Abstract
RapidArc (Volumetric Modulated Arc Therapy) is an advanced radiotherapy technique which can achieve excellent target volume coverage and organ at risk (OAR) sparing. The potential role of RapidArc in the treatment of head and neck cancer (HNC) was evaluated by planning studies comparing RapidArc with fixed gantry IMRT, and a prospective study to assess clinical outcomes in patients treated with RapidArc. In the planning study evaluating oropharyngeal tumours, RapidArc achieved comparable dose distributions to IMRT with significantly improved dose conformity and reduced monitor units (MUs). However, in the study evaluating nasopharyngeal and paranasal sinus tumours, target volume coverage and sparing of some OARs were marginally inferior with RapidArc. Preliminary results have indicated that toxicity is acceptable in patients treated with RapidArc and comparable to fixed gantry IMRT. Further work examined 18F-FDG-PET-guided dose escalation in patients with HNC and was shown to be feasible using RapidArc. A planning study comparing RapidArc with fixed gantry IMRT in this setting showed that RapidArc could achieve significantly improved dose conformity, improved parotid sparing and reduced MUs. Modelling studies evaluating the dosimetric and biological impact of set-up error in RapidArc plans were conducted. The results confirmed that set-up error can result in a reduction in target volume coverage and increase in OAR dose, which could lead to a decrease in tumour control probability and increase in normal tissue complication probability. The effects were greater with systematic compared to random errors, and increased in proportion to the magnitude of the errors. In the study evaluating set-up error in 18F-FDG-PET-guided dose escalation, there was a trend towards a greater dosimetric effect with smaller boost volumes but a weak trend towards a greater biological impact with larger boost volumes. In the study evaluating set-up error in nasopharyngeal and paranasal sinus tumours, even small errors (<3mm) were found to have a detrimental effect for some OARs.