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Background: We have validated the monitor unit calculations from a commercially available treatment planning system (TPS) for three intensity modulated radiotherapy (IMRT) planning techniques for tangential breast irradiation by using ionization chamber measurements. Materials and Methods: Treatment plans were generated for forty-two breast patients by a forward planned field in field technique, electronic tissue compensation (ETC), and an inverse planned sliding window technique. We also performed a reproducibility of delivery and dose linearity analysis for each technique. The treatments were delivered to a phantom using a Varian CL21EX linear accelerator. A 2571 0.6 cm³ Farmer type ionization chamber and Farmer 2570/1 electrometer from NE Technology was used to measure output of the linear accelerator and the dose at predefined point in the verification plan. Results: The agreement between the measured and calculated dose was -0.87% ± 0.54% for field in field technique, -0.74% ± 0.23% for electronic tissue compensators, and -1.26% ± 0.48% for the inverse planning technique and. In terms of reproducibility the mean deviation was -1.10% ± 0.44% for the field in field technique, -0.38% ± 0.42% for electronic tissue compensators, -1.04% ± 0.42% for inverse planning technique. Dose linearity experiments showed no significant variations for clinical situations but a breakdown was observed in relative dose for very low monitor units. Conclusion: We have found that the monitor unit calculations for all three planning techniques are correct to the order of 1%, and that the plans can be delivered in a reproducible and accurate manner. Iran. J. Radiat. Res., 2011 9(3): 145-150