Challenges in the target volume definition of lung cancer radiotherapy

Abstract

Radiotherapy, with or without systemic treatment has an important role in the management of lung cancer. In order to deliver the treatment accurately, the clinician must precisely outline the gross tumour volume (GTV), mostly on computed tomography (CT) images. However, due to the limited contrast between tumour and non-malignant changes in the lung tissue, it can be difficult to distinguish the tumour boundaries on CT images leading to large interobserver variation and differences in interpretation. Therefore the definition of the GTV has often been described as the weakest link in radiotherapy with its inaccuracy potentially leading to missing the tumour or unnecessarily irradiating normal tissue. In this article, we review the various techniques that can be used to reduce delineation uncertainties in lung cancer. The findings of this review indicate that to date, it is still not possible to eliminate interobserver variation in the definition of GTV. Positron Emission Tomography (PET-CT) has an important role in improving the staging accuracy and the definition of the tumour. Various autosegmentation tools have also been proposed to fully or partially automate the delineation process. However, their development is currently hindered by the unavailability of absolute gold standards that can be used to train and validate these algorithms. Hence, manual delineation is still considered to be the gold standard. Nevertheless, auto-segmented contours can provide a good starting point, eventually reducing the delineation time and interobserver variation. Improvements in image quality can also reduce the delineation uncertainty in some cases. The main factor leading to interobserver variation is image interpretation differences between clinicians. Therefore, protocols, training and peer review checks of delineated contours are essential to address this challenge. The development of the MR-linac will also present new challenges and opportunities in optimising the definition of the target volume as well as in the development of adaptive radiotherapy strategies.