Is daily computed tomography image guidance necessary for nasal cavity and nasopharyngeal radiotherapy? An investigation based on helical tomotherapy

To analyze the magnitude of setup errors corrected by helical tomotherapy megavoltage computed tomography (MVCT) on a daily or weekly basis and the impact of those corrections on the delivered dose to the tumor and organs at risk (OARs), we retrospectively analyzed the setup errors for 6 nasal cavity and 4 nasopharyngeal cancer patients treated with helical tomotherapy for 25 – 33 fractions. Each patient had MVCT-guided repositioning for all fractions of treatment. The new dose–volume histograms (DVHs) and equivalent uniform doses (EUDs) for the planning target volume (PTV) and OARs were calculated for hypothetical situations in which no imaging guidance (IG) or once-weekly imaging guidance (WIG) took place. The mean total setup error for treatment without daily IG was 3.6 ± 1.0 mm, which could be reduced to 1.7 ± 0.6 mm if WIG were to be performed. The geometric uncertainties from the absence of IG resulted in a reduction of mean PTV EUD by 2.1% ± 1.0%, which could be reduced to 1.4% ± 1.0% with WIG. The EUDs of the OARs increased to 1.8 ± 2.0 Gy or 0.8 ± 1.3 Gy without and with WIG respectively. Without daily IG, the mean uncertainty in patient position has a relatively small effect on the mean dosimetry for PTV and OARs, and the use of WIG can further reduce those effects by approximately half. On the other hand, because of the large variance, with low probability, substantial deviation from the original planned dosimetry may occur without IG. Therefore, daily MVCT is preferred as an important safety measure in the delivery of intensitymodulated radiation therapy.