Purpose/Objective(s): Grid therapy with megavoltage photon beams has been clinically shown to improve local control of bulky tumors. The purpose of this study is to develop intensity modulated grid therapy (IMGT) with multiple beams. We hypothesize that when compared to conventional single-field grid therapy, IMGT will provide lower spatially fractionated doses to the surrounding organs at risk (OAR) while increasing the minimum dose to the whole tumor volume and thus improve tumor control probability as supported by conventionalradiobiology. Materials/Methods: IMGT was developed with multiple photon beams using dynamic MLC segments and orthogonal blocks. Spatially fractionated fluence patterns for all beams were first optimized by inverse planning. The optimized intensity modulated grid patterns were then segmented by an in-house sliding window technique for radiation dose delivery. To evaluate IMGT, a retrospective planning study was performed on 10 lung patients with bulky tumors (volume _ 500 cc and maximum diameters _ 10 cm). IMGT plans were compared with conventional singlefield grid therapy plans with the prescription dose normalized to a maximum dose of 15 Gy to the planning target volume (PTV) in one fraction. A paired two-tailed t-test was performed on dose volume parameters to identify differences in target volume coverage and OAR sparing. Results: The results of our analysis is summarized in Table 1, where the V7.2Gy and V16.9Gy metrics represent biological equivalent doses (fractions Z 30, a/b Z 3) of V20Gy for lung and V65Gy for skin respectively. Compared to conventional single-field grid therapy plans, IMGT plans improve dose coverage to the target volume significantly. With the exception of the mean dose to the lung, IMGT either maintained or reduced doses to the OARs. Conclusion: Intensity modulated grid therapy improves target volume dose coverage while reducing or maintaining the spatially fractionated dose distributions to most of the surrounding organs at risk.