PREOPERATIVE RADIOTHERAPY IN RECTAL-CARCINOMA - ASPECTS OF ACUTE ADVERSE-EFFECTS AND RADIATION TECHNIQUE

Purpose: To explain a possible association between treatment technique and postoperative mortality after preoperative radiotherapy of rectal carcinoma, the dose distributions were compared in model experiments. Methods and Materials: Preoperative radiotherapy with a three-beam te chnique delivered in five fractions to 25 Gy (5 Gy/daily for 5 or 7 da ys) was given to patients with primary resectable rectal carcinoma. Th e adverse effects of this treatment, both acute and late, have been lo w. In a parallel trial using an identical fractionation schedule and t otal dose but with a two-beam technique, the postoperative mortality w as higher. Two-, three-, and four-beam techniques were analyzed in 20 patients with computed tomography based, three-dimensional dose planni ng. Dose distributions and dose-volume histograms in the planning targ et volume (PTV) and in the organs at risk were considered. A numerical ''biological'' model was used to compare the techniques. Results: The two-beam and the four-beam box techniques give the most homogeneous d ose distributions in the PTV, although all techniques result in dose d istributions that would be considered adequate, provided 16 MV or high er photon energies are used. Three- and four-beam techniques show adva ntages over the two-beam technique with respect to organs at risk, par ticularly the small bowel. With the two-beam technique and the upper b eam limit at mid-IA, the volume of the bowel that receives >95% of the prescribed dose, and hence, is included in the treated volume (TV), i s more than twice as large as that with three- and four-beam technique s, and that of the total body between 1.5 and 2 times as large. The re sults of the analyses using the biological model indicate that the thr ee- and four-beam techniques result in less small bowel complication r ates than the two-beam technique. The integral energy to the total bod y is similar for all treatment modalities compared. Conclusions: The v olume of bowel included in the TV, rather than the energy imparted to the body, influences postoperative mortality, and emphasizes the impor tance of precise radiotherapy planning to minimize normal tissue toxic ity.