Therapeutic advantage of grid irradiation for large single fractions.

PURPOSE: In the present work, we used model calculations of cell survival to compare the effects of single fraction high-dose grid therapy with those of uniform dose delivery on tumor and normal tissues. METHODS AND MATERIALS: The grid consisted of a hexagonal pattern of divergent holes in a Cerrobend block. A linear-quadratic model was used to find the surviving fraction of tumor and normal tissue cells after high-dose irradiation. Equivalent uniform doses were determined according to the tumor cell kill. The ratio of the normal tissue surviving fraction under grid irradiation to that obtained under equivalent uniform dose irradiation was taken as a measure of therapeutic gain.
RESULTS: The therapeutic ratio varied from 0.80 to 13.22 for the range of cell sensitivities investigated, with single fraction doses of 10.0-20.0 Gy. Optimization studies showed no significant dependence of therapeutic gain on hole spacing.
CONCLUSION: With high, single-fraction doses, grid irradiation revealed a therapeutic advantage over uniform dose irradiation whenever the tumor and surrounding normal tissues cells were equally radiosensitive, or, particularly, if the tumor cells were more radioresistant than the normal cells. The therapeutic gain did not appear to be a strong function of grid design.