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Linear accelerator based radiosurgery uses a modified
Brown-Roberts-Wells (BRW) head ring for stereotactic localization of
the lesion. CT, MRI and Angiograms are the imaging modalities used for
treatment planning. A special circular collimator assembly which houses
treatment collimators ranging in diameter from 1.25 cm to 4.00 cm is
attached to linear accelerator for treatment.
Arterio-venous Malformations, Meningiomas, Acoustic
Schwannomas and solitary metastatic tumors are the commonest clinical
cases suitable for radiosurgery.
The procedure starts with the fixation
of BRW headring onto the cranium of the patient using carbon posts and
screws. Stereotactic localisation is achieved by fitting the localiser
frames over the headring exclusively designed for each imaging modality
such as CT,MRI and Angiogram.
CT is the primary image set used for treatment planning
because of its high geometric accuracy. The parameters required
before image acquisition like FOV, Slice thickness and magnification
are carefully noted. The CT scan covers the top of the scalp for better
dosimetric calculations. Two mm slices are taken at the lesion level
and 5mm slice spacing is used for the rest of the skull. 2mm slice
spacing allows for better visualization and contouring of the target
volume. The CT images are transported to the dedicated treatment
planning system via network. MRI and Angiographs are introduced via
Magneto Optical Disk and Film scanner respectively. The CT and MRI
images
reformatted before introduction into the planning module.
The external volume of the patient is reconstructed first by drawing
the outline in all CT slices. Critical structures such as Eyes, Optic
nerves, Optic Chiasm and Brain stem are contoured in CT slices and a 3D
reconstruction enables one to view the complete anatomy from any
angle.
The primary lesion is outlined in the CT slices by the
neurosurgeon. By positioning a suitable collimator size onthe lesion,
the target X,Y,Z coordinates referenced to the head ring are
determined. Based on the size, shape and proximity of the lesion to the
neighboring critical structures, the collimator size and the number of
arcs are chosen. Multiple, non-coplanar arcs are positioned at suitable
angles to avoid overlapping beams. By adjusting the beam parameters
such as arc angle, beam weights, couch angle and collimator size, an
optimal plan can be accomplished. The best plan is selected on the
basis of isodose distributions in the lesion and the dose-volume
histograms (DVH) of lesion and proximal critical structures. . Larger
lesions are treated by judiciously positioning multiple collimators in
the lesion.
Pre-treatment QA checks are carried out to ensure accurate
treatment delivery to the target volume planned. A 3 shot film
test is taken after setting the target coordinates in two gadgets
namely Laser Target Localizer Frame (LTLF) and Rectilinear Phantom
Pointer (RLPP). The positions of radio opaque ball within 1.25 cm
collimator field at 3 gantry angles (0deg,90deg and 270deg) gives the
information of the congruence of laser beams to the isocenter. Film
check tolerance of 1 mm is strictly adhered to for accurate dose
delivery. In addition CT&MRI calibration and absolute dose
verifications are done using Radionics Skull phantom and home-made
Perspex phantoms.
Monitor unit calculations are done manually to verify the
computer generated results. Careful simulation of non-coplanar arcs is
carried out to make sure that there is no collision of couch with
gantry. Prior to treatment Depth helmet readings(fig.5) are taken to
ensure the immobilization of the skull is perfect by comparing the
readings with the ones taken during CT scan. During the treatment the
patient setup is continuously observed via the monitor for smooth
treatment delivery.
Stereotactic Radiosurgery is a non-invasive procedure well
suited for many intracranial lesions which are not manageable by
conventional neurosurgical treatments . The results of treatment
observed over 20 years in many centers have shown good efficacy, low
incidence of treatment complications and no treatment related mortality
for small lesions. Apollo Speciality hospital, Chennai, India started
radiosurgery in 1995. So far 330 patients have been treated with
radiosurgery and 90 patients with stereotactic radiotherapy. The
patients have been followed up every year for assessing the clinical
outcome after radiosurgery. The results of the follow-up done so far
are encouraging and we continue to acquire more data for effective
analysis.
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