Stereotactic radiosurgery (SRS) has been around for over 50 years and is being successfully used for the safe removal of adult gliomas. But its use in children is limited due to concerns of neurological complications. This blog post investigates the application of SRS in pediatric low-grade gliomas such as cerebellar astrocytoma.
What is cerebellar astrocytoma?
Cerebellar astrocytoma is a type of brain tumor found in the lower part of the brain, called the cerebellum. It originates from a type of cells in the brain, known as astrocytes, which do not conduct electric signals but are there to support other neurons. Cerebral astrocytomas may belong to one of the four subtypes depending upon the grade. The most common of these subtypes is a pilocytic astrocytoma, a low-grade tumor that usually shows limited growth and well-defined borders [1]. The majority of pilocytic astrocytomas occur in children and have a favorable prognosis following the standard of care treatment. The current treatment of these low-grade gliomas consists principally of brain surgery to remove the maximum mass of the tumor and is often curative. But the gross total resection of the tumor is not always possible as the tumor can be present near an eloquent part of the brain where removing it could cause severe complications to the patient. In such cases, systemic therapy with chemotherapeutic drugs and some targeted drugs may be used after the surgery.
Why is radiation therapy not common in cerebellar astrocytomas?
In many cancers, the unresectable tumor cells are eliminated through radiation therapy but it is not commonly used in pilocytic astrocytoma because radiations carry a high risk of neurological complications when given to the developing brains of children [2]. Some such complications are:
Cognitive deficits
Hemiparesis (inability to move one side of the body)
Swallowing difficulties
Clumsiness
Spasticity
Hormonal imbalance
Speech impairments
Secondary malignancies
These long-term complications affect glioma survivors' social and functional life and greatly reduce their quality of life. Moreover, any form of radiation therapy requires anesthesia each time a fractioned dose of the overall radiation plan is delivered which puts an additional burden on the children.
How does SRS work?
SRS is a form of radiation therapy in which very fine beams of radiation are precisely directed at the tumor inside the brain from an array of sources outside. A single source does not deliver radiation strong enough to cause damage to the tumor or the brain tissue but radiations from all the sources converge on the tumor and deliver a high dose only to a very precise target. Therefore, SRS does not have the dose limitations as conventional radiation therapy. All the radiation dose required is administered at once. SRS has emerged as a non-invasive way of removing small, deeply seated, or otherwise inaccessible brain tumors.
SRS in cerebellar astrocytomas
SRS is being used as a first-line treatment or add-on therapy for low-grade glioma. The well-circumcised boundaries of pilocytic astrocytoma decrease the chances of exposure of the normal tissue to radiation. A recently published review paper looked at the evidence of the application of SRS in brain tumors from over 15 published research articles between 1978 and 2014 [3]. A majority of patients in these studies were children suffering from pilocytic astrocytoma, and SRS was given as primary or adjuvant therapy. The radiation doses were between 10 to 25 Gy [3]. The patients showed better responses than conventional radiation therapy in terms of clinical symptoms, functional performance, and overall quality of life [3]. The authors of this review concluded that SRS is a safe and effective treatment option for low-grade gliomas. Another review of the more recent studies depicted a similar conclusion [4].
Take home message
SRS is not a standard therapy for low-grade pediatric gliomas like cerebellar astrocytoma but it is being successfully utilized in many clinics to eliminate the tumor which is very deep in the brain, very close to vital parts of the brain or shows growth following primary surgical resection. In conventional radiation therapy, the dose of radiation is limited to protect the normal tissue, but SRS circumvents this limitation by providing the maximum dose only at a point where it is required to destroy the tumor. Ample evidence suggests that SRS is safe in children and it is effective in eliminating unresectable tumors. However, drawing firm conclusions, that can fit all cases, from these data is difficult due to the diversities in patient, tumor, and treatment characteristics. The decision to undergo SRS is highly individualized and should be made by an interdisciplinary team of medical professionals after considering the patient profile.
References
D. Salles, G. Laviola, A. C. de M. Malinverni, and J. N. Stávale, “Pilocytic Astrocytoma: A Review of General, Clinical, and Molecular Characteristics,” J Child Neurol, vol. 35, no. 12, pp. 852–858, Oct. 2020, DOI: 10.1177/0883073820937225.
D. S. Bitterman et al., “Revisiting the Role of Radiation Therapy for Pediatric Low-Grade Glioma,” JCO, vol. 37, no. 35, pp. 3335–3339, Dec. 2019, DOI: 10.1200/JCO.19.01270.
O. Sager et al., “Concise review of stereotactic irradiation for pediatric glial neoplasms: Current concepts and future directions,” World J Methodol, vol. 11, no. 3, pp. 61–74, May 2021, DOI: 10.5662/wjm.v11.i3.61.
N. A, F. A, and L. Ld, “Stereotactic Radiosurgery for Low-Grade Gliomas,” Progress in neurological surgery, vol. 34, 2019, DOI: 10.1159/000493063.
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