|Year : 2021 | Volume
| Issue : 2 | Page : 68-75
Step-by-step stereotactic radiotherapy planning of vestibular schwannoma: A guide to radiation oncologists—the ROSE case (Radiation Oncology from Simulation to Execution)
Kanhu Charan Patro1, Ajitesh Avinash2, Arya Pradhan2, Pamidimukkala Venkatramana3, Chittaranjan Kundu1, Partha Sarathi Bhattacharyya1, Venkata Krishna Reddy Pilaka1, Mrutyunjayarao Muvvala Rao1, Arunachalam Chithambara Prabu4, Ayyalasomayajula Anil Kumar4, Srinu Aketi4, Parasa Prasad4, Venkata Naga Priyasha Damodara1, Veera Surya Premchand Kumar Avidi1, Mohanapriya Atchaiyalingam1, Keerthiga Karthikeyan1, Voonna Muralikrishna1
1 Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and Research Institute, Visakhapatnam, India
2 Department of Radiation Oncology, Acharya Harihar Post Graduate Institute of Cancer, Cuttack, Odisha, India
3 Department of Neurosurgery, Pinnacle Hospital, Visakhapatnam, India
4 Department of Medical Physics, Mahatma Gandhi Cancer Hospital and Research Institute, Visakhapatnam, Andhra Pradesh, India
|Date of Submission||25-Oct-2021|
|Date of Acceptance||17-Nov-2021|
|Date of Web Publication||23-Feb-2022|
Dr. Kanhu Charan Patro
Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and Research Institute, Visakhapatnam, Andhra Pradesh
Source of Support: None, Conflict of Interest: None
Background: Vestibular schwannoma (VS) is a slow-growing tumor that represents 90% of all tumors at the cerebellopontine angle. One of the main modalities of the treatment is stereotactic radiotherapy (SRT). Here, we describe procedure details for stereotactic planning of VS. Methods: The step-by-step procedure for stereotactic planning of pituitary adenoma has been described using a clinical scenario of VS. Results: The stereotactic radiation planning of VS starts with the basic history and relevant evaluation of symptoms such as tinnitus, dizziness, and facial symptoms. Magnetic resonance imaging (MRI) of the brain is the imaging modality of choice. The radiation planning of VS starts with computed tomography (CT) simulation and MRI of the brain that should be performed in prescribed format to achieve uniformity in radiation planning. After CT and MRI fusion, contouring of target, organs at risk (OAR), and radiation planning should be performed. The plan evaluation includes target and OAR coverage index, conformity, homogeneity and gradient index, and beam arrangement. After radiation plan evaluation, treatment is delivered after quality assurance and dry run. Conclusion: The article highlights the sequential process of radiation planning for SRT of VS—starting from simulation to planning, evaluation of plan, and treatment.
Keywords: Acoustic neuroma, radiotherapy planning, SRS, SRT
|How to cite this article:|
Patro KC, Avinash A, Pradhan A, Venkatramana P, Kundu C, Bhattacharyya PS, Pilaka VK, Rao MM, Prabu AC, Kumar AA, Aketi S, Prasad P, Damodara VN, Avidi VS, Atchaiyalingam M, Karthikeyan K, Muralikrishna V. Step-by-step stereotactic radiotherapy planning of vestibular schwannoma: A guide to radiation oncologists—the ROSE case (Radiation Oncology from Simulation to Execution). J Curr Oncol 2021;4:68-75
|How to cite this URL:|
Patro KC, Avinash A, Pradhan A, Venkatramana P, Kundu C, Bhattacharyya PS, Pilaka VK, Rao MM, Prabu AC, Kumar AA, Aketi S, Prasad P, Damodara VN, Avidi VS, Atchaiyalingam M, Karthikeyan K, Muralikrishna V. Step-by-step stereotactic radiotherapy planning of vestibular schwannoma: A guide to radiation oncologists—the ROSE case (Radiation Oncology from Simulation to Execution). J Curr Oncol [serial online] 2021 [cited 2022 May 17];4:68-75. Available from: https://www.journalofcurrentoncology.org/text.asp?2021/4/2/68/338059
| Introduction|| |
Vestibular Schwannoma (VS), also known as acoustic neuroma, is a benign neoplasm with an annual incidence of one in one lakh. It accounts for 6%–7% of all brain tumors. This neoplasm is usually seen at the cerebellopontine angle. MRI of the brain is the best imaging modality for the diagnosis of this tumor. The treatment of VS includes observation, surgery, or stereotactic radiation in the form of stereotactic radio surgery (SRS) or SRT. In this article, the various steps of radiation planning for SRT have been illustrated in an easy way for the beginners who are planning for SRT in a case of VS.
| Methods|| |
In this paper, the various steps of radiation planning for SRT have been illustrated in an easy way for the beginners who are planning for SRT in a case of VS with the help of a clinical case as described below.
A 40-year-old male presented with the chief complaints of tinnitus for six months, a slight decrease in hearing, dizziness, and facial fasciculation and twitching for three months. There was no associated facial numbness.
A patient with symptoms of tinnitus, dizziness, facial fasciculation, and twitching must be evaluated using the Tinnitus Handicap Inventory, Dizziness Handicap Inventory and House-Brackmann Scale for facial nerve functioning, respectively. On evaluation of the present case, the patient had grade II tinnitus using the Tinnitus Handicap Inventory and grade II dizziness using the Dizziness Handicap Inventory, as is depicted in [Table 1]. On speech audiometry, the patient had a speech discrimination score (SDS) of 90% in the right ear and 95% in the left ear. On Pure-Tone Audiometry examination, the pure tone average was 35 dB for the right ear and 22 dB for the left ear. With the above pure tone average (dB) and the SDS, the patient was found to have grade II hearing loss of the right ear, that is, the hearing of the right ear was serviceable as per the Gardner-Robertson grade. The facial nerve functioning was evaluated by House-Brackmann Scale. The patient had a normal resting tone with slight weakness in appearance, that is, grade II whereas forehead, eyes, and mouth were normal, that is, grade I as is shown in [Table 1].
|Table 1: Tinnitus, dizziness grading system Gardner–Robertson grade, and House–Brackmann scale for vestibular schwannoma|
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| Imaging|| |
On imaging by computed tomography (CT) scan of the brain, there was a widening of the right acoustic porous with an ice-cream on cone appearance. The lesion was minimally enhanced, touching the brainstem without any bony involvement [Figure 1]A. The FSPGR sequence of magnetic resonance imaging (MRI) of the brain revealed a fairly well-defined lobulated moderately enhanced extra-axial lesion with altered signal intensity in the right cerebellopontine angle cistern measuring 21 mm x 16 mm x 13 mm, with an intra-canalicular extension being 11 mm x 9 mm [Figure 1]B. The lesion appeared hypointense on both T1- and T2-weighted sequence; there was minimal hyperintensity on FLAIR sequence and it was touching the brainstem. Superiorly, the lesion was indenting the cisternal component of the right V cranial nerve and the right superior cerebellar artery. Laterally, the lesion was indenting the right middle cerebellar peduncle and the adjacent portion of the right cerebellar hemisphere. The right VII and VIII cranial nerves were not separately visualized from the lesion. There was no significant mass effect on the right IX, X, and XI cranial root complex and no significant effacement of the fourth ventricle. The Fast Imaging Employing STeady-state Acquisition (FIESTA) sequence showed the ice-cream cone appearance of the lesion that was impending the fifth cranial nerve [Figure 1]C. The features cited earlier were suggestive of right-sided VS.
|Figure 1: CT and MRI images of Vestibular Schwannoma. (A) CT scan of the case depicting widening of the acoustic porous. (B) FSPGR sequence of the same patient showing the lesion at the right cerebellopontine angle. (C) FIESTA sequence of the same patient showing the lesion impending the fifth cranial nerve|
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| Grading of Vestibular Schwannoma|| |
There are various grading systems for VS, such as Koos grading system, House grading system, and Samii grading system.
In the present case, as the tumor was reaching the brainstem surface but not deforming the brainstem surface or shifting the fourth ventricle, the tumor was grade III as per Koos grading system, grade III as per House grading system, and T3b as per Samii grading system as is seen in [Table 2].
| Line of Treatment|| |
The treatment of VS includes observation, surgery, SRS, or SRT.
| Surgical Consultation|| |
Neurosurgical opinion favored hearing preservation as the VII and VIII cranial nerve roots not separated out on imaging.
| Points in Favor of Choosing Radiation|| |
The tumor was very small (maximum size 2.1 cm), solid and the VII and VIII cranial nerves were not visualized separately. The preservation of hearing and facial nerve functions were the salient points that were the main reasons of choosing radiation for this patient. As per the ISRS practice guidelines, SRS was opted because it fulfilled the criteria of tumor diameter <3 mm, no or mild brain stem compression.
| Treatment Decision by the Tumor Board|| |
The patient details were put in the tumor board for a decision regarding the line to treatment. After a group discussion with the neurosurgeon, the radiation oncologist and the board decided to plan for SRT.
| Discussion with the Patient|| |
The patient was explained about the bouquet of treatment options, such as observation, surgery, and radiotherapy, and the complications and outcome of each procedure. Further, the radiation treatment procedures, hearing preservation, imaging, and follow-up were also explained to the patient.
| Counseling of the Patient|| |
The patient was counseled regarding the tumor response to radiation, the need for surgery in the future, and post-radiotherapy pain.
| Patient’s Preference|| |
The patient opted for radiation, as his major concerns were hearing preservation and trigeminal and facial nerve weakness besides tumor control.
| Dose Selection|| |
As per ISRS practice guidelines, there is a strong consensus to treat a newly diagnosed small size VS with single-fraction SRS. In case of tumor abutting, the trigeminal nerve fractionated SRT can be preferred. As per the University Hospital of Wales protocol, a hypofractionated SRT should be used in a VS case abutting the trigeminal nerve. Thus, it was planned to conduct fractionated SRT for this patient with a marginal dose of 25Gy in 5fractions @ 5Gy/fraction as per the two guidelines cited earlier.
| Decision of Radiotherapy Tumor Board|| |
Fractionated radiotherapy was planned to a marginal dose of 25Gy in 5fractions @ 5Gy/fraction as per the ISRS practice guidelines.
| Radiotherapy Planning|| |
Here, we describe the steps of treatment of VS from simulation to plan execution
Step 1: Computed tomography simulation
During simulation, the patient was set up in the supine position with a neutral neck position and immobilization was done using FRAXION thermoplastic mask and a stereotactic frame [Figure 2]A and B. Fiducials were placed on the thermoplastic mask after proper alignment with the lasers. Intravenous contrast was given at a dose of 1ml per kg body weight. Then, a CT scan was taken from the vertex to the neck with a CT slice thickness of 1 mm, as is depicted in [Table 3]. After simulation, the DICOM CT images were sent to our Oncentra server, which was then imported for delineation of the target and organ at risk.
|Figure 2: Immobilization of the patient using the stereotactic thermoplastic mask and frame during CT simulation in lateral view (A), the cranial view (B), and Fusion of MRI of the patient with planning CT scan (C)|
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|Table 3: CT simulation and MRI Protocol to be followed for vestibular schwannoma|
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Step 2: Magnetic resonance imaging protocol
MRI of the brain of the patient was done using a 512 x 512 matrix in the neutral neck position similar to that of the CT scan during simulation with no gap, no tilt, and a 1-mm slice thickness as depicted in [Table 3]. The field of view included the body contour along with nose, eyes, and skull. The MRI should include the usual T1, T2, FLAIR sequences. In addition, the 3D FSPGR was used for viewing the normal anatomy. The cochlea, brainstem, and cranial nerves were visualized in FIESTA sequence, and GRE sequence was used to find out any cystic changes or hemorrhage. If a dedicated MR simulator is available, MR simulation can be done using this MRI protocol and the simulation process is the same as the CT simulation mentioned earlier.
Step 3: Image fusion
These acquired MRI sequences were fused with the planning CT scan by contouring the eyes, lens, basilar artery, and sinuses, and calcification and matching was done using the auto-fusion technique to help in the target and organ at risk (OAR) delineation [Figure 2C].
Step 4: Target delineation
The gross tumor seen on the CT images that was fused with the MRI images to consider the exact extension of the tumor was delineated as GTV. The PTV was drawn, taking 1 mm around the GTV. Smoothing of the contour was done from the adjacent bone. Multi-planar evaluation, that is, the evaluation of both the GTV in all the three planes—axial [Figure 3]A, coronal [Figure 3]B, and sagittal [Figure 3]C and PTV—was done in all the three planes: axial [Figure 3]D, coronal [Figure 3]E, and sagittal [Figure 3]F.
|Figure 3: Delineation of the GTV (pink) of Vestibular Schwannoma in the axial (A), coronal (B), and sagittal plane (C) and PTV (cyan) generation around the GTV taking 1 mm margin in the axial (D), coronal (E), and sagittal planes (F)|
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In the present case, the GTV volume was 1.682cc and the PTV volume was 2.766cc.
Step 5: Organ at risk delineation
The OARs for delineation included the cochlea, brainstem, trigeminal nerve, optic chiasma, and optic apparatus. The cochlea was contoured in the bone window setting whereas other OARs, that is, the brainstem, trigeminal nerve, optic chiasma, and optic apparatus, were contoured using the MRI that was fused with the planning CT.
Step 6: Radiotherapy technique
Radiation planning can be done using any of the RT techniques, such as Intensity Modulated Radiotherapy (IMRT), Volumetric Modulated Arc Therapy (VMAT), Dynamic Conformal Arc Therapy (DCARC), or 3-Dimensional Conformal Radiotherapy (3DCRT) according to the convenience of the radiation physicist and physician.
In the present case, planning was done using the VMAT technique.
Step 7: Plan evaluation
After the completion of planning by the physicist, the evaluation for the treatment plan is done using the following indices as noted next.
PTV coverage index
After the planning, we need to see the PTV coverage. The prescription isodose level is usually not 100% of the prescribed dose covering 100% of the PTV. Often, the 95% of the prescription dose should cover 95% or higher percentage of the PTV; otherwise, 100% of the prescription dose should cover 95% or higher percentage of the PTV.
In the present case, 95% of the prescription dose covers 100% of the PTV and 100% of the prescription dose covers 97.5% of the PTV, which meets the earlier mentioned parameters for the PTV coverage and is depicted in [Table 4].
|Table 4: Various indices plan evaluation of vestibular schwannoma in the current case|
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Intracranial stereotaxy organ constraints and organ at risk coverage
Keeping in mind the desirable dose constraints to the OAR, we need to check the dose to individual OARs.
The dose desirable and dose achieved for all the OARs in the present case is depicted in [Table 5].
|Table 5: Individual OARS with its desirable dose and dose achieved in the current case of vestibular schwannoma|
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To note the conformity index of the SRS, here we used 2 types of conformity indices, that is, the RTOG conformity index and the Paddick conformity index.,
RTOG Conformity index (CIRTOG) is calculated using the following formula:
CIRTOG = Volume of Prescription Isodose / PTV volume
In this case of VS, the RTOG conformity index was 1.15 [Table 4].
Paddick conformity index (CIPaddick) was calculated using the following formula:
CIPaddick = (Volume of prescription isodose in the area of interest i.e. PTV)2 / PTV volume x Volume of Prescription Isodose
Here in the current case, Paddick conformity index was 1.02 [Table 4].
It is calculated using the formula:
Homogeneity Index = Maximum Dose/ Prescription Dose
In this case, the Homogeneity Index was 1.19 [Table 4].
Dose fall off
The dose fall off observation is very much needed in the plan evaluation under the heading of gradient index. For this we need to calculate the difference between various isodose lines. In order to calculate the difference between the isodose lines, we need to calculate the equivalent radius.
Equivalent radius calculation
To evaluate the dose gradient, we have to find out the difference between the radius of various isodose lines. However, none of the isodoses are spherical. So, we use the following formula to calculate the equivalent radius:
1st: Find out the specified isodose volume
2nd: Calculate the radius of the isodose volume by using the formula:
V = 4/3 π r3
r = (3V/4 π)1/3
The calculation of the volume and radius of various isodose lines in the present case is shown in [Table 4].
The formula for calculating gradient index is as given next.
Gradient Index = Equivalent radius of 50% isodose – Equivalent radius of prescription isodose. Ideally, the gradient index should be between 0.3 mm and 0.9 mm.
In the current case, the gradient index is 1.83 mm–0.91 mm = 0.92 mm, which is close to the ideal gradient index.
Distance between various isodose lines
The various isodose lines are depicted in [Figure 4]A.
|Figure 4: Isodose lines: 100% (red), 80% (green), 60% (yellow), 50% (blue), and 40% (pink) in (A) and beam arrangement in axial (B), coronal (C), and sagittal view (D) for the current case of Vestibular Schwannoma|
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The ideal difference between 80% and 60% isodose lines should be <2 mm.
In the current case, it is 1.65 mm–1.32 mm = 0.33 mm.
The ideal difference between 80% and 40% isodose lines should be <8 mm.
In the present case, it is 2.06 mm–1.32 mm = 0.74 mm.
The arrangement of the beams [Figure 4B]–D] was done such that there is adequate coverage of the target while giving a lower dose to the OARs. It should be noted that the beams should not pass through the ipsilateral eye.
Step 8: Quality assurance
A mechanical isocenter check was done using the Winston Lutz test, and the point dose verification was done while maintaining the tolerance as 1 mm.
Step 9: Dry run
Treatment verification consists of setup reproduction, isocenter verification, and clinically verifying each treatment field: check beam clearance, check any interlock, MLC interlock and potential Monitor Unit (MU) problems, and then clearly mark the immobilization devices after a successful dry run.
Step 10: Premedication protocol
Prior to the start of the treatment premedication was delivered in the form of tablets as described next: all starting the day before the start of RT treatment.
Tablet Dexamethasone 8 mg thrice daily
Tablet Ondansetron 8 mg thrice daily
Tablet pantoprazole 40 mg once daily
If the patient is diabetic, proper diabetic care needs to be done.
Step 11: Set up verification and treatment delivery
It includes cone beam CT correction [Figure 5]A and hexapod corrections [Figure 5]B. After all the corrections are done, the treatment is delivered.
|Figure 5: Treatment verification. (A) Cone beam computed tomography correction of the patient during the treatment. (B) Hexapod correction of the same patient during the treatment|
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Step 12: Postmedication
It is an optional protocol that usually includes antiemetics, proton pump inhibitors, and tapering the dose of steroids over a week.
Step 13: Advice and follow-up
After the completion of the treatment, the patient was usually advised to follow up after six months for imaging. Radiotherapy outcome grading for the VS was done as per the consensus in the 7th International Conference on acoustic neuroma.
Here, we also provide the VS SRS Plan Evaluation sheet as a supplementary file that will help in proper and accurate plan evaluation for every SRS case of VS.
| Conclusion|| |
This article conceptualizes and acts as an easy guide for the beginners for the stereotactic radiation planning for VS.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]