 70 Gy to prostate and 45 Gy to seminal vesicles (SV) in 2.5Gy/fraction with daily CBCTs. Image registrations were used to match anatomy from planning CT to daily CBCT. The rectum was re-contoured on CBCTs and saved with transferred planned dose distribution, generating rectal dose-volume histogram (DVH) of delivered dose for each fraction. Rectal volume, max dose, mean dose and proportion of rectum receiving the full prescription dose (V100%) were estimated for each fraction and compared for all patients using two-sample t-test.
Results: A total 196 fractions were analyzed for seven patients. The three patients with RP presented with rectum contours that were larger for most fractions with mean volume normalized to planning CT of 1.25 (0.18 SD), compared to 1.07 (0.28 SD) for four patients with no RP. The RP patients showed higher rectum V100% of average 12.2% vs. 6.1% from planning, with mean of 13.8% (7.6% SD) found daily compared to 6.3% (5.2% SD) for patients with no RP (P<0.001). While there was no difference for maximum rectal dose (p=0.49), we found that mean rectal dose from daily CBCT was higher for RP patients, average 35.6 Gy (7.8 Gy SD) vs. 30.0 Gy (7.2 Gy SD), p<0.001.
Conclusion: Daily variations in rectal filling were found to be larger for patients with RP, and also had on average higher rectum V100%. Our results suggest that reducing variations in rectal filling and limiting rectum in the high-dose treatment volume will be paramount, especially as hypofractionated treatment for prostate cancer becomes routine.
Cone-beam CT, Radiation Therapy, Radiation Effects