The impact of intratumoral stroma on the expression of the tumor bed effect was studied in R1H rat rhabdomyosarcoma. For this, either R1H tumors growing in subcutaneous tissues (in situ) were irradiated at median volumes of 40 mm3 with a single exposure of 15 Gy (200 kVp X rays), or fresh tumor fragments or cloned tumor cells cultured in vitro were transplanted into subcutaneous tissue 2 days after preirradiation with 15 Gy. The tumor bed effect was evaluated by the time necessary for experimental tumors to grow from 0.1 cm3 to 10 cm3. When irradiated in situ, R1H tumors expressed a clear-cut tumor bed effect. The growth rate was decreased by a factor of 1.8 +/- 0.2 compared to controls. A significant decrease in growth rate by a factor of 1.7 +/- 0.2 was also observed when cultured tumor cells were transplanted into preirradiated tissue. However, when tumor fragments were transplanted into preirradiated sites, no significant decrease in growth rate occurred in two independent experiments (1.0 +/- 0.1; 1.2 +/- 0.2). Our results indicate that vessels originating from the intratumoral stroma can modulate the expression of the tumor bed effect in R1H tumors.
