Helical TomoTherapy (R) is a radiation delivery technique that uses the superposition of many small fields to precisely deliver the prescribed dose to the patient. This work presents a dose verification tool that can be used as part of a quality assurance program for a tomotherapy system. This tool is based on a small field model that takes into account the two main effects that influence the dose distribution in small fields: the extended shape of the radiation source and the loss of lateral charged particle equilibrium (CPE) within the field. The dose verification tool was implemented for simple beam configurations and used to study the influence of temporal beam parameter variations on the delivered dose. After comparing measured and calculated output factors (OFs) and dose profiles for different field configurations, it was found that they agree well to within the globally-defined gamma acceptance criteria of 2%/2mm. The study demonstrated that none of the studied systematic and random variations applied resulted in failed gamma scores using gamma acceptance criteria of 3%/3mm. The developed model implemented in the verification tool allows to evaluate the performance of devices applying narrow photon beams in the treatment delivery and, in particular, to evaluate the delivery performance of a tomotherapy unit.