Technical note: 9-month repositioning accuracy for functional response assessment in head and neck chemoradiotherapy

The use of thermoplastic immobilisation masks in head and neck radiotherapy is now common practice. The accuracy of these systems has been widely studied, but always within the context and time frame of the radiation delivery—some 6–8 weeks. There is growing current interest in the use of functional imaging to assess the response to treatment, particularly in the head and neck. It is therefore of interest to determine the accuracy with which functional images can be registered to baseline CT over the extended periods of time used for functional response assessment: 3–6 months after radiotherapy. In this study, repeated contrast-enhanced diagnostic quality CT and mid-quality localisation CT from a positron emission tomography/CT scanner were available for five time points over a period of 9 months (before, during and up to 6 months after chemoradiotherapy) for a series of eight patients enrolled in a clinical pilot study. All images were acquired using thermoplastic immobilisation masks. The overall set-up accuracy obtained from this 9-month study of 5.5±3.2 mm (1 standard deviation) and 1.9±1.3° (1 standard deviation) is in agreement with published data acquired over 6–8 weeks. No statistically significant change in set-up error was seen with time. This work indicates that thermoplastic immobilisation masks can be used to accurately align multimodality functional image data for assessment of the response to treatment in head and neck patients over extended follow-up periods.Head and neck cancer accounts for 5% of cancers worldwide [1] with approximately 7000 new cases being diagnosed each year in the UK. The majority of patients with squamous cell carcinoma of the head and neck (SCCHN) present with locally advanced disease. Despite recent advances in multimodality therapy and technical delivery of radiotherapy, outcomes remain suboptimal with 5-year survival rates of 50–60%. However, functional image data, provided by positron emission tomography (PET) and dynamic contrast-enhanced MRI (DCE-MRI) or diffusion-weighted MRI (DW-MRI), have been shown to have a number of potentially important applications in external beam radiotherapy for head and neck cancer [2,3]. Functional imaging is routinely used in diagnosis and staging [4], and there is increasing current interest in its application to localisation and delineation of target volumes [5] and normal tissues [6]. There is also growing evidence to support the use of functional imaging for early assessment of the response to therapy [7].Ensuring accurate registration between functional and anatomical data is clearly of paramount importance and much work has been done to date developing and testing immobilisation systems for use in radiotherapy planning and delivery. Thermoplastic mask systems have been described providing set-up accuracy in the head and neck of 2.5±1.4 mm, with no increase in systematic error seen over an 8-week period [8]. Similar accuracy was demonstrated with a polyvinyl chloride mask system [standard deviation (SD)=2.1 mm], with cut-outs in the mask to improve dose sparing to the skin but not affecting accuracy [9]. Studies using cone-beam CT (CBCT) to assess set-up accuracy have shown mean vector lengths of 4.7±1.7 mm intercranially and 7.3±4.5 mm in the neck for the thermoplastic mask [10]. Very similar SDs have been shown by other groups using thermoplastic shells and repeat CT during therapy (1.9 mm, 1 SD, for the upper neck and 5.7 mm, 1 SD, for the lower neck) [11]. Set-up error with thermoplastic masks has been shown to increase linearly with treatment time, with a SD of 1.2 mm calculated for 32 intercranial patients treated supine over 15 min [12]. In a large recent study, 762 CBCT scans were analysed from 11 patients using standard and skin-sparing nine-point thermoplastic masks [13]. The interfraction population (SD) was 1.6 mm (1.1°) (random) and 1.0 mm (1.4°) (systematic). All set-up errors >2 mm for three fractions were corrected before calculating these figures.However, if functional image data are to be accurately aligned with baseline (pre-treatment) imaging to assess response at 3 or 6 months after therapy, use of immobilisation systems over longer time intervals than the 6 or 7 weeks of radiotherapy is clearly of interest. It is well known that head and neck patients often experience dramatic shrinkage of nodal masses in the neck and general weight loss during treatment. This in turn can mean that the thermoplastic shells no longer fit perfectly and may be expected to lead to increased set-up error over longer periods of time; alternatively, weight gain following treatment may potentially reduce immobilisation accuracy. In this paper, we describe an investigation of the accuracy of head and neck patient set-up using a standard five-point thermoplastic shell system during and up to 6 months after the end of induction chemotherapy and definitive chemoradiotherapy (CRT).