Description of technique of SRS by LINAC in AVM

1. STEREOTACTIC RADIOSURGERY IN
ARTERIO VENOUS MALFORMATIONS
Dr Umesh V
Department of Radiation Oncology
KMC MANIPAL

2. Introduction
• Arteriovenous malformations (AVMs) are congenital vascular anomalies comprised of an
abnormal number of blood vessels that are abnormally constructed.
• The blood vessels directly shunt blood from arterial input to the venous system without an
intervening capillary network to dampen pressure.
• Both abnormal blood vessel construction and abnormal blood flow lead to a risk of rupture
and intracranial hemorrhage.
• In addition, patients with lobar vascular malformations may suffer from intractable vascular
headaches or develop seizure disorders.

3. Patient characteristics & Epidemiology
• Patient’s age – even though it is congenital in origin they usually present in young
adults
• Brain AVMs occur in about 0.1 percent of the population, one-tenth the incidence of
intracranial aneurysms.
• Supratentorial lesions account for 90 percent of brain AVMs; the remainder are in
the posterior fossa. They usually occur as single lesions, but as many as 9 percent are
multiple.
• Brain AVMs underlie 1 to 2 percent of all strokes, 3 percent of strokes in young
adults, and 9 percent of subarachnoid hemorrhages

4. Pathophysiology
• AVMs are congenital lesions composed of a complex tangle of arteries and veins
connected by one or more fistulae.
• The vascular conglomerate is called the nidus.
• The nidus has no capillary bed, and the feeding arteries drain directly to the draining
veins.
• The arteries have a deficient muscularis layer.
• The draining veins often are dilated owing to the high velocity of blood flow through
the fistulae.
• Deranged production of vasoactive proteins is under investigation as the angiogenetic
link to pathophysiology.

5. Types of AVM
•Lobar AVMs
•Pial AVM

6. Presentation
• Brain AVMs usually present between the ages of 10 and 40 years
• AVMs produce neurological dysfunction through 3 main mechanisms.
 Hemorrhage may occur in the subarachnoid space, the intraventricular space or,
most commonly, the brain parenchyma.
 In the absence of hemorrhage, seizures may occur as a consequence of AVM:
approximately 15-40% of patients present with seizure disorder.
 Progressive neurological deficit may occur in 6-12% of patients over a few months to
several years. These slowly progressive neurological deficits are thought to relate to
siphoning of blood flow away from adjacent brain tissue (the "steal phenomenon”).

7. Risk of Hemorrhage
• Hemorrhage as the initial clinical presentation was
the strongest predictor for subsequent hemorrhage
in patients with untreated brain AVMs.
• According to the systemic review, the annual rate
of hemorrhage is 2.2 percent (95% CI: 1.7 – 2.7) for
unruptured AVMs and 4.5 percent (95% CI: 3.7 –
5.5) for ruptured AVMs.
• Anatomic and vascular features of the AVM also
appear to influence risk of subsequent
hemorrhage. These include the presence of
associated aneurysms (HR 1.8), exclusive deep
venous drainage (HR 2.4), and deep brain location
• AVM size do not seem to influence the risk of
hemorrhage

8. Diagnosis
• Computed Tomography- Flow voids may be identified on CT with contrast
administration in and around the region of the nidus of the brain AVM. CT
characteristically demonstrates intraparenchymal hemorrhage without significant
edema in patients who present with hemorrhage.
• Magnetic resonance imaging- MRI is very sensitive for delineating the location of the
brain AVM nidus and often an associated draining vein. It also has unique sensitivity in
demonstrating remote bleeding related to these lesions. Dark flow voids are
appreciated on T1 and T2-weighted studies
• Angiography

10. Angiography
• Angiography — Angiography is the gold standard for the diagnosis, treatment
planning, and follow-up after treatment of brain AVMs.
• ●Anatomical and physiological information such as the nidus configuration, its
relationship to surrounding vessels, and localization of the draining or efferent
portion of the brain AVM are readily obtained with this technique.
• ●The presence of associated aneurysm suggests a lesion at higher risk for subsequent
hemorrhage.
• ●Contrast transit times provide additional useful information regarding the flow state
of the lesion which is critical for endovascular treatment planning.

12. Management

13. Surgery
• Open microsurgical excision offers the best chance
immediate cure in patients considered to be at high
risk of hemorrhage.
• An important factor in recommending therapy is an
assessment of surgical risk.
• Multiple or large lesions, those in eloquent brain
areas, and those with deep venous drainage are
more difficult to safely resect.
• Many surgeons use a classification system (Spetzler-
Martin grading scale) that assesses the surgical risk .

15. Interpretation of Spletzer Martin Grade
• Microsurgery is an effective and relatively safe option for patients with SM Grade I or
II AVMs.
• In contrast, Grade IV and V AVMs are associated with higher risks and less success
regardless of the option selected.
• The SM Grade III AVMs are a heterogeneous group that includes different subtypes of
AVMs according to their size, location in critical brain regions, and venous drainage
• Stereotactic radiosurgery (SRS) has been widely used to manage SM Grade III AVMs.

17. Radiosurgery
•Radiobiology
•Indications
•Pre treatment Assessment
•Techniques
•Complication
•Future

18. Stereotaxis
• Stereotactic radiotherapy dates back more
than 50 years; however, this form of treatment
has entered the domain of radiation oncology
only in the past 10–15 years
• Stereotaxy (stereo + taxis – Greek, orientation
in space) is a method which defines a point in
the patient’s body by using an external three-
dimensional coordinate system which is rigidly
attached to the patient.
• This results in a highly precise delivery of the
radiation dose to an exactly defined target
(tumor) volume.

19. Radiobiology
• In radiobiology, tissues are divided into 2 broad categories, namely, early- and late-
responding tissues. Early-responding tissues, such as skin, mucosa, and
gastrointestinal epithelium, tend to respond acutely to radiation exposure, whereas
radiation-induced effects are not immediately observed in late-responding tissues,
such as vascular tissue, nerves, brain parenchyma, and spinal cord. Most malignant
tumors behave like early-responding tissue, whereas benign tumors behave like late-
responding tissue.
• Late-responding tissues are more susceptible to a single, high dose of radiation
compared with early-responding tissues, and this factor has to be considered in the
delivery of SRS.

20. Rationale
• The immediate effect of SRS is damage to the endothelial cells of the vessels in the
nidus, probably mediated by release of tissue-specific cytokines.
• This is followed by initiation of a chronic inflammatory process, with formation of
granulation tissue that has fibroblasts and new capillaries.
• Myofibroblasts, which are actin-producing fibroblasts, have been detected in the
region of radiation, and these have been postulated to exert contractile properties
and facilitate AVM obliteration.
• The ensuing radiation-induced vasculopathy results in progressive occlusion of
vessels within the AVM nidus.
• This process takes from 1 to 3 years.

21. Indications
• Successful AVM obliteration with SRS depends on lesion size and radiation dose.
• An overall 80% obliteration rate by 3 years occurs with lesions 3 cm or smaller, while
larger lesions have obliteration rates of 30-70% at 3 years
• However, some amount of lesion volume reduction (mean, 66%) typically occurs in
larger lesions (>3 cm) treated with SRS, and retreatment is effective in about 60% of
patients with residual AVMs.
• A dose response has been demonstrated for radiographic AVM obliteration, with doses
of 16, 18, and 20 Gy associated with obliteration rates of about 70%, 80%, and 90%,
respectively
• Treatment of AVMs with SRS may improve seizure control in patients with comorbid
epilepsy

22. Pre Treatment Evaluation
• Consent
• Look for signs of hemorrhage in brain
• Any allergies for Intravenous contrast
• Renal function tests
• Ophthalmic evaluation
• Pure tone audiometry
• Hormonal analysis in children
• In Young females rule out pregnancy
• Patients with lobar AVMs were placed prophylactically on anticonvulsants for a period
of 2 to 4 weeks around the time of the procedure.

23. Techniques
• Immobilisation
• CT simulation
• Image acquisition and registration
• Contouring
• Beam Placement
• Plan evaluation
• Set up verification
• Treatment

24. Immobilisation
•Frame immobilization
•Frameless immobilisation

25. Frame
• Stereotactic radiotherapy is based on the rigid connection of the stereotactic frame
to the patient during CT, MRI, and angiography imaging
• The stereotactic frame is the base for the fixation of the other stereotactic
elements (localizer and positioner) and for the definition of the origin (point 0) of
the stereotactic coordinates.
• During the whole treatment procedure, from the performance of the stereotactic
imaging to the delivery of the irradiation treatment, the stereotactic frame must not
be removed from the patient.
• In case of relocatable frames it must be assured that the position of the patient is
exactly the same relative to the frame after reapplication of the relocatable frame

26. Different types of Frame systems
• There are different stereotactic frame systems described in detail in the literature:
 the BRW system
 the CRW system
the Leksell system
the BrainLAB system
Each system is different with regard to material of the stereotactic frame, design, and
connection with the localizer and positioner and accuracy of repositioning

33. Simulation
• Patient will be immobilized with either a frame based or frameless stereotactic
method
• CT scanning was done in spiral mode using a pitch of 0.75, 512 × 512 pixel size, and
slices in thickness and spacing of 1.2 mm acquired throughout the entire cranium.
Tube voltage and tube potential were set at 130 kV and 300 mA to obtain high quality
reconstructed slices
• Assessment of images after acquiring is a must.
• If the site of the lesion is supratentorial and close to pituatory flexing of the neck is
recommended
• If infratentorial neutral spine positioning is advised.

36. • In addition, a mouth bite positioned against the upper dentition attached to the
stereotactic frame was applied to prevent any head tilt movement
• If any head tilt the imaging must be repeated
• If there are head tilts in more than 3 times the procedure is abandoned and
remoulding or re fixation of the frame is advised
• A localizer is mounted over the frame in order to provide a three dimensional (3D)
stereotactic coordinate array for target localization.

38. Image registration
• The Ct Angiography , MRI and planning CT datasets
were imported into the planning system and
stereotactic coordinates localization were
performed by the software by identifying the
location of six localizer rods on the outside surfaces
of the right, left, and anterior walls of the localizer
box.
• Localization establishes the 3D stereotactic
coordinate system for treatment planning and
delivery

41. Target Delineation
• Organ at risk( OAR ) need to be contoured first in T1 weighted MRI – CT fused images
• OARS which need to be contoured are
Whole Brain
Bilateral Optic nerve
Optic Chiasma and a 5 mm PRV
Brain stem and PRV
Bilateral Cochlea
Hippocampus
3 mm of skin needs to be contoured

42. Contouring of the AVM
• On Planning CT- Following contrast administration, and especially with CTA with
feeding arteries, draining veins, and intervening nidus visible in the so-called "bag of
worms" appearance.
• DSA-Remains the gold standard to exquisitely delineate the location and number of
feeding vessels and the pattern of drainage.
• On angiography, an AVM appears as a tightly packed mass of enlarged feeding arteries
that supply a central nidus.
• Fusion of Planning CT , CT angiography , MRI and DSA correlation are required for
accurate delineation of a AVM by radiotherapy.

45. Dose Prescription
• The K index—calculated as the prescribed minimum dose of radiation delivered x
(AVM volume)1/3—has been proposed to guide the dose of radiation delivered.
• However, its use may be limited to SRS for small AVMs, with obliteration rates
increasing linearly up to a value of 27
• Various studies have prescribed various doses based on the volume of AVM
• Conventionally Volumes of AVM 12-14cc can be prescribed a SRS doe of 20-24 Gy
single fraction
• Volumes of 14 to 20cc can be prescribed a dose of 15-18 Gy
• Volumes above 20cc it is better to go with Staged Radiosurgery

51. •Usually at the geometrical center of the PTV
•Collimator size is set to encompass most of the target
volume
• Multiple non-coplanar beams (8-12) or 4-5 arcs used
•Limit no of beams/arcs in ANT/POST directions
Planning

55. Isodose prescription
• Dose prescribed to an isodose line (shell) that conforms to the periphery of the
target
• Typically 80% line (sharper dose fall-off outside the target)for LINEAR ACCELERATOR
based SRS with single isocentre
• Multiple isocentres 70% isodose line
• 50% isodose lines for Gamma Knife based SRS systems

58. Dose constraints

59. Results
• MRI has been shown to have a reliability of 97% in documenting AVM obliteration.
• Most authors recommend an yearly follow-up with MRI after SRS, and DSA may be
used to confirm its obliteration once the MRI shows evidence of obliteration.
• MRI also aids in assessing radiation-induced changes in the vicinity of the nidus.
• Obliteration of AVMs after SRS has been reported to range from 35% to 92%, with the
obliteration rate exceeding 70% in most series
• The interval-to-obliteration after SRS could be from 1 to 4 years or even longer
• A minimum of 3- to 4-year follow-up of the AVM is required, before SRS can be
deemed as a failure

60. Prognosis
• Smaller AVM volume
• Higher marginal dose of radiation
• Smaller maximal diameter
• Smaller number of isocenters,
• Radiosurgery-based AVM score <1
• Lower modified Pollock–Flickinger score
• Lower Spetzler–Martin grade,
• Younger age
• Absence of a history of embolization
• Have all been documented to have correlation with higher obliteration rates in series
reporting on SRS for AVMs

61. Adverse effects
• Neurological deficits- 0-17%
• Seizures- 0-9%
• Radiation Induced imaging changes- In MRI upto 30%
• Rebleed or hemorrhage- 60-70%
• Cyst formation- 1.5-3.4%
• Radiation induced neoplasms- 0.64% at 10 years
• Very Rarely cognitive changes

62. Radiosurgeries in different scenarios
• Recurrence-Repeat radiosurgery is an option when patients have persistent or
residual nidus 3 or more years after the initial SRS.
• Large AVMS more than 15cc- Volume staged SRS, Dose Staged SRS both can be tried.
• Children- the risk of bleeding in children is rare and the obliteration rate is very high
in children

63. Summary
• SRS has been proven to be effective in the management of ruptured as well as
unruptured AVMs.
• A higher marginal dose of radiation is the most important factor in predicting AVM
obliteration after SRS.
• Obliteration and complication rates reported in literature suggest that there is no
difference in the efficacy and safety of different delivery systems.
• While it is most effective in the management of small AVMs, treatment paradigms for
larger AVMs include multiple-session SRS or SRT.
• Even after obliteration has been achieved, these patients need a long-term
follow-up to determine the cognitive sequelae, delayed complications, and rarely,
AVM recurrence

64. Questions

65. Thank u