Fifty-eighth annual meeting of the american association of physicists in medicine
SU-F-J-69: The Dosimetric Impact of Interfraction Anorectum and Bladder Variability in Prostate Radiation Therapy
In the era of dose escalation and numerous protocols evaluating radiation delivery to the prostate, it is imperative to achieve accurate and standardized daily set up. At the Sylvester Comprehensive Cancer Center, patients are instructed to drink 8 ounces of water 30 minutes prior to RT and follow a low residue diet to ensure that the anorectum is not distended and the bladder is adequately filled. If daily CBCT imaging shows any variation, the patient is removed from the table and drinks water or evacuates their rectum prior to a repeat CBCT. Here we attempt to quantify the efficacy of this procedure.
CBCTs were collected for 5 patients receiving 40 fractions of definitive treatment for prostate cancer. CBCTs were imported into MIM (v6.5.7, Cleveland OH) and the bladder, anorectum, and prostate were contoured. Using the daily registration reviewed by the attending physician, the planning dose was rigidly transferred to the daily CBCT. On days that multiple CBCTs were performed due to inadequate anorectum or bladder preparation, the repeated and final CBCTs were evaluated for variations in V40Gy and V65Gy to both the anorectum and bladder.
A high level of variability in doses to the anorectum and bladder was found in the scans that were not utilized for treatment. The aggregate lower quartile for the unused versus used CBCTs was 27.2% vs. 16.83% for V40Gy and 8.53% vs. 5.66% for V65Gy bladder. The upper quartiles showed to be 48.88% vs. 41.92% and 21.05% vs. 20.55%. The combined lower quartile for the unused vs. used CBCTs was 8.24% vs. 5.49% for V40Gy and 0.57% vs. 0.0% for V65Gy anorectum. The upper quartiles were 34.35% vs. 33.25% and 18.37% vs. 16.11%.
This study shows that daily imaging is insufficient and that proper bladder and anorectum preparation are essential to deliver proper treatment.