New dosimetric parameters to predict ano-rectal toxicity during radiotherapy treatment.

ABSTRACT

PURPOSE: Radiotherapy is essential in the treatment of locally advanced rectal cancer. Side effects of radiotherapy in the treatment of rectal cancer have a great effect on quality of life. The aim of this retrospective study is to evaluate the correlation between dosimetric parameters and acute toxicity in rectal cancer patients. METHODS: We analyzed the Dose Volume Histogram parameters for both the target structures and the Organs at risk of 89 patients. A dedicated statistical analysis was performed for all the acute toxicities showing a frequency rate higher than 20%. A linear logistic regression model was elaborated using the variable showing the highest level of significance at the univariate analysis. RESULTS: The occurrence of proctitis was significantly correlated with three dosimetric parameters D98% of low ano-rectum, D98% and Dmean of low ano-rectum wall. A predictive linear logistic regression model reports that the D98% of the wall of the low ano-rectum must be < 38.5 Gy to decrease the rate of proctitis. A general analysis on grade 2 acute toxicity occurrence reported that it was correlated with D98% of low ano rectum. CONCLUSIONS: Two dose constraints were elaborated D98%<33.5 Gy for low ano rectum to prevent grade 2 acute toxicity and D98%<25 Gy for low ano-rectum wall to prevent proctitis (grade 1 or superior).