Variability in target delineation and assessment of the average target in stereotactic radiosurgery - a multiobserver study

Keywords: Brain Tumors, dose planning, radiosurgery, technique, survey

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     Target delineation and planning can vary between individuals and centers.
     Four brain disorders of a general complex nature, anaplastic astrocytoma, AVM, meningioma and vestibular schwannoma were analysed with respect to the variability of target delineation with the aim of determining the average target and the average target coverage for each of them.
     Anaplastic astrocytoma, AVM, meningioma and vestibular schwannoma, were chosen because they are prone to pose difficulties in delineation. Twenty centres chosen for their high experience with Leksell Gamma Knife® participated in the study which resulted in a total of 60 treatment plans. The analysis of the delineated targets was based on a calculated average target which was compared to each delineated target by the concordance index and discordance index to evaluate the coverage of average target to resemble a true target. The difference in target structure location, size and shape was analyzed using the encompassing volume compared to the common volume of all delineated structures.
     Target delineation resulted in considerable differences in the planned target volume regarding size, position and shape. These variations translated into corresponding discrepancies in average target for each disorder. The largest difference regarding the indexes between the average target assumed to resemble the accurate target and the planned one was observed for anaplastic astrocytoma with a concordance index of 0.45 (0-0.85) and for vestibular schwannoma with a discordance index of 1.13 cm3 (0.16-4.67 cm3).
     This was a limited survey of a small number of cases.
     The differences between the targets delineated by various observers for the investigated four brain disorders appear to be clinically significant with respect to the average target.
     Understanding how targets are defined can help to refine planning techniques.


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