A wide range of publications has documented the value of external beam radiation for the treatment of meningioma. However clinical practice is more likely to show that only those cases suffering from complex meningioma are referred to radiotherapy. This included patients with relapse after surgery, large tumors or complexly growing tumor. Thus the treating physician is frequently faced with the dilemma to spare as much of critical normal tissue without missing gross tumor. The use of highly conformal treatments including IMRT even increases the need for optimal target volume delineation.
In the present study we evaluated the value of the [68Ga]-DOTATOC-PET for treatment planning of intracranial complexly shaped meningiomas. Up to now the follow up time in our cohort is all too short to give some information about local control after IMRT treatment. However, our data show clearly that the use of [68Ga]-DOTATOC-PET improved target volume delineation in a larger proportion of our patients schedule for an IMRT based radiation approach when compared to MRI based planning alone. Particularly bony lesions or direct bone infiltration by adjacent meningioma tissue were more likely to be detected with PET. Basically we found a geographical miss in 50% of the patients and - on the other hand - were able to reduce the CTV in 38% of the patients. When compared to other observations using [68Ga]-DOTATOC-PET,11C-Methionine [30, 31] or18F-Tyrosine , similar ranges were reported in the term of PET scanning offering additional information. In this regard Milker-Zabel reported relevant information in 19 out of 26 patients using DOTATOC , Astner reported additional information in 29 out of 32 patients  and Rutten reported changes in 6 out of 13 lesions in 11 patients using18F-Tyrosine . In our series in 17 out of 26 patients PET scanning offered relevant complementary information.
When one analyzes the pattern of changes in more detail, Milker-Zabel and Rutten reported larger proportions of potential geographical misses avoided by PET scanning (38% in both studies) [32, 33]. This is in accordance with our findings where the CTV was increased after inclusion of the PET data (50%). In contrast, the study by Astner reported a larger proportion of GTV/PTV reductions after inclusion of11C-Methionine-PET data when compared to MRI scanning alone (75%) .
The reasons for these differences are not readily deducible from the reported data. However, it may be speculated that the inherent bias of patient selection and strategies employed for MRI-GTV definition may be the underlying reason. This assumption is supported by the fact that at least comparable volumes were treated in all three studies excluding the possibility that differences in tumor volume are responsible for the differences in target volume changes.
An important consideration in this context is the open question if there is s SUV-threshold to define the GTV-PET. In our study for the target volume definition the windowing of the DOTATOC-PET was determined by the matching between the PET-positive areas and the viewable tumor margins determined by CT/MRI. The physiological signal of the bony skull and the air-filled nasal cavity was masked out via windowing. Although a SUV-threshold would be helpful for the GTV-DOTATOC-PET delineation in meningiomas, up to know clear evidence for a SUV cut-off is missing. Astner et al.  reported an interesting phantom study in 11 patients with glomus tumors and revealed that a value of 32% of the maximum standardized uptake was an appropriate threshold for tumor delineation. At the moment we do not have this information for meningiomas in DOTATOC-PET imaging. However, in regard to IMRT planning for meningiomas special biological characteristics of microscopic tumor growth have to be taken into account especially for CTV delineation. Hence in our opinion we have to be cautious in reducing target volumes along an experimental SUV-threshold alone.
From the data currently available it seems that either [68Ga]-DOTATOC,11C-Methionine or18F-Tyrosine are useful tracers for target volume definition in patients with meningioma. Up to now there is no clear evidence available supporting the superiority of any of the given tracers.
As stated above, meningiomas particularly show high levels of somatostatin receptor expression (SSTR2) resulting in a high tracer uptake. The usefulness of [68Ga]-DOTATOC-PET for a distinction of meningioma from other brain tumors has been well documented [35–37]. In several disorders including metastasis or glioma Methionine or Tyrosine may produce false positive results. However, amino acid tracers like Methionine or Tyrosine are markers of amino acid transport and give some more information in regard to metabolic activity of several tumor tissues. At least Methionine-PET may help to judge the aggressiveness of meningioma since the uptake has been reported to correlate with the proliferative activity measured by the KI-67 index [38–40]. In our opinion for IMRT planning it seems reasonable to use the tracer with the highest inherent specificity which - by means of its mechanism of action - is [68Ga]-DOTATOC-PET.
A given disadvantage of DOTATOC is the fact that the pituitary gland is generally highly positive and limits the precision of target volume definition in this area.
Although PET/CT images have reached a considerable level of spatial discrimination the current technology does not allow for the visualization of microscopic tumor growth along the dural membranes. Thus it will be still necessary to add empirical margins to cover all areas.