[en] Therapeutic effects of preoperative radiotherapy (42 Gy/14 F/5 Weeks or 42.6 Gy/21 F/5 weeks) on rectal cancer were determined by comparing surgical specimens from 55 patients treated with radiation and 13 controls. The histologic radiation effects according to Ohboshi-Shimosato's classification were as follows: 7.3% (4/55) of patients were in Grade I (no effect), 49% (27/55) in Grade II a (loss of tumor cells but still many viable tumor cells remained), and 34.5% (19/55) in Grade IIb (loss of most cells but a few viable tumor cells remained), and 9% (5/55) in Grade III (no viable tumor cells detected). As a method of quantitative evaluation of therapeutic effects, Tumor Nest Index (TNI=SAxΣ NA/D) was calculated. TNIs in the irradiated groups of patients were significantly smaller than those in the non-irradiation group. TNI was supposed to be a valuable parameter for evaluating therapeutic effects of radiotherapy. (author)