Carcinoma of the salivary glands constitutes 1–3% of head and neck cancer (HNC), with most tumours arising in the parotid glands. The mainstay of treatment is surgery, with post-operative radiotherapy in selected cases. Post-operative radiotherapy reduces local recurrence rates from 30% to 10% but has not been shown to impact on overall survival [1
]. The aim of radiation therapy is to deliver homogeneous high-dose radiation to the target area with relative sparing of the healthy surrounding tissues. The physical delivery of radiotherapy has improved with the introduction of intensity-modulated radiotherapy (IMRT), which allows greater sparing of the surrounding healthy tissue and reduces the risk of long-term treatment morbidity [2
], and image-guided radiotherapy (IGRT), which helps to improve geometric localisation of the high-dose volume to the target.
Sources of geometric uncertainty in dose delivery have been well characterised in modern IMRT workflows [3
]. Despite this, geometric uncertainty due to target volume and normal tissue structure segmentation has escaped close scrutiny. It has been suggested in a number of tumour sites that the largest systematic error in radiotherapy planning today is related to the radiation oncologist's ability to localise and delineate the target volume [4
]. When using IMRT and inverse planning, accurate and consistent segmentation of target and normal tissues impacts not only on the spatial localisation of the high-dose volume, but also on accuracy of dose calculation to organs at risk (OARs). Inaccuracies in segmentation can compromise treatment, either by unnecessarily treating the healthy surrounding tissue (which may increase treatment morbidity) or by inadequate target segmentation risking marginal tumour recurrence.
Previously published studies have assessed techniques to reduce interobserver and intraobserver variation in segmentation of gross tumour volume (GTV) and clinical target volume (CTV). These include the use of multimodality imaging such as MRI and positron emission tomography (PET)–CT [10
], multidisciplinary team contouring [5
], use of segmentation guidelines [22
] and using auto-delineation software [23
]. Studies have looked at interobserver and intraobserver variation in GTV (primary tumour) segmentation for HNC [10
]. Changes in the patient anatomy post surgery introduce difficulties for post-operative CTV segmentation. There is limited data on interobserver variation in CTV segmentation in HNC. Rasch et al [9
] found significant interobserver variation in CTV segmentation in patients with paranasal sinus tumour, especially in the anterior border of the nasal cavity, possibly due to lack of anatomical landmarks to define the edge of the volume.
We studied interobserver variation in the segmentation of CTVs and OARs for patients receiving post-operative radiotherapy after parotidectomy and the impact of introducing segmentation guidelines for the CTV. We tried to replicate the clinical setting by asking radiation oncologists to outline both the primary tumour bed CTV and high-risk nodal groups (if appropriate). Currently, there are well-established guidelines for segmentation of CTV nodal volumes in HNC [27
], but there are no standard segmentation guidelines for patients receiving post-operative radiotherapy to the parotid bed. Cochlear sparing therapy and conventional radiation (COSTAR) is a UK National Cancer Research Institute-supported head and neck trial, designed to demonstrate a difference in sensori-neural hearing loss with cochlear sparing IMRT compared with conventional radiotherapy [28
]. The COSTAR trial management group created a segmentation protocol for the parotid bed CTV, which includes landmarks for defining extent of microscopic disease and recommends guidelines for elective and post-operative nodal irradiation. We obtained written permission to use the COSTAR segmentation guidelines for this study.