Target volume delineation and margins in the management of lung cancers in the era of image guided radiation therapy

The goal of radiation therapy is to deliver a lethal dose of radiation to diseased tissue while minimising dose to surrounding healthy structures. Prior to the actual treatment delivery, treatment planning is the most critical part of a patient’s radiation treatment management. The most crucial step is the accurate localisation and delineation of the target volume. Advances in tumour localisation and treatment delivery capabilities are limited by the inability to deliver treatment with complete precision to the localised tumour on a day-to-day basis over an entire course of radiation treatment. Most solid tumours are soft tissue masses, so the lack of inherent soft tissue contrast within images of intrathoracic regions can result in reduced visualisation and distinction of tumour boundaries from surrounding structures such as blood vessels, fatty tissues and lymph nodes. The ability to deliver an intended radiation dose to the tumour and to minimise the radiation dose to the healthy surrounding structures is related to the accuracy of the set up, tumour delineation, treatment plan generation and treatment delivery. These uncertainties significantly impact on the ability to deliver the intended dose to the target volume while minimising the dose to the surrounding structures. Due to these uncertainties, margins need to be added to the target volume as a buffer to accommodate the variation and uncertainties, and to ensure that the localised tumour receives the full intended dose. One of the most challenging aspects is the definition and delineation of the target volume. There are many sources of uncertainty in radiation therapy which impact on treatment accuracy and patient outcome. Geometric uncertainties cause deviations between the intended dose and the actual dose received by the tumour volume. These uncertainties consist of both external and internal factors. The external factors relate to the external patient set up displacements and the internal influences are due to organ motion and respiration. The external set up displacement can be reduced with the application of immobilisation devices to fix the patient in the treatment position and by employing image guided radiation therapy (IGRT). The implementation of IGRT in radiation treatment has reduced the impact of organ motions and set up errors. Can the increased precision mitigate the issues associated with target volume delineation? I do not think so because IGRT is only as precise as the accuracy of the delineated target volume. The precision of IGRT and the steep dose gradient of the intensity modulated radiation therapy (IMRT) technique made accurate target volume delineation ever more important. The recent article by Liang et al. investigating the effect of IGRT on the margin between the clinical target volume (CTV) and planning target volume (PTV) in lung cancer found that the application of IGRT reduced the geometric uncertainties, but was unable to completely mitigate the errors. Proper identification and precise delineation of target volume improves accuracy whereas IGRT only improves precision. Current target volume delineation protocol is based on the guideline of International Commission on Radiation Units and Measurement (ICRU) recommendations for consistent definition of target volumes, but individualised based on tumour location, size, proximity to dose-limiting structure and the probability of high-grade treatmentrelated toxicities occurrence. The largest target volume variance tends to occur in the delineation of CTV because it includes the visible gross tumour as well as subclinical and microscopic invasions which are currently below the resolution limits of anatomical imaging techniques. This problem is exacerbated by adding margins based on assumptions derived from clinical experiences based on the known pathological pattern of spread and has high degree of uncertainty. Ketting et al. reported on a study assessing the consistency of the delineation of 3D target