R1H tumors were irradiated with a single dose of 15 Gy X rays using varying sizes of treatment fields. Damage to tumor cells and tumor stroma was determined separately by analysis of growth delay to ten times treatment volume (GD10vo) and net growth delay. GD10vo comprises irradiation effects on tumor parenchymal cells and on tumor stroma, whereas net growth delay only measures effects on tumor parenchymal cells. Stromal damage was observed to increase with increasing field size; the effect on the tumor parenchymal cells, however, was independent of the field size. An increase of GD10vo of 13 days per cm increase of field size diameter was observed. From this the velocity of neovascularization of the irradiated tumor bed was calculated to be 0.30 to 0.38 mm per day.
