Contouring variability in radiosurgery - dosimetric and radiobiological implications

The use of Stereotactic Radiation Therapy (SRT) employing one large fraction of radiation, as in stereotactic radiosurgery (SRS), or few fractions of high doses, has continuously increased due to the technical development and the progress in dose delivery complemented by the positive clinical experience. The success of stereotactic radiation therapy depends on many clinical, dosimetric and radiobiological factors. For SRS in particular, the delivery of a highly conformal dose distribution to the target in one fraction allowing at the same time the sparing of the normal tissue and the critical structures is part of the basic concept of the technique. Provided that the highly accurate radiosurgical equipment available today is used, planning and delivering the prescribed dose distribution is an achievable goal, and therefore the main issue to be solved is the definition of the target. As the target volume in radiosurgery is usually defined without margins, the success of the stereotactic approach critically depends on the accurate delineation of the target which could be identified as a factor of key importance. In addition, the delineation of the Organs At Risk (OAR) is also critical. The purpose of this work was to evaluate the current degree of variability for target and OAR contouring and to establish methods for analysing multi-observer data regarding structure delineation variability. A multi-center target and OAR delineation study was initiated. Two complex and six common cases to be treated with SRS were selected and subsequently distributed to centers around the world performing Gamma Knife® radiosurgery for delineation and treatment planning. The resulting treatment plans and the corresponding delineated structures were collected and analysed. Results showed a very high variability in contouring for four complex radiosurgery targets. Similar results indicating high variability in delineating the OAR and reporting the doses delivered to them were also reported. For the common radiosurgery targets however, a higher agreement in the delineation was observed, although lower than expected. The assessment of the quality of treatment planning for radiosurgery is usually performed with respect to the coverage of the target, the planning specificity, and dose to the sensitive structures and organs close to the target. However, physical dose conformity to the target does not guarantee the success of the treatment. The assessment of the plan quality should also be performed with respect to the clinical outcome expressed as probability of controlling the target that should be irradiated. In this respect, this study also aimed to create the framework for assessing the impact of the inaccuracy in delineating the target on the predicted treatment outcome for radiosurgery targets known for their high potential to invade the neighbouring normal tissue, using radiobiological models. In addition, radiobiological models have also been used to determine the tumour control probability accounting for the oxygenation for stereotactic radiation therapy targets. The results suggest that radiobiological modelling has the potential to add to the current knowledge in SRS by theoretically assessing the key factors that might influence the treatment outcome