An assessment of action levels in imaging strategies in head and neck cancer using TomoTherapy. Are our margins adequate in the absence of image guidance?

AIMS: To assess the effectiveness of different on-treatment correction strategies on set-up accuracy in patients with head and neck cancer (HNC) treated on a TomoTherapy HiArt system. To assess the adequacy of clinical target volume (CTV) to planning target volume (PTV) treatment planning margins when treating with intensity-modulated radiotherapy without daily image guidance.
MATERIALS AND METHODS: The set-up accuracy measured by daily online volumetric imaging was retrospectively reviewed for the first 15 patients with HNC treated on the TomoTherapy unit at Addenbrooke's Hospital. For each fraction, megavoltage computed tomography was carried out, any discrepancy from the planning scan was noted, and corrected, before treatment. These data were used to evaluate imaging correction protocols using three different action levels. The first three fractions were imaged and used to correct for systematic error, using a 5 mm action level (5 mmAL), a 3 mm action level (3 mmAL), and no action level (NAL). All imaging strategies were applied, to assess the number of fractions that would potentially have exceeded a 5 and 3 mm margin. Systematic and random errors were calculated for the population, assuming the NAL protocol had been applied, and minimum CTV-PTV margins, required to allow for errors attributable only to set-up, were calculated using van Herk's formula.
RESULTS: In total, 490 fractions were analysed. Using a 5 mmAL imaging protocol, potentially 198/490 fractions (40%) were outside a 5 mm CTV-PTV margin and 400/490 (82%) were outside a 3 mm margin. Using a 3 mmAL imaging protocol, potentially 67/490 fractions (14%) were outside a 5 mm CTV-PTV margin and 253/490 (52%) were outside a 3 mm margin. A small systematic error was identified in the system; once corrected this would improve these results. Using the NAL imaging protocol, potentially 31/490 fractions (6%) were outside a 5 mm CTV-PTV margin and 143/490 fractions (29%) were outside a 3 mm margin. Estimated minimum CTV-PTV margins to account only for set-up errors, with three-fraction image-guided radiotherapy and a NAL protocol, were 2.8, 3.1 and 4.1 mm in the mediolateral, superior-inferior and anterior-posterior directions, respectively.
CONCLUSION: Reducing the action level at which the systematic error is corrected improves the probability of treatment delivery accuracy. Using the NAL correction protocol reduces the number of fractions that have set-up displacements outside a 5 mm CTV-PTV margin. Although a 5 mm margin is probably sufficient for standard HNC radiotherapy, change to a 3 mm margin is not favoured at our centre without access to daily image-guided radiotherapy.