Purpose: Conformal radiotherapy (RT) has allowed radiation dose escalation to improve the outcome of prostate cancer. With higher doses, concern exists that rectal injury may increase. This study analyzed the utility and limitations of the widely used Lyman-Kutcher- Burman (LKB) normal tissue complication probability model in projecting the hazards of rectal complication with high-dose RT.
Methods and materials: A total of 128 patients were included in this study. These patients were treated with three-dimensional conformal RT alone at the University of Texas M.D. Anderson Cancer Center between 1992 and 1999. Patients were treated to 46 Gy with a four-field box technique followed by a six-field arrangement to boost the total dose to 78 Gy. All doses were delivered at 2 Gy/fraction to the isocenter. The minimal follow-up was 2 years. The end point for analysis was Grade 2 or worse rectal bleeding by 2 years. The LKB model was fitted to the data using the maximal likelihood method.
Results: Of the 128 patients, 29 experienced Grade 2 or worse rectal bleeding by 2 years. For the entire cohort, the parameters obtained from the fit of the LKB model were as follows: the volume factor was n = 3.91 (95% confidence interval [CI] 0.031 to infinity ), dose associated with 50% chance of complication for uniform whole rectal irradiation [TD50(1)] was 53.6 Gy (95% CI 50.0-75.1), and a determinant of the steepness of the dose-response curve, (m), was 0.156 (95% CI 0.036-0.271). A statistically significant difference was found in the rate of postradiation rectal bleeding in patients with hemorrhoids vs. those without hemorrhoids. The parameters obtained for the patients without hemorrhoids were as follows: n = 0.746 (95% CI 0.026 to infinity ), TD50(1) 56.7 Gy (95% CI 49.9-75.2), and m 0.092 (95% CI 0.019-0.189).
Conclusion: Our analysis suggests a dose response for rectal bleeding probability along with a volume effect. We found that the LKB model might have limited utility in determining a large volume effect. We further suggest that LKB model should be used with caution in clinical practice.