Gamma Knife

Question 1

Stereotactic radiosurgery (SRS):

Answer 1

Precise administration of radiation in a single session to control the tumor by ablation.

Question 2

Stereostatic radiotherapy (SRT):

Answer 2

Precise administration of radiation over multiple fractions for cranial diseases.

Question 3

Sterostatic body radiotherapy (SBRT)

Answer 3

The application of SRT-like tech to the treatment of extra-cranial diseases.

Question 4

Radiosurgery:

Answer 4

Surgery using precisely targeted radiation to destroy tissue without cutting.

Question 5

Stereotactic Irradiation

Answer 5

–Focal irradiation techniques using multiple non-coplanar beams to deliver a prescribed dose of ionizing radiation to preselected and stereotactically localized lesions

Question 6

Characteristics of Stereotactic Irradiation

Answer 6

• Total prescribed dose: 10 - 50 Gy
• Planning targets: 1 - 35 cm³
• Modalities:

1. ⁶⁰Co gamma rays (Gamma Knife, ViewRay)
2. Megavoltage x rays (linac-based, CyberKnife, TomoTherapy)

Question 7

Requirements of SRS, SRT:

Answer 7

1. Accurate knowledge of the total dose and fractionation scheme required to treat the particular disease
2. Target volume accurately determined via stereotactic techniques
3. Calculation of 3D dose distributions that conform to target shapes, with sharp dose fall-off outside the target volume
4. Minimal dose to radiosensitive structures
5. Precise delivery of dose to the target:

• 1 mm for SRS
• 2 mm for SRT, SBRT

Question 8

Gamma Knife Specifics:

Answer 8

• Source: Co-60 with initial activy of ~3 Ci
• Number of sources: 201
• Ability to treat very small fields (down to 4mm), which most linac-based systems cannot
• High accuracy 0.5 mm (no rotating or moveable parts)
• Larger treatment volumes require multiple isocenters, increases overall treatment times
• Can only be used for intracranial lesions:
• APS positioning challenged in lesions proximal to skull
• Target-dose distributions tend to be more non-uniform than for linac-based SRS

Question 9

Gamma Knife: Stereotactic Frames

Answer 9

Leksell frame:

• Titanium
• CT, MR compatible

Question 10

Gamma Knife: Localizers

Answer 10

Localizers (fiducial box):

• CT:
  
  • Aluminum wires
• MR:
  
  • Hollow plastic tubes filled with copper sulfate doped saline

Question 11

Gamma Knife: Applications

Answer 11

Treatment of small (<4 cm), spherical

intracranial diseases:

• Multiple isocenters required for irregular targets

Question 12

Gamma Knife: Prescription Doses

Answer 12

About 15 -25 Gy