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

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.

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.

The occurrence of proctitis was significantly correlated with three dosimetric parameters: D98% of low ano-rectum, D_98% and D_mean of low ano-rectum wall. A predictive linear logistic regression model reports that the D_98% 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.

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).