Download Frameless Stereotactic Radiosurgery (SRS / X

Frameless Stereotactic Radiosurgery (SRS / X-Knife)
Stereotactic radiosurgery refers to a single treatment with high radiation doses. The
treatment planning margins for the treatment targets are usually 2mm or less to spare
normal healthy tissues. Since high doses are used, a precise positioning mask is
necessary to immobilize the patient. ExacTrac is also used for further monitoring
the intra-fractional patient position. This treatment is not only suitable for malignant
tumors, but also for benign lesions, such as meningiomas, pituitary adenomas and
trigeminal neuralgia.
The above figure shows the patient immobilization method used.
Stereotactic Radiotherapy (SRT)
Stereotactic radiotherapy refers to fractionated frameless radiosurgery with a higher
accumulated total dose compared to SRS. It also has precise patient positioning,
however SRT has the added benefit of sparing more normal tissue than SRS due to the
nature of fractionating the treatment.
Stereotactic Body Radiation Therapy (SBRT)
Stereotactic body radiation therapy (SBRT), also known as stereotactic ablative
radiotherapy (SABR), refers to the precise and focused delivery of a hypofractionated
radiation in the ablative dose range to extracranial targets. It is typically delivered in
1 to 5 fractions; although, in some cases, 10 fractions may be used. SBRT is a
specialized technique used to treat early-stage localized tumors (up to 6-7 cm) and
metastases of the lung, liver, pancreas, spine and prostate.
Four Dimensional Computer Tomography (4DCT) & Real-time Position
Management (RPM)
Our center has recently acquired the Four Dimensional Computer Tomography
(4DCT), which monitors the breathing cycles of patients during the CT scans. With
this technique, the tumor position at each phase of the breathing cycle can be closely
monitored.
The 4DCT can be used in conjunction with the Varian Real-time Position
Management (RPM) system. It allows correlating the tumor position in relation to
the patient’s breathing cycle using an infrared tracking camera and a reflective marker.
The patient’s respiratory pattern and range of motion can be measured and showed in
a waveform.
There are two ways to reduce the treatment margin using 4DCT. Both methods do
not require controlled breathing;
1. Maximum Intensity Projection (MIP)
Maximum intensity projection refers to the projection of voxels with the highest
attenuation value on every slice throughout the volume onto a 2D image. The
internal tumor volume can be generated from the 4DCT using MIP and thus, the
treatment margin can be reduced accordingly compared with conventional CT
scanning.
2. Respiratory Gating
Respiratory Gating refers to a technique that synchronizes the radiation beam with the
patients’ breathing cycles. Oncologists will then determine which phase of the
breathing cycle is optimum for treatment to be given. The treatment machine will
track the patients’ respiratory patterns and automatically turn on the radiation during
the chosen treatment phase.
Respiratory Gating is an advantageous technique that treatment margins can be
reduced. Therefore, healthy tissues can be spared while higher doses of radiation are
delivered to tumors. As a result, a better treatment outcome would be expected.
The above figure shows the respiratory patterns with the selected phase for treatment.
Implementation of advanced image-guided systems
With the goal to improve the accuracy and precision of the treatment, sophisticate
image-guided systems have played an essential role in radiotherapy. Recently, the
integrated BrainLab ExacTrac x-ray 6D system and Varian PerfectPitch couch have
been successfully installed and implemented in our centre as well as our current
image-guided system, Cone Beam Computed Tomography and On-Board Imaging
kilovoltage imaging system.
ExacTrac x-ray six dimensions (6D) imaging system
This enables fast and accurate patient positioning and provides high-resolution x-ray
imaging for intra-fraction verification. It also comes with an integrated optical
infra-red tracking system for continuous monitoring of patient position throughout
treatment.
The above figure shows the integrated optical infra-red tracking system and the ExacTrac
x-ray detector.
PerfectPitch couch
The PerfectPitch couch enables the adjustment of the patient positioning in six
dimensions. With the six dimension of freedom couch, CBCT image-guided target
location and isocenter correction can be done remotely. As a result, treatment
accuracy can be improved, together with a shorter treatment time.
The figures above show two additional axes of couch rotations: roll (left) and pitch (right).
Deformable Registration/Adaptive Radiotherapy with Velocity
Furthermore, Varian Velocity is currently available in our centre. This powerful
software allows deformable registration, which is a method used to integrate different
images – whether it is attained from the same or different imaging modality. It is
able to register the image by locally warping the target image to align with the
reference image. With the aid of deformable registration, the past dose distributions
from different treatment fractions can be added up and calculated as non-rigid dose
accumulation. Oncologists can use this information to modify the original treatment
plan and counteract any changes in body shape or tumor size during a course of
therapy, known as adaptive radiotherapy. This would result in a better treatment
outcome.