Radiation treatment for intracranial tumorsMedical editor Knut Lote MD
Oslo University Hospital
Radiation therapy is used in the curative treatment of medulloblastoma, germinoma and lymphoma tumors. For a number of patients this may be sufficient to cure their cancer.
There is a major risk of spinal fluid metastases in the central nervous system with medulloblastoma or germinoma tumors. Radiation therapy to the entire central nervous system with a subsequent boost to the primary area is usually required for curing the patient. Radiation therapy to the ventricle system supplied with a boost to the main tumor is sufficient for germinomas.
Postoperative radiation therapy, limited to the tumor area with 2–3 cm margins, is routine for high-grade malignant glioblastoma gliomas (grade IV) and for anaplastic gliomas (grade III). It can also destroy infiltrating cancer cells around the main tumor. Radiation therapy will have a life-extending, but generally not a curative, effect on such conditions.
Radiation therapy limited to the tumor area will relieve the symptoms for low-grade (grade 1–2) gliomas, but will probably not have a life-extending effect. Indications for radiation therapy for low-grade gliomas must, therefore, be individually evaluated.
Radiation therapy limited to the tumor area will often stop the growth of local, aggressive, inoperable hypophyseal adenomas and meningiomas. These tumors will most often persist after external radiation therapy, but this is rarely of clinical significance provided that further tumor growth is prevented.
In the case of brain metastasis, often the entire brain is subjected to radiation. However, stereotactic treatment - either as the only modality or as a boost to macroscopic metastasis - is an alternative for conditions with a maximum of 3 metastases and where the largest single metastasis has a diameter of < 3 cm.
For malignant infiltrating tumors (grade III–IV gliomas), the addition of local radiation therapy can destroy infiltrating cancer cells around the main tumor and thereby extend the patient's life.
An individually adapted target volume and dosage is administered. CT and MRI-based dosage planning, risk assessment with regard to normal tissue structures, as well as informing the patient of the value of radiation therapy when weighed against any potential risk for delayed damage to normal tissue must all be included as routine preparation for radiation therapy.
If the patient cannot be operated on, either for medical reasons or due to the location of the tumor - close to or in important nerve centers in the brain, then radiation therapy as the sole treatment method may be an alternative.
- Intracranial tumors that are difficult to access for surgery
- Other intracranial tumors
- Tumor reduction, local tumor control or cure.
The various target volumes for radiation therapy are defined according to ICRU standards.
The margins of a macroscopically definable tumor to the outer limit of the radiation fields vary significantly depending on the type of tumor; the biology of the respective tumor must be considered.
Definitions of target volumes in accordance with the ICRU (International Commission on Radiation Units and Measurements)
|GTV (Gross tumor volume)
Gross palpable or visible/identifiable area of malignant growth.
|CTV (Clinical target volume)
Macroscopic tumor volume including any remaining tumor tissue.
|ITV (Internal Target Volume)
Volume containing CTV and internal margin to allow for internal movements and changes to CTV.
|PTV (Planning Target Volume)
||Geometric volume containing ITV with set-up margin taking into account patient movements, variations in patient positioning, and field settings.
||Normal tissue senstive to radiation that may significantly affect planning and/or dose.
PRV (Planning organ-at-risk volume)
|Geometric volume containing risk volume with set-up margin.
|TV (Treated Volume)
||Volume within an isodose surface considered sufficient based on the treatment intention.
|IV (Irradiated Volume)
||Volume-to-receive dose that is of significance with regard to normal tissue tolerance.
|CI (Conformity Index)
||Relationship between the planning target volume and treated volume (PTV/TV).
The patient's head is immobilized with a head support/mask.
A CT scan is taken, possibly with the addition of an MR image, for dosage planning.
Based on the CT dosage plan, the tumor area to be treated with radiation therapy is defined, and in addition, sensitive normal tissue structures that should preferably not be subjected to radiation are marked (eye, inner ear, chiasma).
The optimal radiation dosage and radiation field for the respective patient are calculated.
The radiation fields are marked on the patient.
Radiation therapy can be administered as a single fraction dose, or daily, 5 times a week, over a period of 2–6 weeks.
The fraction dose is generally 1.8–2.0 Gy, with an accumulated total dose of 50-60 Gy. For brain metastases,10 fractional doses of 3 Gy each, to a total dose of 30 Gy, are often used.
Administering the dose of radiation itself only takes around 1-2 minutes, but positioning the patient on the treatment apparatus and setting the radiation fields often takes 10–15 minutes. In certain complicated situations, this can take more time.
During radiation therapy
The patient is routinely seen for weekly consultations during the radiation therapy treatment period for follow-up purposes. If necessary, the patient can be seen more frequently.
After radiation therapy
Usually the patient is seen at their local hospital. The frequency of follow-ups varies according to the disease group and symptoms. Long-term follow-up after radiation therapy to the central nervous system must include yearly endocrine basal monitoring for all axes, as a minimum. The reason for this is that patients treated with radiation therapy to the hypophysis/hypothalamus can develop endocrine dysfunction many years after treatment.
Side effects of radiation therapy
Hair loss in the radiation field, usually temporary
Possible fatigue and loss of initiative
Normally, treatment results in few acute side effects.
There is a risk of delayed cognitive damage in the form of increased fatigue, loss of initiative, reduced short-term memory and learning difficulties. The risk of and degree of such delayed damage depends both upon the volume of the treated tumor and the radiation dose, but most of all upon the age of the patient. For adult patients, the risk of significant delayed cognitive damage is small (probably less than 5 %). After radiation therapy to the hypophysis and hypothalamus, endocrine dysfunction requiring treatment (STH, TSH, FSH, LH, ACTH) can occur, and there may be a need for endocrine substitution.