Purpose: Tomotherapy is a form of intensity modulated radiation therapy (IMRT) that utilizes rotating fan beams modulated by a binary‐multileaf collimator. The radiation is delivered either serially or helically as the patient is moved in a craniocaudal sequence for tumor coverage. While Tomotherapy can deliver highly conformal dose distributions, it yields the lowest delivery efficiency (tumor dose per MU) of current IMRT‐delivery options. This relatively low efficiency has the potential for delivering high total‐body doses due to head leakage, so a quantitative evaluation of the whole‐body dose is warranted. Methods and Materials: We conducted this evaluation for a dedicated helical Tomotherapy delivery device (Hi‐Art System®, Tomotherapy Inc) and compared the results against the previously published serial Tomotherapy system (Corvus, NOMOS Corporation) and traditional IMRT whole‐body data. A typical head‐and‐neck treatment plan (2Gy per fraction, 6622MU) was prepared and delivered to a large water‐equivalent phantom. An ADCL‐calibrated large‐volume ionization chamber (A17 Exradin) was used to measure the low doses. The dose was measured at both 1.5cm (dmax) and at the center of the phantom. Results: From 10cm to 48cm from the inferior target edge (the most proximal serial tomotherapy point was at 10 cm), the helical tomotherapy dose was less than 0.5% of the target dose, and was between 20% and 30% of the serial tomotherapy leakage dose. This study showed that the whole‐body dose for the 70Gy is approximately 140mSv. This dose is less than the 560mSv for a 70Gy treatment as published by Followill, et al. (Int.J.Radiat.Oncol.Biol.Phys. 38, 667). Conclusions: This study indicates that the commercial helical Tomotherapy system provides less whole‐body dose than serial Tomotherapy or conventional IMRT. This is probably due to the internal linear accelerator shielding design and the use of 6 MV photons. This work was supported in part by funding from Tomotherapy, Inc.