Abstract
Gamma angle plays a major role in Gamma Knife Radiosurgery (GKRS) treatment planning. Selecting an appropriate gamma angle may help in mitigating unnecessary radiation exposure to organs at risk (OARs). The aims in GKRS of vestibular schwannoma (VS) is to deliver sufficient radiation to the tumor extending into internal auditory canal (IAC) while keeping basal turn of cochlea and brain stem away from 4 and 12 Gy radiation exposure, respectively. This study analyses the optimal gamma angle in GKRS for VS treatment planning.
The study was performed using old MRI datasets of 16 patients of VS in Leksell GammaPlan version 10.1.1. T2 weighted contrast MRIs were used for the planning purposes. Three different plans were made for each patient at gamma angles 90°, 110° and 70° using hybrid inverse planning technique. Dynamic shaping was used to achieve as low as reasonably achievable (ALARA) doses to the cochlea without compromising target coverage (i.e. coverage of more than 97% of tumor volume).
This comparative analysis shows minimal radiation exposure to cochlea for plans made at gamma angle 110° compared to 90° and 70°. Average percentage volume of cochlea receiving 4 Gy were 9.63 ± 12.32%, 6.19 ± 8.24%, and 25.25 ± 31.82% at gamma angles 90°, 110° and 70°, respectively (one-way ANOVA p = 0.0247). The average selectivity indices were 83.44 ± 7.13, 84.06 ± 7.84 and 83.56 ± 7.22 at gamma angles 90°, 110° and 70° respectively. Similarly, the gradient indices and beam on time were 2.80 ± 0.23, 2.81 ± 0.23 and 2.80 ± 0.25 and 120.65 ± 59.63, 117.95 ± 58.06 and 123.99 ± 61.61 min, respectively, at 90°, 110° and 70°. The selectivity index, gradient index and beam on time were minimal at gamma angle 110° compared to the other two angles, but not statistically significant (one-way ANOVA p-values were 0.9686, 0.9942 and 0.9598, respectively). The gamma angle of 110° is a good choice for treatment planning of VS patient in Gamma Knife as it gives better treatment plans (minimal cochlea doses).
Keywords: Gamma angle, vestibular schwannoma, gamma knife radiosurgery, selectivity index, gradient index, beam on time
INTRODUCTION
Gamma Knife radiosurgery (GKRS) is a stereotactic radiosurgery device dedicated to treat intracranial pathologies for various benign, malignant, and functional disorders. It was first developed by Lars Leksell of the Karolinska Institute in Stockholm and the medical physicist Börje Larsson of the Gustaf Werner Institute, University of Uppsala [1]. It has 192 precisely focused Co-60 beams of radiation (Perfexion and Icon models) [2]. Old models like U, B and 4C had 201 encapsulated Co-60 [3-4]. A vestibular schwannoma (VS) is a benign, slow-growing tumor that develops from the vestibular devision of the eighth cranial nerve [5]. The advocated radiation prescription dose for GKRS for VS remains 11-13 Gy at 50% isodose level [6]. It remains challenging to achieve appropriate target coverage while preserving basal turn of cochlea from 4Gy radiation exposure in a single fraction [7]. Selecting an optimal gamma angle among 90°, 110° and 70° (Figure 1A) can significantly alter the radiation spillage to cochlea without compromising on other indices viz. coverage, selectivity index (SI), gradient index (GI), beam on time (BT) etc. Selectivity is defined by how much normal tissue around the target is spared and SI is the volume of the target covered by the peripheral isodose divided by the total peripheral isodose volume [8-9]. The GI defined the degree of dose fall off outside the target volume [10-11].
Figure 1.
a) Patient docking at various gamma angles γ (90°, 110°, and 70°) and cochlea volume involvement in the path of beam and b) the Leksell Gamma Knife coordinate system and the patient coordinate system with the gamma angle γ.
METHODS
The treatment planning algorithm uses patient coordinate system (X, Y, Z) and Leksell Gamma Knife coordinate system (P, Q, R) [12]. The origin of the P, Q, R coordinate system coincide with the focus of the radiation beam from all 192 sources. The axis ‘R’ is directed into the radiation unit along the couch, ‘C’. The gamma angle is the angle between ‘R’ axis and the X-Y plane of the patient coordinate system (Figure 1B). This retrospective study was performed on T2 weighted MR image of 16 cases of VS treated with frame based GKRS with Leksell Perfexion model. These patients were randomly selected from the database of the LeksellGammaPlan (version 10.1.1). An independent neuroradiologist defined the target volume (TV), ipsilateral basal turn of cochlea, and brain stem. Three different plans were made for each patient at gamma angles 90°, 110° and 70° using hybrid inverse planning technique [13]. We used dynamic shaping to achieve ALARA (As Low As Reasonably Achievable) doses to the cochlea without compromising on target coverage. In all the cases, brain stem was saved from 12 Gy radiation exposure. These three plans of the same patient (total 48 plans for 16 cases) were made to have similar target coverage (e.g., 97% of the target volume cover by the prescription isodose).
RESULTS
The results of this study is shown in figure 2. We observed that the cochlea dose was minimal for plans made at gamma angle 110° in comparison to plans at 90° and 70°. The average percentage volume of cochlea receiving 4Gy from 16 plans were 9.63 ± 12.32%, 6.19 ± 8.24% and 25.25 ± 31.82% respectively at gamma angles 90°, 110° and 70° (statistically significant as One-way ANOVA p=0.0247). The median values were 6%, 3% and 7.5% respectively (Figure 2). The average SI were 83.44 ± 7.13, 84.067.84 ± and 83.56 ± 7.22 with median values of 85, 87.50 and 85.50 at gamma angles 90°, 110° and 70° respectively. Similarly, the GI were 2.80 ± 0.23, 2.81 ± 0.23 and 2.80 ± 0.25 respectively at these angles with median values of 2.78, 2.74 and 2.75. The BT were 120.65 ± 59.63 minutes, 117.95 ± 58.06 minutes and 123.99 ± 61.61 minutes respectively with median value of 117.40 minutes, 114.75 minutes and 118.60 minutes at these gamma angles. The selectivity index, gradient index and beam on time were also minimal at gamma angle 110° in comparison to other two angles but were not statistically significant as One-way ANOVA p-values were 0.9686, 0.9942 and 0.9598 respectively.
Figure 2.
Box and whisker plot showing different Gamma Knife plan indices of vestibular schwannoma patients at different gamma angles.
DISCUSSION
The results of the study show that gamma angle 110° is the optimal choice for treatment planning in GKRS of VS. It is an optimal angle to spare cochlea from undesirable radiation exposure (i.e., ≥ 4 Gy) (Figure 2) and statistically significant as p<0.05. In most of the cases, the basal turn of cochlea lies anterior and inferior to the target volume. Planning at 110° leads to lesser involvement of the cochlea along the path of the primary beam (Figure 1A) as it keeps the cochlea oriented away from the beam. The average cochlea volume receiving 4Gy at 110° was 6.19% which was 3.44% lesser than plans at 90°, and 19.06% lesser volume than plans at 70°. The maximum and median 4 Gy exposure to cochlea was minimal with plans made at gamma angle 110° than 90° and 70° (Figure 2). On the other hand, plans made at gamma angle 70° lead to maximum radiation delivery to cochlea (Figure 1A, 2). At the same time, the average length of the beam path from skull surface to the target get reduced at gamma angle 110° in comparison to 90° and 70°angles. This causes the average and median beam on time (BT) or the treatment time gets minimal at this particular gamma angle. The other plan index like SI and GI were also better at gamma angle 110° relative to other angles, but they were not statistically significant as p>0.05. From the perspective of patient, the optimal and comfortable frame fixation includes frame fixed parallel to floor, without any angulation. Among the three gamma angles, 90° is the most comfortable as the patient lies in anatomical position with no undue strain on the neck. Gamma angle of 70° remains the most uncomfortable as the fixed head lies in a flexed duration for the time of treatment. With 110°, most of the patients remain comfortable with slight extension in the neck however some patients do not tolerate it citing pain in the neck and shoulder. Additionally, the base ring of the frame gets close to the shoulder leading to discomfort. With frame fixed with slight downward angulation from anterior to posterior the desired effect can be achieved. The further effectivity can be proven in prospective studies concentrating on long term hearing preservation.
CONCLUSIONS
The gamma angle 110° may be an optimal choice for treatment planning of VS patient in GKRS to preserve cochlea from 4 Gy exposure compared to gamma angle 90° and 70°. It may help in achieving the goal of effective target coverage while avoiding radiation exposure to OARs.
NOMENCLATURE
ALARA as low as reasonably achievable
ANOVA analysis of variance
BT beam on time
GI gradient index
GKRS Gamma Knife radiosurgery
MRI magnetic resonance imaging
SI selectivity index
SRS stereotactic radiosurgery
VS vestibular schwannoma
ACKNOWLEDGMENTS
Authors’ disclosure of potential conflicts of interest
The authors have nothing to disclose.
Author contributions
Conception and design: Ngangom Robert
Data collection: Ngangom Robert, Manjul Tripathi
Data analysis and interpretation: Ngangom Robert, Budhi Singh Yadav
Manuscript writing: Ngangom Robert
Final approval of manuscript: Ngangom Robert, Budhi Singh Yadav, Manjul Tripathi
REFERENCES
- 1.University of Virginia, School of Medicine. Neurosurgery: History and technical overview. https://med.virginia.edu/neurosurgery/services/gamma-knife/for-physicians/history-and-technical-overview/. Accessed 2020-10-13. [Google Scholar]
- 2.Elekta Gamma Knife Treatment Process. https://www.elekta.com/patients/gammaknife-treatment-process/. Accessed 2020-10-13.
- 3.Asgari S, Banaee N, Nedaie HA. Comparison of full width at half maximum and penumbra of different Gamma Knife models. J Cancer Res Ther. 2018;14:260-266. [DOI] [PubMed] [Google Scholar]
- 4.Coffey RJ, Lunsford LD. Stereotactic radiosurgery using the 201 cobalt-60 source Gamma Knife. Neurosurg Clin N Am. 1990;1(4):933-954. [PubMed] [Google Scholar]
- 5.National Institute on Deafness and other Communication Disorders. Vestibular schwannoma (acoustic neuroma) and neurofibromatosis, NIH Pub No 99-580. Bethesda, MD: US Department of Health and Human Services - National Institutes of Health, 2015, 4 pp. https://www.nidcd.nih.gov/sites/default/files/Documents/health/hearing/VestibularSchwannoma-FactSheet.pdf. Accessed 2020-10-13. [Google Scholar]
- 6.Lunsford LD, Niranjan A, Flickinger JC, Maitz A, Kondziolka D. Radiosurgery of vestibular schwannomas: Summary of experience in 829 cases.. J. Neurosurg. 2005;102(Suppl):195-199. [PubMed] [Google Scholar]
- 7.Tamura M, Carron R, Yomo S, Arkha Y, Muraciolle X, Porcheron D, Thomassin JM, Roche PH, Régis J. Hearing preservation after Gamma Knife radiosurgery for vestibular schwannomas presenting with high-level hearing. Neurosurgery. 2009;64:289-96; discussion 296. [DOI] [PubMed] [Google Scholar]
- 8.Lomax NJ, Scheib SG. Quantifying the degree of conformity in radiosurgery treatment planning. Int J Radiat Oncol Biol Phys. 2003;55:1409-1419. [DOI] [PubMed] [Google Scholar]
- 9.Torrens M, Chung C, Chung HT, Hanssens P, Jaffray D, Kemeny A, Larson D, Levivier M, Lindquist C, Lippitz B, Novotny Jr J, Paddick I, Prasad D, Yu CP. Standardization of terminology in stereotactic radiosurgery: Report from the Standardization Committee of the International Leksell GammaKnife Society. J Neurosurg. 2014, 121(Suppl 2):2-15. [DOI] [PubMed] [Google Scholar]
- 10.Wagner TH, Bova FJ, Friedman WA, Buatti JM, Bouchet LG, Meeks SL. A simple and reliable index for scoring rival stereotactic radiosurgery plans. Int J Radiat Oncol Biol Phys. 2003, 57:1141-1149. [DOI] [PubMed] [Google Scholar]
- 11.Yomo S, Tamura M, Carron R, Porcheron D, Régis J. A quantitative comparison of radiosurgical treatment parameters in vestibular schwannomas: The Leksell Gamma Knife Perfexion versus Model 4C.. Acta Neurochirurg. 2010;152:47-55. [DOI] [PubMed] [Google Scholar]
- 12.Elekta. Online reference manual: Leksell GammaPlan 8.3, Document number 1008129. Stockholm: Elekta AB, 2008. 336 pp. [Google Scholar]
- 13.Robert N, Tripathi M, Trivedi G, Chauhan RP, Oinam AS, Singh R, Tomar P. A technique to increase the treatment plan indices in GammaKnife: A retrospective study. J Radiosurg SBRT. 2021;7:245-248. [PMC free article] [PubMed] [Google Scholar]