Chapter 21: Stereotactic Radiosurgery
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What is Stereotactic Radiosurgery (SRS)? Pg. 453
Stereotactic radiosurgery (SRS) is a single-fraction radiation therapy procedure for treating intracranial lesions using a combination of a stereotactic apparatus and narrow multiple beams delivered through noncoplanar isocentric arcs. The same procedure when used for delivering multiple dose fractions is called stereotactic radiotherapy (SRT).
What are the three types of radiation used for SRT/SRS? Pg. 453
Currently there are three types of radiation used in SRS and SRT:
- 1.) heavy-charged particles.
- 2.) cobalt-60 gamma ays.
- 3.) megavoltage x-rays.
*Gamma knife is approximately 10 times greater in costs than x-ray knife.
What are two types of SRS techniques? Pg. 453
- 1.) x-ray knife (linac based)
- 2.) gamma-knife.
Briefly describe the generalities of the linac-based SRS techniques? Pg. 453
The linac-based SRS technique consists of using multiple noncoplanar arcs of circular (or dynamically shaped) beams converging on to the machnie isocenter, which is stereotactically placed at the center of imaged target volume.
What are the two basic linac-based SRS systems? Pg. 454
There are basically two linac-based SRS systems:
- 1.) pedestal-mounted frame
- 2.) couch-mounted frame
The frame in this case refers to an apparatus called the stereotactic frame, which is attachable to the patient's skull as well as to the couch or pedestal. This provides a rigidly fixed frame of coordinates for relating the center of imaged target to isocenter of treatment.
What is an essential element of the SRS procedure? Pg. 456
An essential element of the SRS procedure is the alignment of stereotactic frame coordinates with the linac isocenter (point of intersection of the axes of rotation of the gantry, collimator, and couch). Acceptable specification of linac isocentric accuracy requires that the isocenter (mechanical as well as radiation isocenter) remains within a sphere of radius 1.0 mm with any combination of gantry, collimator, and couch rotation.
What is the verification device of the BRW frame system called? Pg. 456
The BRW frame system includes a verification device called the phantom base. It has identical coordinates (anteroposterior, lateral, ad vertical) to those of the BRW frame.
Why should the accuracy of the phantom base used to verify the BRW frame system important? Pg. 456
The accuracy of the phantom base should be carefully maintained because it serves as a reference standard for all other steps in the stereotactic localization process.
How is the overall (geometric) accuracy tested? Pg. 458
This can be accomplished by usimg a suitable head phantom with imageable hidden targets. The test phantom and the targets must be compatible with the imaging modality used. One such test phantom for CT and MRi is commercially available and is shown in Figure 21.6. The phantom contains test objects: a cube, sphere, cone, and cylinder. The top center point of each of these objects is identified in the CT and MRI images and the BRW coordinates are reconstructed by the treatment-planning software. The comparison of these coordinates with the known coordinates of these points in the phantom gives the geometric accuracy. The localization error, LE, is the quantitative description of the system's geometric accuracy.
What is SRS and SRT normally used for? Pg. 458
SRS/SRT is normally used for small lesions requiring much smaller fields than those for conventional radiation therapy.
Knowing that minimizing the penumbra is significantly important, how is that achieved? Pg. 458
This has been achieved, for example, by using 15-cm-long circular cones made of Cerrobend lead, encased in stainless steel. The cones are mounted below the x-ray jaws, which provide a square opening larger than the inside diameter of the cone, but small enough to prevent radiation escape from the sidewalls of the con. A range of cone diameters from 5 to 30 mm is needed for treating SRS lesions. A few cones of larger diameter may also be available for treating larger lesions with SRT.
How is the gamma-knife applied? Pg. 459
The gamma-knife delivers radiation to a target lesion in the brain by simultaneous irradiation with a large number of isocentric gamma-ray beams. In a modern unit, 201 Co-60 sources are housed in a hemispherical shield (central body) and the beams are collimated to focus on a single point at a source to focus distance of 40.3 cm.
What are the differences between gamma- and x-ray knife? Pg. 460
As stated previously, there are no significant clinical differences between gamma knife and x-ray knife treatments. However, the gamma knife can only be used for small lesions because of its field size limitation (maximum diameter 18mm), although several isocenters can be placed within the same target to expand or shape dose distribution. For treating multiple isocenters or targets, the gamma knife is more practical than the x-ray knife because of its simplicity of setup. For the same reasons, the gamma knife can produce a more conformal dose distribution than that possible with the x-ray knife unless the latter is equipped with special field-shaping collimators such as the dynamic MLC. On the other hand, the x-ray knife is far more economical because it is linac based. Besides, the linac can be used for all kinds of radiation therapy techniques including SRS, SRT, intensity modulated radiation therapy, and conventional radiation therapy.
What are the three quantities of interests when dealing with SRS dosimetry? Pg. 460
- 1.) central axis depth distribution (%dd or tissue-maximum ratios [TMRs])
- 2.) cross-beam profiles (off-axis ratios)
- 3.) output factors (Sc,p or doe per monitor unit [MU]).
Why is it hard to measure the three quantities of interests when dealing with SRS dosimetry? Pg. 460
Measurement of those quantities is complicated by two factors:
- 1.) detector size relative to the field dimensions.
- 2.) possible lack of charged particle equilibrium.
In either case, the detector size must be as small as possible compared to the field size.
What are the three different types of detectors used for SRS dosimetry and when are they used? Pg. 460
- 1.) ion chambers.
- 2.) film.
- 3.) TLDs.
The choice of any detector system for SRS dosimetry depends on the quantity to be measured and the measurement conditions.
When dealing with cross-beam profiles in SRS dosimetry, what detector size will render accurate measurements within 1mm? Pg. 461
The effect of detector size on accuracy of beam profiles has been investigated and it has been shown that with a detector size of 3.5 mm diameter, the beam profiles of circular fields in the range of 12.5 to 30.0 mm in diameter can be measured accurately within 1 mm. Because cross-beam profiles involve relative dose measurement (doses are normalized to central axis value) and there is little change in the photon energy spectrum across small fields, diodes and film are the detectors of choice.
What are some requirements for accurate measurements when dealing with depth dose distribution? Pg. 461
Measurement of central axis depth dose in a small field requires that the detector dimensions be small enough so that it lies well within the central uniform area of the beam profile. For field sizes of diameter 12.5 mm or greater, it has been shown that the central axis depth dose can be measured correctly with a parallel plate ionization chamber of diameter not exceeding 3.0 mm. Smaller diameter chambers will be required for smaller field sizes.
When making measurements for output factors, similar requirements for cross-beam profiles and depth dose distributions are needed. What is the most significant factor? Pg. 461
As with the other two properties that are measured, output factor measurements require the detector to be small relative to the field size.
When dealing with dose calculation for SRS, what methods can be used? Pg. 461
Any of the dose calculation methods discussed in Chapters 10 and 19 can be adopted for SRS dose calculations. The approximate spherical geometry of the human head and homogeneity of tissue density greatly simplify the demands on a dose calculation algorithm.
What is the simplest method of beam modeling dependent on? Pg. 462
One of the simplest methods of beam modeling is based on TMRs, OARs, exponential attenuation, output factors, and inverse square law.
- 1.) TMR
- 2.) OAR
- 3.) Exponential attenuation.
- 4.) Output factors.
- 5.) Inverse square law.
What are the two types of QAs used for SRS/SRT? Pg. 462
- 1.) Treatment quality assurance: this involves checking or double-checking of the procedures and treatment parameters pertaining to individual patients.
- 2.) Routine quality assurance: this is designed to periodically inspect the hardware and software performance to ensure compliance with original specifications.
When treating malignant brain tumors with SRT, what are the four important R's regarding radiobiology? Pg. 463
- 1.) Repair.
- 2.) Reoxygenation.
- 3.) Redistribution.
- 4.) Repopulation.
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