Evaluation of late rectal toxicity after conformal radiotherapy for prostate cancer: a comparison between dose-volume constraints and NTCP use

PURPOSE: To analyze the reliability of different methods used in evaluating the risk of late rectal toxicity. PATIENTS AND METHODS: The treatment plans of 57 patients treated at the authors' institute between September 1999 and September 2000 for localized prostate cancer using three-dimensional conformal radiotherapy (3D-CRT) were analyzed retrospectively. The expected rate of late rectal toxicity was analyzed (a) by means of the dose-volume histogram (DVH) constraints; (b) by calculating the normal-tissue complication probability (NTCP) using the Lyman-Kutcher-Burman (LKB) model with the radiobiological parameters of either Emami (1991; for toxicity of grade >or= 2) or Rancati (2004; for toxicity of grade >or= 2 and >or= 3). Patients were divided into high-/low-risk (HR/LR) groups and the results were compared to the clinical outcome. RESULTS: (a) The HR percentages were 24% and 5% for radical and postsurgical 3D-CRT, respectively. When applying high-dose constraints only, HR percentages were 18% and 5%, respectively. (b) In the case of the NTCP (grade >or= 2), Emami (1991) HR rates were 16% and 11%, and Rancati (2004) HR rates 29% and 11%, for radical and postsurgical treatment, respectively. Only one case with higher-grade toxicity was found. The reported clinical toxicity was 17.8% and 6.7% for grade >or= 2 toxicity, and 3.7% and 0.7% for grade >or= 3 toxicity, for radical and postsurgical treatment, respectively. CONCLUSION: This study demonstrated that there is an agreement between the toxicity rate evaluated by DVH constraints and by the LKB model and the clinical outcome. In this case, the use of the LKB model can be as reliable as the use of DVH constraints