Assessment of residual setup errors for anatomical sub-structures in image-guided head-and-neck cancer radiotherapy

Abstract

Background: To quantify residual setup errors (RSE) and required planning target volumes (PTV) margins in head-and-neck cancer (HNC) radiotherapy when using daily image guidance (IG) and less-than-daily IG protocols.

Material and methods: Daily on-line kV-image registrations of 80 HNC patients (2640 imaged treatment fractions) were retrospectively studied to analyze RSE. Less-than-daily imaging protocols, using different action levels, were simulated on the data. To quantify local RSE; single rigid bony structures were defined as landmarks. The RSEs and required PTV margins were computed for each sub-structure with and without daily IG.

Results: For less-than-daily IG protocols the setup accuracy was more dependent on frequent imaging throughout the treatment course than the number of initially imaged fractions. With daily IG the RSE of the sub-structures ranged from 0.6 mm to 2.3 mm (systematic) and from 1.0 mm to 1.7 mm (random). Required PTV margins for the sub-regions ranged from 4.5 mm to 9.3 mm with no IG and from 2.3 mm to 6.8 mm with daily IG.

Conclusion: Anatomical changes over the treatment course require frequent IG to achieve accurate dose delivery using highly conformal radiotherapy techniques. The current study shows that considerable local RSE may remain even with daily IGRT. The comprehension of local RSEs in HNC radiotherapy is important when designating PTV margins as well as tolerance levels for couch correction and plan adaption.