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Purpose: To evaluate in stereotactic body radiotherapy (SBRT) setting the impact of the spatial relationship between the target volume and main organ at risk (mOAR) using different target-multi-leaf collimator (MLC) block margins with or without fluence intensity modulation (IMRT technique).Methods/Patients: Dosimetric evaluation of 3 scenarios was performed.In the first one, the target and the main organ at risk (mOAR) had a partial overlap.In the second, the target was completely surrounded by the mOAR and in the third the target was near without touching the mOAR.Results and Conclusions: Maximal mOAR sparing was obtained with 6 mm (patients 1 and 3) and 4mm (patient 2) block margins.Concerning irradiation technique, IMRT plans showed a significant mOAR sparing in patients 1 and 3.In conclusion, a 5mm standard block margin can be proposed as optimal.Moreover, the use of IMRT in the SBRT setting may carry out a potential benefit, except when the mOAR surrounds the target.