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 four rectal symptoms with the dose to rectum in patients, who received SBRT with Cyberknife for prostate cancer. Also, to fit those dosimetric and outcome data by using a normal tissue complication (NTCP) model.
Methods: 230 patients underwent prostate CyberKnife SBRT, receiving 35-36.25 Gy in 5 fractions. Patient reported symptoms were collected using the expanded prostate cancer index composite (EPIC-26) score before treatment and 12 months post-SBRT. Toxicities were reported on a 0–4 scale. Patients with reported scores of 3 or above at baseline were excluded. The symptoms evaluated were urgency, frequency, bloody stool, and abdominal/pelvic/rectal pain. Score differences of ≥2 between follow-up and baseline were defined as toxic for bloody stools and pain and ≥3 for urgency. The clinical data was fitted using the Lyman-Kutcher-Burman (LKB) NTCP model.
Results: Of the examined patients, 5 (2.2%) reported increased urgency, 11 (5.0%) reported increased frequency, 10 (4.3%) reported bloody stools, and 17 (7.7%) reported pain. Urgency, bloody stools and pain showed statistically significant correlations with dose. For those three symptoms, the area under curve (AUC) values were 0.63, 0.70 and 0.66, the odds ratio (OR) values were 6.3, 6.2 and 3.0, which are associated with the threshold values of V25<20%, V30<10% and V20<35%, with p-values of 0.05, 0.01 and 0.03, respectively. The parameter values for the LKB model, TD50 (Gy), m, and n were as follows: 34.1Gy, 0.12, 0.10 for urgency, 31.9Gy, 0.24, 0.10 for bloody stool and 24.0Gy, 0.32, 0.66 for pain.
Conclusion: Urgency, bloody stool, and abdominal/pelvic/rectal pain showed high correlations with rectal dose-volume metrics. The dose-response curves of three rectal symptoms could be derived by the LKB NTCP model. These findings may help guide clinicians on assessing dosimetric goals that can reduce the rates of rectal complications in patients receiving prostate SBRT with CyberKnife.