The lungs are the primary dose limiting organ in high dose total body irradiation (TBI). Accurate prediction of lung dose can dictate treatment success by preventing pulmonary toxicity. Normal treatment planning consists of basic hand calculations and potentially in vivo dosimetry. The use of a treatment planning system (TPS) is an unverified method at the extended treatment distance required by TBI. In this study, we attempted to analyze the Eclipse TPS for used in a TBI setup. Both its ability to determine the required monitor units (MU) and its ability to compensate for tissue heterogeneity in lung dose estimates were compared to traditional techniques and dosimetry.