stereotactic radiotherapy

1. STEREOTACTIC
RADIOSURGERY (SRS)
AND RADIOTHERAPY
(SRT)
Dr. purvi rathod

2. Definitions
■ Stereotactic- using a precise 3D mapping technique to guide a procedure.
■ Stereotactic radiosurgery (SRS)- stereotactically guided conformal RT of a
defined target volume in a single session with gamma knife/modified
LINAC/ cyberknife/ tomotherapy/proton beam systems.
■ Stereotactic radiotherapy (SRT)- stereotactically guided delivery of highly
conformal radiation to a defined target volume in multiple #’s using
noninvasive positioning techniques.

3. ■ Fractionated stereotactic radiotherapy- stereotactically guided high
dose conformal RT administered to a precisely defined target volume in
2-5 #’s ( hypofractionated SRT).
■ SIMRT- stereotactic intensity modulated radiotherapy. Using multiple
isocenters to perform radiosurgery of a single target volume.

5. SRS
■ Hallmarks of SRS-
1) High degree of dose conformity- achieved by
- using appropriate circular beams to fit the tumor
- optimizing arc angles and weights
- using multiple isocenters
- dynamically shaping the field using mini MLC.
2) Accuracy of beam delivery- using specially designed stereotactic
apparatus.

6. ■ Currently there are 3 types of radiations being used for SRS and SRT
■ Heavily charged particles, cobalt 60 gamma rays and megavoltage
Xrays (most commonly used ).

7. Key requirements for optimal
stereotactic irradiation

8. Radiobiology of SRS.
■ Acute reduction in the vessel length
density and blood flow over first 24hrs
post radiation.
■ Park et.al concluded that there is a
threshold effect occurring after 10Gy,
above which there is substantial vascular
damage that contributes indirectly to
tumoricidal effect.
■ The toxicity increases rapidly once the
volume of brain exposed to >12Gy is >5cc
to 10cc.

9. Radiosurgery techniques.
1) Gamma knife-
■ Leksell and Larson created the first
prototype in 1967.
■ There are hemispherical arrays of
multiple fixed cobalt 60 beams (201).
■ Sharply collimated to create very small
and spherical target volume with sharp
dose fall off.
■ Currently Perfexion model introduced in
2006 is in use.
■ It has a larger patient aperture (treat all
intracranial and cervical spine targets
quickly with robotic positioning).

11. 2) Rotating gamma system
■ There are 30 cobalt 60 radiation sources revolving in hemispherical shell
with 6 groups of 5 different collimators.
■ These system provides spherical system volumes of different diameters.
■ There is limited experience with this system.

12. 3) proton surgery
■ The chief advantage is that the beam
stops at a depth related to the
beam’s energy.
■ Lack of exit dose and sharp beam
profile provides target irradiation
with lower integral doses than are
required in photon.
■ The bragg peak or bragg gray peak is
an area of increased ionization in the
last few millimeters of the path
length.
■ First functional neurosurgery with
proton was done in 1958 for
Parkinson disease.

13. 4) LINAC radiosurgery
■ Improved hardware and advanced dose planning
software have been developed for enhancing the
conformity of radiosurgery with LINAC.
■ Beam shaping with multileaf collimators and intensity
modulation with inverse treatment planning
algorithms.
■ LINAC based systems- Xknife, Novalis, the peacock
system, Cyberknife.

15. 5)Tomotherapy
■ It combines the precision of Ct imaging
with the radiotherapy technique.
■ A LINAC megavoltage treatment beam is
added to a rotating Xray source with
moving table of the diagnostic CT.
■ The beams rapidly rotate around the
patient thus allowing treatment from
many angles.
■ There is precise localization of the target
before every treatment setting.

16. 6) LINAC image guided radiotherapy-
■ These systems use non invasive immobilization devices and patient
position tracking systems.
■ Egs-Trilogy and SynergyS equipped with cone beam CT imaging
capability.

17. Normal tissue tolerance in SRS and SRT
■ The recommended tolerance dose from the
RTOG dose escalation study (graph).
Optic nerve tolerance for SRS- 8Gy as safe
maximum dose limit for optic chiasm /nerve.
Other cranial nerves- special sensory nerves
(optic and auditory) are the most radiosensitive.
Followed by trigeminal and then the motor
Nerves (II,IV,VI,VII,IX to XII.)
Radiosurgery dose of 12 to 13Gy, FRS 18Gy/3#,
SRT to 45 to 50Gy/25 to 28#.

18. ■ Spinal cord- tolerance dose depends on the volume of irradiation,
distribution of radiation, dose of radiation and time interval between
initial treatment and reirradiation.
■ Gersten et.al reported 125 patients study on cyberknife spine surgery.
■ Mean dose of 14Gy received by mean volume of 27.8ml with 80%
isodose treatment volume showed no acute radiation toxicity after a
mean follow up of 18mths.

19. Xray knife
■ In linac based SRS technique multiple non coplanar circular/dynamic beams
are focused on the machines isocenter.
■ This isocenter is placed at the center of the imaged target volume.
■ Spherical dose distribution is obtained to fit the lesion.
■ Manipulation of several parameters-
1) Selectively blocking parts of circular field.
2) Shaping the beams eye aperture dynamically with MLC.
3) Changing the arc angle and weights.
4) Using more than one isocenter.
5) Combining stationary beams with arcing beams.
6) Help ofTPS.

21. 1) Stereotactic frame
■ 1) pedestal mounted frame.
■ 2) couch mounted frame.
■ The stereotactic frame is attached to the patients skull and the couch or
the pedestal.
■ This provides a rigidly fixed frame of coordinates for relating the center
of imaged target to isocenter of treatment.
■ Srs frames- Leksell, Riechert-Mundinger,Todd-Wells, Brown- Robert-
Well(brw)

23. Basic stereotactic system
■ BRW frame
■ CT localizer
■ Angiographic localizer
■ Device for fixing the frame to the patient support table.
■ The BRW frame has 3 orthogonal axes- anterior, lateral, axial.
■ The axes intersect in the center of the circular frame and 80mm on top
of the ring surface.

25. A
1- ct localizer
2- angiographic localizer
3- patient positioning mount
4- head ring with post and pins.
B- angiographic localizer.
C- CT localizer.

26. CT localizer
■ There are 9 fiducial rods on the localizer that look like dots in the CT
transaxial slice image.
■ These dots are called frame co ordinates.
■ A patient docking device joins the frame to the LINAC through patient
support system.
■ The origin of the frame is aligned with the LINAC isocenter within
0.2mm to 1mm.

28. ■ The angiographic localizer consist of 4 plates and attaches to the BRW
head ring.
■ Each plate is embedded with 4 lead markers that act as the fiducial
markers for the angiographic images.
■ MRI localizer is a slightly modified version of CT localizer and
compatible with MRI images.

29. GillThomas Cosman headring
■ A special relocatable ring designed for fractionated SRT
■ There is bite block system, headrest bracket andVelcro straps attached
to the BRW frame.

30. Linac isocentric accuracy
■ there should be an alignment of the stereotactic frame coordinates with
the LINAC isocenter.( intersection point of axes of rotation of gantry,
collimator and couch.)
■ The LINAC isocenter must remain in a radius of 1mm with any
combination of rotations.
■ The stereotactically determined tumor isocenter and the LINAC
isocenter must be coincidental with +/-1mm.

31. Stereotactic accuracy
■ Phantom base- verification device in the BRW frames with identical
coordinates to the frame.(ap, lat, vert).To check the alignment of
radiation isocenter with the target point defined by BRW frame
coordinates.

32. Overall accuracy
■ Geometric accuracy- by using a suitable head phantom with imageable
targets.
■ Test objects- cubes, sphere, cone cylinder.
■ The comparison of there coordinates with the coordinates on the
phantom give the geometric accuracy.

33. Beam collimation
■ The geometric penumbra must be as small as possible because the
treating volume is small itself.
■ Tertiary collimator system is designed to bring the collimator
diaphragm close to the surface.
■ 15cm long circular cones made of cerrobend encased in stainless steel.
■ They are mounted over the xray jaws that provide a square opening
larger than the cones but small enough to prevent radiation escape
from the side walls of the cone.

34. Gamma knife
■ Delivers irradiation to a brain lesion by production of large number of isocentric beams
simultaneously.
■ The sources are contained in a very heavy shielded body >20tons with shielded
entrance door.
■ Hydraulic systems controls the opening and closing of the door.
■ Most recent model is Perfexion by Elekta.
■ Gamma rays are delivered by 192 cobalt60 sources housed in cylindrical configuration
in 5 rings.
■ Each ring has 8 movable sectors with each sector housing 24 sources.

36. ■ The patient positioning system(pps) moves the whole patient into the
stereotactic coordinates.
■ The patients head can be locked in 3 possible positions called gamma angles.-
70’ chin up, 90’ chin horizontal, 110’ chin down.
■ The target is localized with a Leksell frame attached to patient and using a
CT,MRI or Angiography.
■ It has the potential to treat 1 or more lesions in a single session.
■ Also has the potential to treat lesion of the orbits, paranasal sinuses, cervical
spine.
■ Gamma knife can produce more conformal dose distribution.
■ It can be used to treat only small lesions due to field size limitations.

38. QualityAssurance
■ Keeping ALARA (as low as reasonably achievable) principle in mind.
■ To maintain its original accuracy specifiations.
1) treatment quality assurance-
■ Checking or double checking the procedures and treatment parameters.
2) routine quality assurance-
■ Periodically inspect the hardware and software performance to ensure
compliance with the original specifications.

39. Clinical applications
1) functional radiosurgery for trigeminal neuralgia refractory to medical
therapy-
■ 4mm collimators are used with a maximum dose of 80Gy.
■ Response rate is 85% in first 1 week to 4 months, sometimes 6 months.
■ About 50% patients stay pain-free and off medications for 5 years.
2) arteriovenous malformations-
■ Radiosurgery obliterates the AVM nidus in approx 75% patients within 3
years of the procedure. Marginal dose of 23Gy approx.
■ Irradiation can be repeated if the avm fails to obliterate within 3years
with acceptable morbidity.

41. 3) Benign tumors-
■ Vestibular schwannomas, meningioma, pituitary adenoma, craniopharyngioma.
With a prescribed dose of 12 to 14Gy.
4) brain metastases-
■ Most common and best studied indication of radiosurgery.
■ A phase 3 RTOG trial established that radiosurgery immediately following wbrt
(37.5Gy/15#) improves local control and quality of life in patients with 1 to 3
metastases.
■ Radiosurgery can preserve wbrt full dose for later on progression of disease.
■ There is no limit as to how many metastases and what total volume should be
treated by radiosurgery.

42. 5) glioblastoma –
■ Radiosurgery is a reasonable option for small, well circumscribed, high grade
gliomas that recur after prior conventional large field radiotherapy and
chemotherapy.
6) spinal metastases-
■ Pain is the most common indication for radiosurgery.
■ In a study by Gerszten et.al mean dose of 19Gy was given to a mean volume of 27.7
ml of tumor on a Cyberknife. No radiation induced toxicity occurred during follow
up period.
■ Radiosurgery has been used as a substitute for brachytherapy in the management
of recurrent head and neck tumors.
■ Liver metastases and prostrate radiosurgery is being explored.

43. Thank you