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?

An assessment of the magnitude of intra-fraction movement of head-and-neck IMRT cases and its implication on the action-level of the imaging protocol

“…Thirdly, there would be a need to pilot the suggested 2 mm-AL for its appropriateness and effectiveness. Similar to Houghton et al [20] findings, there were other sources of errors which were not considered in this study. For instance, phantom transfer errors, target delineation errors and change in target shape or position due to tumour regression or growth were not evaluated [6].…”

“…Consequently, a smaller action-level or intra-fraction movements would be valuable in maintaining treatment accuracy. Houghton et al [20] highlighted an important point that currently there were no dosimetric studies undertaken to investigate the effect of geometrical misses over a small number of treatment fractions within a course of radiotherapy. Hence, the study had only addressed treatment accuracy from the perspective of a technical viewpoint in terms of margins, to maintain an optimally corrected and accurately delivered treatment [20].…”

“…Houghton et al [20] highlighted an important point that currently there were no dosimetric studies undertaken to investigate the effect of geometrical misses over a small number of treatment fractions within a course of radiotherapy. Hence, the study had only addressed treatment accuracy from the perspective of a technical viewpoint in terms of margins, to maintain an optimally corrected and accurately delivered treatment [20]. At present, it is not possible to quantify the clinical impact of those highlighted potential geometrical misses due to the lack of evidence.…”

“…Houghton et al [20] mentioned that reducing the action-level at which the systematic errors of the pre-treatment deviations were corrected improved the probability of accurate treatment delivery. Considering that CTV-PTV margin of 3-5 mm is employed for most head-and-neck cases [4], online correction with 2 mm-AL could be employed to correct for any existing deviations at the primary target volume and account for possible intra-fraction movement.…”

Computer Simulation and Analysis of Three-Dimensional Tumor Geometry in Radiotherapy

“…Image-guided radiation therapy (IGRT) uses advanced imaging technology such as on-board imaging system to achieve precise and accurate dose delivery. Many studies have reported inter-fractional organ motions and efficacy of IGRT in reducing targeting errors using daily CT images (Den et al 2009, Wang et al 2009, Houghton et al 2009, Pawlowski et al 2010, Varadhan et al 2009, Greene et al 2009). Owing to these techniques, errors in patient set-up and dose delivery can be reduced to some extent.…”