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?

Abstract 

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 5mm action level (5mmAL), a 3mm action level (3mmAL), and no action level (NAL). All imaging strategies were applied, to assess the number of fractions that would potentially have exceeded a 5 and 3mm 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 5mmAL imaging protocol, potentially 198/490 fractions (40%) were outside a 5mm CTV–PTV margin and 400/490 (82%) were outside a 3mm margin. Using a 3mmAL imaging protocol, potentially 67/490 fractions (14%) were outside a 5mm CTV–PTV margin and 253/490 (52%) were outside a 3mm 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 5mm CTV–PTV margin and 143/490 fractions (29%) were outside a 3mm 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 4.1mm 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 5mm CTV–PTV margin. Although a 5mm margin is probably sufficient for standard HNC radiotherapy, change to a 3mm margin is not favoured at our centre without access to daily image-guided radiotherapy.

Key words: Head and neck cancer, IGRT, imaging protocols, IMRT, tomotherapy