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To develop a specialist-based consensus of cochlear contouring to be used in patients undergoing stereotactic radiosurgery (SRS) treatment for vestibular schwannoma.
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 7 radiation oncology department physicians and reviewed by neuroradiologists. A total of 12 cases accrued from a single academic institution were studied for a similarity analysis by the Dice coefficient.
Results
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. This could be a useful tool even for non-vestibular schwannoma radiation therapy. The Dice coefficient suggests reproducible results as long as the following contouring recommendations are observed.
Conclusions
The template for vestibulocochlear delineation may be useful for an adequate organs at risk definition. Future studies are required to find specific constraints for each segment of the vestibulocochlear system, and to mitigate interobserver variations.
Introduction
Radiation therapy is an option for the management of vestibular schwannoma (VS). Moreover, ablative treatments have become more common and can be performed in 1 to 5 fractions.
Congress of Neurological Surgeons systematic review and evidence-based guidelines on hearing preservation outcomes in patients with sporadic vestibular schwannomas.
To ensure the preservation of the organs at risk (OAR), the precise contouring of them is mandatory. Because of its proximity to the VS, vestibulocochlear contouring requires discussion. There are few contouring guidelines published, and most of them are OAR guidelines for cancers of the head and neck.
CT-based delineation of organs at risk in the head and neck region: DAHANCA, EORTC, GORTEC, HKNPCSG, NCIC CTG, NCRI, NRG Oncology and TROG consensus guidelines.
The vestibule contains the utricle and therefore the saccule. There are 3 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 structure
EPOS – European Society of Radiology. Lost in the labyrinth: Let's find the way out. Published March 2, 2016. Available at: https://epos.myesr.org/poster/esr/ecr2016/C-1951. Accessed September 20, 2022.
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 modiolus.
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 vestibuli.
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 endolymph.
The lowermost of the 3 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 2 chambers along most of their length (Fig. 2).
Figure 2(A, C) Illustration of normal cochlear anatomy and (B) cochlear turn anatomy. Abbreviations: AT = apical turn; BT = basal turn; CN = cochlear nerve; MD = mdiolus; MT = middle turn; OC = organ of Corti; SM = scala media; ST = scala tympani; SV = scala vestibuli.
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 apparatus.
The majority of guidelines for OAR contouring recommend the inclusion of only the cochlea. Therefore, the accepted constraints were developed excluding the vestibular component,
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 performed.
This paper intends to create a detailed step-by-step contouring guideline for the entire vestibulocochlear apparatus for patients referred to stereotactic radiosurgery (SRS) treatment.
Methods and Materials
A panel of 7 physicians (FCFR, LHB, FHY, APAP, DRFN, MTMS, SAH) went through cochlear anatomy training and received a total of 12 patients with a radiologic diagnosis of VS who underwent ablative radiation therapy, which was performed with 1 to 5 fractions. Two of the panel members (SAH and GNM) were radiation oncologists with expertise in neuro-oncology, and 5 were final-year resident physicians in radiation oncology. The cases were contoured and verified by senior radiation oncologists as well as 2 neuro-radiologist (GWOG and ULP).
The gross tumor volume was contoured and received a 1-mm margin for planning target volume. The slice thickness of a computed tomography (CT) scan should be 1 mm.
It is essential to have a high-resolution, 1-mm thick, T2-weighted magnetic resonance imaging (MRI) scan for endolymphatic visualization. Possible geometric distortions may occur and must be carefully recognized and corrected.
Task group 284 report: Magnetic resonance imaging simulation in radiotherapy: considerations for clinical implementation, optimization, and quality assurance.
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 examinations 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
1.
Creation of the OAR structure set using a high-resolution matrix. One of them will represent the entire vestibulocochlear system.
2.
Proceed with the registration between planning CT and MRI sequences. The registration must prioritize the internal acoustic meatus.
3.
Switch to a T2-weighted MRI.
4.
The contour of the vestibulocochlear system must include the anterior semicircle, viewable at the upper portion of the temporal bone (Fig. 3A).
Figure 3The figures A, B and C are axial slices that demosntrate the contouring steps using T2 weighted MRI. Red denotes the semicircles, blue for vestibular component, and green, the cochlea. The orange volume represents the VS, defined as GTV. For anatomic purposes, each component was contoured separately but in clinical practice, they can be contoured together. Abbreviations: MRI= magnetic resonance imaging; GTV = gross total volume; VS = vestibular schwannoma.
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 (Fig. 3B).
6.
The contour must be extended until the final viewable portion of the posterior semicircle (Fig. 3C).
Figure 4 shows with more anatomic details the structures within each component contoured as detailed previously.
Figure 4Axial slices of magnetic resonance imaging (MRI), which are helpful to distinguish the structures when merged with a computed tomography scan.
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.
Figure 5From A to F the figures in craniocaudally sense shows the axial slices of a computed tomography scan showing the semicircles in light blue, cochlea (yellow), the vestibule (brown), the middle ear (dark blue), and external acoustic meatus (EAM) (gray). All these structures, except for the middle ear and EAM must be included in the final volume.
Once the volumes were contoured by 7 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 as follows: DICE = 2.(X n Y)/X + Y, in which X and Y represent 2 different volumes.
As expected, that the proposed contouring method leads to the detection of greater doses, the paired 1-tailed Student t test was performed for the normalized maximum and medium doses variables.
Results
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 greater with the conventional contouring method, we found a statistically significant difference between the medium dose of +15.62% to –25.99% (P < .008) and no significant difference in maximum dose +12.78 to –12.78% (P = .612). The 3-dimensional reconstruction (Fig. 6) shows the difference in the volumes of OAR expected by comparing the proposed guideline with the other guidelines used for cochlear contouring.
Figure 6Final 3-dimensional reconstruction comparing the entire vestibulocochlear system (red volume) with the cochlea according to most guidelines (light green volume).
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 = .612). This can be explained by the fact that the maximum dose received depends on the location and size of the planning target volume. However, the medium dose is smaller with conventional contouring (P = .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 ear,
It is presumed that the stria vascullaris cells are responsible for producing the endolymph, and the ischemia of those might lead to hearing loss, vertigo, and dizziness.
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 greater 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 because 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 radiation therapy. The mean dose received by the labyrinth with SRS by those patients was 5.7 Gy and the dose related to gait uncertainty was 11.5 Gy in SRS treatments.
Outcome after radiotherapy for vestibular schwannomas (VS)-differences in tumor control, symptoms and quality of life after radiotherapy with photon versus proton therapy.
Dizziness is another toxic endpoint that must be evaluated. Retrospective data support that there is a correlation between dose and dizziness worsening; these data suggest doses of 5 Gy were statistically related to this side effect.
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.
Impact of cochlear dose on hearing preservation following stereotactic radiosurgery and fractionated stereotactic radiotherapy for the treatment of vestibular schwannoma.
reported hearing loss in 37% of patients who received doses greater than 60 Gy to the inner ear compared with 5% of those who received lower doses. The formal recommendation by The Quantitative Analyses of Normal Tissue Effect in the Clinic (QUANTEC) would be a median dose <45 Gy, a maximum dose <60 Gy, and the volume that receives 40 Gy corresponding to less than 2% of the volume of organ at risk.
As demonstrated, there are more data and recommendations for cochlear constraints than for the vestibulocochlear system, and retrospective data show that vestibular toxicity also plays an important role in the quality of life as well.
Another subject must be formally addressed. Approximately 90% of patients with VS present some degree of hearing loss, and impaired balance in up to 60% at diagnosis.
Rizk A. Vestibular Schwannoma: Microsurgery or Radiosurgery. Brain Tumors - An Update [Internet]. 2018 Aug 22. Available at: https://doi.org/10.5772/intechopen.74508. Acessed on February 22, 2022.
The worsening of these symptoms could be associated with tumor regrowth or radiotoxicity, although differentiating between the 2 might be challenging.
To help in this matter, baseline audiometry and vestibular tests, for example, cervical vestibular–evoked myogenic potentials and bithermal caloric testing, are recommended.
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 effect, 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.
Conclusion
With this report, we formulated an easy-to-access and easy-to-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.
Congress of Neurological Surgeons systematic review and evidence-based guidelines on hearing preservation outcomes in patients with sporadic vestibular schwannomas.
CT-based delineation of organs at risk in the head and neck region: DAHANCA, EORTC, GORTEC, HKNPCSG, NCIC CTG, NCRI, NRG Oncology and TROG consensus guidelines.
EPOS – European Society of Radiology. Lost in the labyrinth: Let's find the way out. Published March 2, 2016. Available at: https://epos.myesr.org/poster/esr/ecr2016/C-1951. Accessed September 20, 2022.
Task group 284 report: Magnetic resonance imaging simulation in radiotherapy: considerations for clinical implementation, optimization, and quality assurance.
Outcome after radiotherapy for vestibular schwannomas (VS)-differences in tumor control, symptoms and quality of life after radiotherapy with photon versus proton therapy.
Impact of cochlear dose on hearing preservation following stereotactic radiosurgery and fractionated stereotactic radiotherapy for the treatment of vestibular schwannoma.
Rizk A. Vestibular Schwannoma: Microsurgery or Radiosurgery. Brain Tumors - An Update [Internet]. 2018 Aug 22. Available at: https://doi.org/10.5772/intechopen.74508. Acessed on February 22, 2022.
Sources of support: This study was supported by the Radiotherapy Department of Sírio Libanês Hospital.
Disclosures: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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