Results of a two-year quality control program for a helical tomotherapy unit.

BACKGROUND AND PURPOSE Image-guided helical tomotherapy (HT) is a new modality for delivering intensity modulated radiation therapy (IMRT) with helical irradiation: the slip ring continuously rotates while the couch moves into the bore. The radiation source (Linac, 6 MV) is collimated into a fan beam and modulated by means of a binary multileaf collimator (MLC). A xenon detector array, opposite the radiation source, allows a megavoltage-CT (MVCT) acquisition of patient images for set-up verification. The aim of this paper is to report the results of a two-year quality control (QC) program for the physical and dosimetric characterization of an HT unit installed at our Institute and clinically activated in November 2004, in order to monitor and verify the stability and the reliability of this promising radiation treatment unit. MATERIALS AND METHODS Conventional Linac acceptance protocols (ATP) and QC protocols were adapted to HT with the addition of specific items reflecting important differences between the two irradiation modalities. QC tests can be summarized as: (a) mechanical and geometrical characterization of the system's components: evaluation of alignment among radiation source-gantry rotation plan-jaws-MLC-MVCT; (b) treatment beam configuration in static condition: depth dose curves (PDD) and profiles, output factors, output reproducibility and linearity; (c) dynamic component characterization: accuracy and reproducibility of MLC positioning; rotational output reproducibility and linearity, leaf latency, couch movement constancy; (d) gantry-couch and MLC-gantry synchronization; and (e) MVCT image quality. Peculiar periodicity specific tolerance and action levels were defined. Ionization chambers (Exradin A1SL 0.056 cc), films (XOmat-V/EDR2), water and solid water phantoms were used to perform quality assurance measurements. RESULTS Over a two-year period the final average output variation after possible beam output adjustment was -0.2+/-1% for the static condition and equal to 0+/-1% for the rotational condition: around 98% of the collected output data was within the action level compared to 94% if no beam output adjustment was considered. An average energy variation of -0.4+/-0.4% was found. The daily absolute dose verification of IMRT plans showed a dose reproducibility of -0.5+/-1.2% and -0.4+/-2.2%, for low and high dose gradient regions, respectively. Source-jaws-MLC and MVCT alignment results and jaw and leaf positioning accuracy were +/-1mm. Couch-gantry-MLC synchrony tests showed good stability level (<or= +/-2mm). CONCLUSIONS QC results indicated good reproducibility of all HT mechanical-dosimetric performance.