Author: Nicholas Joseph Casteloes, Brian T Collins, Sean P Collins, Shiva K. Das, Panayiotis Mavroidis, Michael Repka, Simeng Suy 👨🔬
Affiliation: University of South Florida, University of North Carolina 🌍
Purpose: To correlate two urinary symptoms with dose to bladder and urethra in patients who received SBRT with Cyberknife. Also, to use a normal tissue complication (NTCP) model to fit the dosimetric and outcome data.
Methods: 226 patients underwent prostate CyberKnife SBRT, receiving between 35-36.25 Gy in 5 fractions. Patient reported symptoms were collected using the expanded prostate cancer index composite (EPIC-26) before treatment and 12 months post-SBRT. Toxicities were reported on a 0 – 4 scale and patients with reported scores of 3 or above at baseline were excluded. The symptoms evaluated were pain during urination, and weak or incomplete emptying. Score differences of ≥2 between follow-up and baseline were considered toxic for pain and ≥3 for weak or incomplete emptying. The clinical data was fitted using the Lyman-Kutcher-Burman (LKB) NTCP model. Area under the receiver operating characteristic curve (AUC) and odds ratios (OR) were used to evaluate the goodness-of-fit of the model.
Results: Of the patients examined, 9 (4.0%) reported pain during urination and 5 (2.4%) reported weak or incomplete emptying. Incomplete emptying showed statistically significant correlations with dose to bladder and urethra. For this symptom, the AUC values for bladder and urethra were 0.81 and 0.74, whereas the OR values were 6.8 and 8.8, respectively. These correspond to associated threshold values of V15 < 95% for bladder and V40Gy < 95% for urethra, with p values of 0.04 and 0.05, respectively. The LKB model parameters of bladder for weak or incomplete emptying are as follows: TD50=44.9Gy, m=0.27, n=1.00.
Conclusion: Weak or incomplete emptying showed high correlations with dose to bladder and urethra. The dose-response curves of two urinary symptoms could be derived for bladder by the LKB NTCP model. These findings may help in reducing the rates of urinary complications for patients receiving prostate SBRT with CyberKnife.