Original articleTarget Volume Definition for Head and Neck Intensity Modulated Radiotherapy: Pre-clinical Evaluation of PARSPORT Trial Guidelines
Introduction
Xerostomia is the most prevalent long-term complication after radiotherapy for head and neck cancer in patients who require bilateral neck irradiation and is associated with significant deterioration in the patient's quality of life 1, 2, 3. Intensity modulated radiotherapy (IMRT) has been shown to achieve significant reductions in the dose delivered to the parotid glands and several phase II studies have suggested lower xerostomia rates and improvements in quality of life 4, 5, 6, 7, 8. Most of these early clinical reports fail to give clear protocols for target volume definition, making reproducibility difficult. The ability to reduce the radiation dose to the parotid gland is largely determined by its proximity to the planning target volume (PTV) [9] and therefore is significantly affected by differences in target volume definition.
In the UK, a multicentre randomised controlled trial of parotid-sparing IMRT vs conventional radiotherapy is underway (PARSPORT). The trial aims to compare the incidence of LENT SOM (late effects on normal tissue: subjective, objective, management) grade 2 xerostomia 1 year after treatment between patients treated with parotid-sparing IMRT and conventional radiotherapy Agreement has been reached among the trial participants as to the methods of target volume definition. The aim of this current study was to calculate the theoretical outcomes of the application of our detailed target volume definition for oropharyngeal tumours in terms of dosimetric and calculated normal tissue complication probability.
Section snippets
Patients and Radiotherapy Planning
Five patients with histologically proven locally advanced squamous cell carcinoma of the oropharynx (T1-4, N1-2b M0) who met the entry criteria for the PARSPORT trial were evaluated. All patients were immobilised using a custom-made cabulite head and neck mask in the conventional treatment position, i.e. neutral neck with a straight spine. They then underwent radiotherapy planning computed tomography of the head and neck, from the supra-orbital ridge to the carina. Both non-contrast-enhanced
Target Volumes
Typical dose distributions for the three-dimensional radiotherapy and IMRT plans are shown in Fig. 3 and typical DVHs in Fig. 4. Plan objectives were achieved for all IMRT plans, but not all three-dimensional radiotherapy plans.
Table 3 shows the mean (± standard deviation) dose statistics, inhomogeneity coefficient, D95% and conformity indexes for the three-dimensional radiotherapy and IMRT plans. On average, maximum doses to both PTVs were higher for the three-dimensional radiotherapy plans (
Discussion
A number of phase II trials of parotid-sparing IMRT have been reported. These studies contained small numbers of patients, but concluded that sparing the parotid gland contralateral to the primary tumour reduces the rates of xerostomia, and increases salivary flow from the spared gland. Patterns of recurrence reported in these trials show that a proportion of tumours recur in the locoregional area, mostly within the areas treated with a high radiation dose 24, 25, 26. These recurrences are
Acknowledgements
The authors would like to thank F. M. Buffa, Gray Laboratories, Mount Vernon Hospital, Rickmansworth, UK.
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