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01.12.2014 | Clinical Article - Brain Tumors | Ausgabe 12/2014

Acta Neurochirurgica 12/2014

Variability in target delineation for cavernous sinus meningioma and anaplastic astrocytoma in stereotactic radiosurgery with Leksell Gamma Knife Perfexion

Zeitschrift:
Acta Neurochirurgica > Ausgabe 12/2014
Autoren:
Helena Sandström, Håkan Nordström, Jonas Johansson, Per Kjäll, Hidefumi Jokura, Iuliana Toma-Dasu
Wichtige Hinweise
Portions of this work were presented as an oral presentation and posters at the 16th Leksell Gamma Knife Society Meeting in Sydney, March, 2012.

Comments

In this potentially very important paper, the authors compared the outlines of two tumors targeted with stereotactic radiosurgery as performed in 20 radiosurgery units and demonstrated that there was significant disagreement between the delineated volumes and thus the eventual treatment plan. As all departments had the same radiological material available, the differences resulted from different levels of experience (of the departments or at least the individuals), personality of the practitioner, desired radicality, acceptability of complications, etc. As the authors acknowledge, even with the same equipment, different practitioners would deliver different treatments for the same pathology. This is of course analogous to the differences of how the same tumor would be operated on by different surgeons. Furthermore, the difference starts already at the stage when the radiological material is analyzed, even more in the case of radiosurgery than in the case of open surgery where direct vision is also applied during the procedure. Just as the different neurosurgeons reading this paper, the practitioners in this study will have different personality profiles. As the authors implied, a radiation oncologist may be more willing to use a wider margin than a neurosurgeon and even in a multidisciplinary team such differences do not disappear. Even amongst neurosurgeons, some are more radical than others. Ideally, the radiology should be interpreted by a radiologist and radicality should only appear at this stage of choosing a radiation dose, but there is a temptation not even to mark as pathology something that one does not intend to treat. It is almost arbitrary how far, for instance, one plans to follow the dural tail of a meningioma. Naturally, these considerations would not arise in a well-defined pathology, e.g., vestibular schwannoma. Also, even a Gamma knife treatment with the sharpest dose fall-off has a small marginal effect beyond the prescription. The results presented in this paper could be taken as quite shocking, particularly for the anaplastic astrocytoma where some delineated 13 times as much volume as the most conservative team. However, one also has to be aware that the next step, deciding the dose, may dramatically diminish this difference, because the team outlining a large volume with a margin around the tumor may prescribe a very low dose and vice versa. The very interesting new indices the authors introduced, CCI and DCI, would not take this into consideration, and perhaps a further index incorporating a treatment plan may be developed. At the end, all that matters is the final clinical outcome and of course the same patient will not be treated by 20 different departments. One has to acknowledge that radiosurgery, just as open surgery, is not an absolute exact science, but an art, and in spite of technological advances, careful judgment based on experience will remain the determinant of what we achieve. Andras Kemeny, Sheffield, UK
The authors present an interesting and important study on target contouring in radiosurgery. They asked several centers to contour and do the treatment planning for two identical targets. The various centers received identical imaging and the required prescription doses were the same for all centers. The authors then compared the results. Unfortunately, they chose two cases that were not really typical, considering everyday radiosurgical practice. The cavernous sinus meningioma is rather large, receiving a comparatively low dose and the anaplastic astrocytoma is rather small, receiving a comparatively high dose. In fact, the mean volume of the cavernous sinus meningioma is smaller than that of the anaplastic astrocytoma. In addition, the authors chose with the cavernous sinus meningioma, a case which is best contoured in a coronal plane and with the anaplastic astrocytoma, a case which may as well be contoured in an axial plane. The input of an experienced neurosurgeon when selecting the exemplary cases would have been of great advantage. However, setting aside these shortcomings, the study's results are interesting and important enough to justify their publication. The differences in target volumes are surprisingly large, with a standard deviation of ±0.74 of the mean volume in the meningioma case and ±6.21 in the astrocytoma case. Not surprisingly, the differences are much smaller in the meningioma case where there is less room for ambiguity concerning target definition. What can we learn from this study's results? (1) Target contouring in radiosurgery needs our attention. Contouring should be done by a specialist with the necessary knowledge of anatomical and disease-specific details. Only a neurosurgeon or a neuroradiologist has that knowledge. (2) Contouring needs to be clearly separated from treatment planning. The target should be delineated uncompromised by possible treatment plans. (3) Some targets are better contoured in coronal than in axial planes. (4) Imaging modalities are important for contouring. In my view, FLAIR sequences are crucial in target contouring of gliomas. (5) Not all the differences in target volumes are bad. Some may just reveal different treatment strategies. Some radiosurgeons may prefer to treat gliomas according to C. Duma's “Leading Edge” principles while others may not. None of the resulting treatment plans are wrong according to today's standards of care. (6) This study exemplifies the importance of contouring guidelines for target volumes according to the contouring guidelines for organs at risk (OAR). OAR guidelines are still missing, but they are worked out by the international OAR group, which met in Toronto in 2013 and in New York City in 2014. A target-contouring group still needs to be launched. Thomas Mindermann, Zurich, Switzerland

Abstract

Background

Radiosurgery clinical practice relays on   empirical observations and the experience of the practitioners involved in determining and delineating the target and therefore variability in target delineation might be expected for all the radiosurgery approaches, independent of the technique and the equipment used for delivering the treatment. The main aim of this study was to quantify the variability of target delineation for two radiosurgery targets expected to be difficult to delineate. The secondary aim was to investigate the dosimetric implications with respect to the plan conformity. The primary aim of the study has therefore a very general character, not being bound to one specific radiosurgery technique.

Materials and methods

Twenty radiosurgery centers were asked to delineate one cavernous sinus meningioma and one astrocytoma and to plan the treatments for Leksell Gamma Knife Perfexion. The analysis of the delineated targets was based on the calculated 50 % agreement volume, AV50. The AV50 was compared to each delineated target by the concordance index and discordance index. The differences in location, size, and shape of the delineated targets were also analyzed using the encompassing volume compared to the common volume, i.e., the AV100, of all delineated structures.

Results

Target delineation led to major differences between the participating centers and therefore the AV50 was small in comparison to each delineated target volume. For meningioma, the AV50 was 5.90 cm3, the AV100 was 2.60 cm3, and the encompassing volume was 13.14 cm3. For astrocytoma, the AV50 was 2.06 cm3 while the AV100 was extremely small, only 0.05 cm3, and the encompassing volume was 43.27 cm3. These variations translate into corresponding discrepancies in plan conformity.

Conclusions

Significant differences in shape, size, and location between the targets included in this study were identified and therefore the clinical implications of these differences should be further investigated.

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