Verification dosimetry during treatment for helical tomotherapy using radiographic film

Helical tomotherapy (HT) is a novel radiotherapy treatment modality that allows the delivery of intensity modulated radiation in a rotational fashion. Due to the complexity of the treatment approach, it is desirable to have a simple tool for treatment delivery verification. Radiographic film placed under the patient is exposed to dose from most of the possible beam projections and therefore constitutes a useful in vivo dosimetry record of the whole treatment. Measurements were performed during the initial clinical implementation of HT at the London Regional Cancer Centre on all patients during the first treatment fraction. It was possible to predict the optical density of the film using a dose calculation on a phantom of similar size to the patient. The comparison of expected and delivered dose allows the verification of dose delivery patterns which was found to be particularly useful in the case of treatment interruptions. The absolute dose measured with film differed in general by less than 10% from the expected one despite the fact that no build-up was used on the film. The agreement improved with proximity of the primary target to the location of the film on the treatment couch. Due to the rotational delivery mode, radiographic film was shown to be a useful, cheap and convenient method to verify dose delivery in helical tomotherapy.