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Stereotactic radiation of intracranial tumors


Medical editor Knut Lote MD
Oncologist
Oslo University Hospital

General

For stereotactic radiation treatment, high radiation doses can be delivered with great precision to intracranial target areas, without the surrounding normal brain tissue being irradiated above the tolerance limit. Often a single radiation fraction is used, or possibly a few. The radiation dose in focus is then so high that the tissue in the small high-dose volume will die.

There are multiple equally effective techniques for stereotactic radiation treatment:

  • Gamma Knife (radiation knife)
  • linear accelerator
  • CyberKnife   

A gamma knife is mostly used for small tumors. The treatment is started and completed during one hospital stay. Haukeland University Hospital is the only hospital in Norway with a Gamma Knife. The technique will not described further here.

At Oslo University Hospital, the Radium Hospital, radiation therapy is carried out with a linear accelerator. This technique can also be used for larger tumors. The treatment is given in the form of several smaller radiation doses distributed over a number of weeks or as a single fraction.

A CyberKnife is a form of stereotactic treatment for delivering radiotherapy in which a linear accelerator is operated by a robot. The type of radiation and effect are otherwise equal to a gamma knife and/or linear accelerator. This system, however, is currently not available in Norway.

Indications

  • Intracranial metastases (max. number, 2-3 metastases; max. diameter, approximately 3 cm)
  • Small (max. 3 cm in diameter) Schwannomas or meningiomas 
  • Residual tumors after earlier operations for pituitary gland tumors
  • Other benign tumors where operation involves risks

Goal

  • Destroying the cancer cells' DNA and destroying the cancer cell
  • Neutralize the intracranial cancer disease of intracranial metastases
  • Limit radiation strain on surrounding brain tissue

Definitions

Target volume and doses

Dose to the target volume's periphery, doses in the vicinity of 12–24 Gy are usually given. Tumors in the centre can receive even higher doses. These are radiation doses that will destroy the irradiated tissue.

Definitions of target volumes in accordance with the ICRU (International Commission on Radiation Units and Measurement)

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 accound patient movements, variations in patient positioning, and field settings.
OAR (Organ-at-Risk) 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).

  


Preparation

  • The patient's head is immobilized in a stereotactic frame or a mask.
  • The tumor's exact localization is calculated with the aid of MRI/CT
  • Dose planning is performed on the basis of the CT and MR images.
  • The patient is given a sedative before the radiation.
  • The patient is placed in the dorsal position.   
  • The metal frame is fastened to the associated top.
  • The lasers are aimed according to the reference point on the frame and relative table adjustments are performed according to calculations.
  • The frame is then removed and the patient is ready for treatment.

Implementation

  • The treatment is given in the form of 1–5 fractions.
  • The treatment will usually be given over the course of 1–2 weeks.
  • The total dose to the tumor's periphery can be around 12–24 Gy or higher. 

Follow-up

  • Stereotactic radiation treatment usually produces little or no acute subjective side-effects.
  • Most patients go home the same day that they receive the treatment.

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