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To develop a specialist-based consensus of cochlear contouring to be used in patients undergoing stereotatic radiosurgery (SRS) treatment for vestibular schwannoma (VS).
Methods and Materials
Representative computed tomography (CT) and magnetic resonance imaging (MRI) were used for cochlear contouring. The semicircles, cochlea, vestibule, and internal acoustic meatus were delineated by seven radiation oncology department physicians and reviewed by a neuro-radiologists. A total of 12 cases accrued from a single academic institution were studied for a similarity analysis by Dice coefficient.
The suggested guideline is an easily reproductive tool that allows radiation oncologists to accurately contour the vestibulocochlear system to avoid toxicity due to inadequate dosimetry of organs at risk (OAR). This could be a useful tool even for non-VS radiotherapy. The Dice coefficient suggests reproducible results as long as the following recommendations are observed.
The template for vestibulocochlear delineation may be useful for adequate OAR definition. Future studies are required to find specific constraints for each segment of the vestibulocochlear system, and to mitigate interobserver variations.
Radiotherapy is an option for the management of vestibular schwannoma (VS). Moreover, ablative treatments became more common and can be performed in 1 to 5 fractions1. Reports of sensorineural hearing loss vary between 50 to 70%, but data is controversial2. Systematic analyses report better results of retained hearing with doses received by vestibulocochlear apparatus below 12.5Gy3. The dose range can vary between 11 to 16Gy when treated with ablation 4.
To ensure preservation of the organs at risk (OARs), the precise contouring of them is mandatory. Due to its proximity to VS the vestibulocochlear contouring requires discussion. . There are few contouring guidelines published, and most of them are OAR's guidelines for head and neck cancers5.
The vestibulocochlear system consists of the vestibule, cochlea, semicircular canals, vestibular aqueduct, and cochlear aqueduct6. The vestibule contains the utricle and therefore the saccule. There are three semicircular canals, the superior, lateral and posterior. The space between the bony labyrinth and structure contains a fluid called perilymph. The same fluid (endolymph) fills the space within the body structure6,7 (figure 1).
The cochlea is a shell-shaped spiral that turns between two-and-a-half and two-and-three-quarters times around the modiolus, which is a central column of porous bone. Branches from the cochlear portion of the vestibulocochlear (VIII) nerve are found at the base of the modiolus8. The cochlea spiral is divided by a delicate osseous lamina (spiral lamina) projected from the modiolus and divides the cochlear canal into a lower portion named scala tympani, scala media, and the upper portion scala vestibuli9. The scala tympani and scala vestibule communicate through an opening called the helicotrema, at the cochlear apex. The scala tympani and scala vestibuli are filled with perilymph and the scala media with endolymph9. The lowermost of the three chambers, the scala tympani, has a basal aperture, the round window, and the upper one, the scala vestibule, has another opening called the oval window. The scala media or cochlear duct separates these two chambers along most of their length (figure 2).
Even with the use of guidelines, the interobserver variation of contouring is expressive, however, those guidelines are not widely accepted10. Specific guidelines of OARs for patients with brain neoplasms usually cite the importance of cochlear contouring but there is no step-by-step description of each component of the vestibulocochlear apparatus11,12. The majority of guidelines for OAR contouring recommend the inclusion of only the cochlea. Therefore the accepted constraints were developed excluding the vestibular component11,13 whose radiation-induced toxicity might be clinically relevant. There is one guideline that successfully includes the vestibular apparatus in contouring recommendation however no dosimetric consideration was performed14.
This paper intends to create a detailed step-by-step contouring guideline for the entire vestibulocochlear apparatus for patients referred to SRS treatment.
A panel of 7 physicians ((anonymized for review), (anonymized for review), (anonymized for review), (anonymized for review), (anonymized for review), (anonymized for review), (anonymized for review)) went through cochlear anatomy training and received a total of 12 patients with a radiological diagnosis of vestibular schwannoma who underwent ablative radiation therapy, which was performed with 1 to 5 fractions. Two of the panel members ((anonymized for review), (anonymized for review)) were radiation oncologists with expertise in neuro-oncology, and five were final-year resident physicians in radiation oncology. The cases were contoured and verified by senior radiation oncologists as well as two neuro-radiologist (anonymized for review) and (anonymized for review).
The gross tumoral volume (GTV) was contoured and received a 1mm margin for planning target volume (PTV).
The slice thickness of CT scan should be 1mm thickness15. It is essential to have a high-resolution, 1mm thickness, T2 weighted MRI for endolymphatic visualization is recommended. Possible geometric distortions may occur and must be carefully recognized and corrected16. Our general recommendation for window setting is to use window width of 3000 to 4500 and window level of 400 to 80014.
For immobilization, we used thermoplastic masks, commonly used in radiation oncology practice. It is recommended to ensure that the masks are designed for SRS frameless procedures, although SRS with frame is another possible option.
After adequate simulation, the registration of these exams is preferably performed using bony structures. It is also important that all volumes be in 512×512 matrix resolution in the axial plane.
Recommended steps for contouring
Creation of the OAR structure set using a high-resolution matrix. One of them will represent the entire vestibulocochlear system.
Proceed with the registration between planning CT and MRI sequences. The registration must prioritize the internal acoustic meatus.
Switch to T2-weighted MRI.
The contour of the vestibulocochlear system must include the anterior semicircle, viewable at the upper portion of the temporal bone (Figure 3A).
The vestibular components and cochlea can be contoured at the level of the internal acoustic meatus, as long as part of the semicircles and cochlea (Figure 3B).
The contour must be extended until the final viewable portion of the posterior semicircle (Figure 3C).
Figure 4 shows with more anatomic details the structures within each component contoured as detailed previously.
The CT scan is also helpful to double-check the contoured structures although the use of MRI is recommended and was essential to create this tool. Figure 5 shows the anatomic landmarks that allow the identification of components of the vestibulocochlear system.
Once the volumes were contoured by seven physicians at different times, the use of a magnitude that allows comparisons was needed. To do so, we used the Dice coefficient which could be interpreted as percentages of intersections between volumes. The Dice formula is represented by: DICE = 2. (X n Y)/ X + Y, in which X and Y represent two different volumes.
As it was expected that the proposed contouring method leads to the detection of higher doses, the paired one-tailed T-Student test was performed for the normalized maximum and medium doses variables.
We reached a Dice coefficient of 0.75 +/- 0.03 which indicates a significant correlation between the contoured volumes.
Under the alternative hypothesis that both maximum and medium doses would be higher with the conventional contouring method, we found a statistically significant difference between the medium dose of +15.62% to -25.99% (p<.008). No significant difference in maximum dose +12.78 to -12.78% (p.612).
The 3D reconstruction (Figure 6) shows the difference in the volumes of OAR expected by comparing the proposed guideline to the other guidelines used for cochlear contouring
This study provides an easy way to access vestibulocochlear anatomy and can be used for daily practice as a tool for OAR delineation. The Dice coefficient mathematically demonstrates that this guideline was easily reproduced in our group and might be as easy for other physicians.
From the results, we observed no statistical difference between the maximum dose received by any of the volumes, conventionally, or the contouring method proposed by this study (p = 0.612). This can be explained by the fact that the maximum dose received depends on the location and size of the PTV. However, the medium dose is smaller with conventional contouring (p = 0.008). This could represent that, without the inclusion of vestibular component, these structures might be receiving more dose than expected and not been identified by planning software.
Radiation might cause progressive degeneration and even ossification of the structures of the inner ear17, which can lead to hearing loss. This is due to an endothelial reaction occurring in small vessels leading to a vascular insuficiency18. It is presumed that the stria vascullaris cells are responsible to produce the endolymph and the ischemia of those might lead to hearing loss, vertigo and dizziness19.
Most guidelines recommend contouring only the cochlea as an OAR, excluding the vestibular component11. The era of highly conformal treatments leads to the need for better comprehension of the dose received by surrounding structures. With reverse planning, what is not contoured is not used in the optimization strategy, leading to uncontoured structures receiving higher doses. This raises special concern because there is increasing evidence that the radiation to the vestibular component of the vestibulocochlear system can lead to clinically significant symptoms, and since it is not contoured, there are no well established constraints. Therefore, the first step to addressing this issue is to standardize vestibular contour.
Recently, retrospective series reported rates of 24.1% of patients developing imbalance after SRS and 15% with gait uncertainty after SRS or fractionated radiotherapy. The mean dose received by the labyrinth with SRS by those patients was 5.7Gy and the dose related to gait uncertainty was 11.5Gy in SRS treatments20.
Dizziness is another toxic endpoint that must be evaluated. Retrospective data support that there is a correlation between dose and dizziness worsening, this data suggest doses of 5Gy were statistically related to this side effect21. These data were extracted by retrospective series that also conclude that more data are needed to develop a reliable constraint for the vestibular component of the vestibulocochlear system. It is also important to note that hearing loss continues to be the most studied side effect.
Timmerman et al. showed that doses delivered to the cochlea over 12Gy increase the risk of hearing loss22. Although authors such as Chung et al. reported significant hearing loss with doses above 6.5Gy for SRS regimens with p=0.0423. Bhandare et al. reported 100% serviceable preservation of hearing in patients submitted to SRS for vestibular schwannoma whose cochlea received a maximum dose below 4.2Gy24.
For fractionated treatments, Marks et al. reported hearing loss in 37% of patients who received doses greater than 60Gy to the inner ear compared to 5% of those who received lower doses25. The formal recommendation by The Quantitative Analyses of Normal Tissue Effect in the Clinic (QUANTEC) would be a median dose < 45Gy, a maximum dose < 60Gy, and the volume that receives 40Gy corresponding to less the 2% of the volume of organ at risk15.
As demonstrated, there is more data and recommendations for cochlear constraints than for the vestibulocochlear system and retrospective data shows that vestibular toxicity also plays an important role in the quality of life as well.
Another subject must be formally addressed. About 90% of patients with VS present some degree of hearing loss and impaired balance in up to 60% at diagnosis26. The worsening of these symptoms could be associated with tumor regrowth or radiotoxicity, though differentiating between the two might be challenging.
To help in this matter, baseline audiometry and vestibular tests e.g. cervical vestibular evoked myogenic potentials and bithermal caloric testing are recommended21,27,28. Changing in those baseline parameters must be investigated with brain MRI although physicians must be aware of pseudoprogression after SRS29. Continued growth during follow-up may be the best indicator of toxicity secondary to tumor regrowth. However, this is still a challenging topic and must be addressed in an individualized and multidisciplinary manner.
It is important to point out that this study did not intend to evaluate clinical outcomes, which would require a larger number of patients and longer follow-up. However, it is our intention to facilitate the standardization of vestibulocochlear contouring.
This study has its limitations: first, the retrospective design leads to reduced statistical impact, second, the reduced number of observers and cases, and third, this paper was not validated by other institutions, although this is one of our objectives by sharing it.
With this report, we formulated an easy-to-access and use tool for vestibulocochlear contouring that could be used for OAR delineation. We believe this guideline might improve the chances of including all the vestibulocochlear components, allowing more precise dosimetric readings. In addition, further studies can be developed to determine specific constraints for each component of the vestibulocochlear system.
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Founding statement: The Radiotherapy department of Sírio Libanês Hospital funded this project.
Data Sharing: It is the Hospital's statement not to public share patient's data for public access even if a Health Authority approval has been granted based only on data analyses. However, the author agrees to share the data with the editorial office if requested.