[Volume of the irradiated small intestine during pelvic radiotherapy. Comparison of various calculation methods]

In pelvic irradiation, the small bowel portion included in the planning treatment volume is one of the major factors of acute enteropathy. Three different methods are used to calculate the bowel volume: Gallagher's grid method and two systems based on specific algorithms using CT data. We compared the results of these different methods in a series of nine patients submitted to treatment volume planning simulation for pelvic irradiation, after oral barium administration. The small bowel volumes were calculated with the grid method on orthogonal radiographs. About one hour later, the patients were submitted to CT for radiotherapy planning. The small bowel regions to be irradiated were drawn manually on all CT slices on a Varian Cadplan 2.62 console. Two different algorithms were used to calculate the small bowel volumes: one of them based on polyhedral and the other on cylindric approximation. The average volumes, the variance and the determination coefficient with linear and polynomial regression were in substantial statistical agreement in the three series; the correlation index between the grid and the CT methods ranged 0.84-0.87. Therefore, the authors believe that enteric side-effects can be correlated with the irradiated small bowel volume, independent of the calculation method.