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Head and Neck Cancer Radiation Therapy Atlas-based Auto-contouring – Balancing Accuracy with Efficiency in OnQ rts ® Susan Barley, 1 Clare Antoine, 2 Gareth Webster, 2 Marie Tiffany, 2 Navinah Nundlall, 2 Rosemary Simmons 3 and Andrew Hartley 4 1. Lead Clinical Scientist, Oncology Systems Limited, Shrewsbury, UK; 2. Medical Physicist; 3. Specialist Radiographer; 4. Clinical Oncologist, Queen Elizabeth Hospital, Birmingham, UK Abstract Atlas-based auto segmentation with OnQ rts ® has been shown to deliver time-savings for the delineation of organs at risk in head and neck patients being treated with intensity-modulated radiotherapy. However, as the initial time to set up atlases can be high in busy departments the optimal number of atlas cases needed for auto-contouring was investigated. Using conformity index and mean distance to conformity to compare automatically generated with gold standard clinical contours, it was found that the majority of contours were unaffected by reducing the number of atlas cases from 30 to 10. The optimum number of atlas cases, however, was considered to be 20 due to the reduction in accuracy of the mandible, larynx and brain, below this level. Keywords Radiation therapy, atlas-based segmentation, radiotherapy planning, deformable image registration, OnQ rts Disclosures: Susan Barley is an employee of Oncology Systems Limited. Clare Antoine, Gareth Webster, Marie Tiffany, Navinah Nundlall, Rosemary Simmons and Andrew Hartley have no conflicts of interest to declare. Compliance with Ethical Guidelines: All procedures were followed in accordance with the responsible committee on human experimentation and with the Helsinki Declaration of 1975 and subsequent revisions, and informed consent was received from the patients involved in this study. Received: 13 October 2014 Accepted: 2 December 2014 Citation: European Oncology & Haematology, 2014;10(2):98–101 Correspondence: Susan Barley, Oncology Systems Limited (OSL), 14 Longbow Close, Shrewsbury, SY1 3GZ, UK. E: susan.barley@osl.uk.com Support: The publication of this article was supported by Oncology Systems Limited. Delineation of contours remains the one part of radiotherapy that is completely manual in the majority of centres and therefore the potential for time-saving is huge, 1 being estimated to range from 23 % 2 to 41 %. 3 One of the additional benefits of auto-contouring is the reduction in inter-observer variation, which in general is not quantified in radiotherapy. Balik et al., 4 showed that, when comparing the demons and small deformation inverse consistent linear elastic (SICLE) algorithm for propagating contours between cone beam computed tomography (CBCT) scans of non-small cell lung carcinoma (NSCLC) patients taken weekly, over 7 weeks that the Dice similarity coefficient (DSC) for the two algorithms was similar to the DSC seen for manual contouring between different trained observers. This shows that all contours should be independently checked by a separate clinician. 3,5 However, if atlas-based contours are computer generated, the need for the second clinician is replaced by one clinician checking the contours produced by the software: 3 another potential manpower saving of an auto-contouring system. create customised atlases, knowing what will work and how many atlases are needed in the library is too much of a challenge to already over-stretched centres, despite the time savings of a fully operational system being well cited in the literature. One cancer centre that has embraced the use of atlas-based segmentation to outline the normal anatomy for all radical head and neck radiotherapy treatment plans is Queen Elizabeth Hospital (QEB), Birmingham, UK. Following on from research that showed that the use of OnQ rts (Oncology Systems Limited, UK) (one of a number of commercially available software products that can be used for atlas-based auto segmentation) could reduce outlining time by 36 minutes – from 90 down to 54 minutes – work has now been performed to establish the optimal number of atlases required in the atlas library for the contouring of head and neck structures and which structures are most affected by the number of atlases used. This work will be reported in this paper. Background Sharp et al. 6 performed an overview of the current status of auto segmentation in clinical radiotherapy. One of the main findings of the paper was that the atlases used should be customised and department specific. The slow uptake in clinical use by centres that have purchased commercially available software products that perform atlas-based segmentation is in many cases linked to the need for atlases to be customised and department specific. The initial input of time to 98 OnQ rts is a stand-alone software product that has been designed for the display, evaluation, co-registration and fusion of medical images, auto-contouring of anatomical structures and display of radiation therapy dose distributions to aid in radiation therapy planning. The auto-contouring protocol uses atlas-based segmentation of CT image data and is available for the following anatomical sites: head and neck, male pelvis, and thorax. Eur op ean On c ol og y & Ha e matolog y