PurposeTransitQA is an innovative method for Tomotherapy transit dosimetry using the on‐board detector (OBD). Our previously published model for Tomotherapy treatment plan verification (AirQA) has been enhanced to take into account patient and couch transmission. AirQA estimates the OBD signal during irradiation with nothing in the beam path from the leaf control sinogram, allowing us to check whether the planned treatment is correctly delivered by the machine. TransitQA allows us to check the treatment delivery with the patient on the couch, potentially showing the effects of changes in the patient anatomy and delivery errors.MethodsPatient and couch transmission have been added to the model using the OBD projections of pretreatment megavoltage computed tomography (MVCT). The difference in the energy spectra between the imaging and treatment beams has been corrected by an exponent from the MVCT projections consisting of the ratio of the mass attenuation coefficients. This exponent has been found to not vary significantly with the atomic number Z, allowing us to apply this procedure to heterogeneous media, such as patients. The attenuated OBD projections acquired during the treatment are compared to the model via a signed global γ‐index analysis. The dose criterion was 5% of the 95th percentile of the dose distribution, and the distance to agreement (DTA) was 4 mm.ResultsOur method has been applied to a heterogeneous phantom with 98.1% of the points passing the γ‐evaluation test, showing that the model can predict the attenuated OBD projection. The method has been applied to two representative patients throughout the whole treatment, highlighting variations in the signal transmission and γ‐index.ConclusionThis paper establishes the proof‐of‐concept of transit dosimetry for all patients treated by Tomotherapy. Moreover, this method can be used as a surrogate for in vivo dosimetry.