High Energy Treatment machines, pt 1
The dose at dₘₐₓ depth in a patient from a single MV portal image acquired on an amorphous silicon EPID is approximately:
5.0 - 8.0 cGy Modern EPIDs can create an image with about 1 - 2 MU, corresponding to a dose in the range of a couple of cGy. MV imaging dose may not be negligible compared to the prescribed per fraction if imaging is performed every fraction.
A linac has a source size of 2.1 mm. The SDD of the unit is 34.5 cm. The physicist measures the penumbra of the unit to be 0.45 cm at a depth of 10 cm. What is the SSD of the unit?
98 cm
The penumbra is the sharpest for a _____ photon beam.
Among the linac-based beams listed, the 6 MV beam will have the sharpest penumbra. As the beam energy increases, the average energy of the secondary electrons increases, increasing their range beyond the edge of the field, thus increasing the beam penumbra.
What technology is used to create an intensity modulated radiotherapy (IMRT) beam in helical tomotherapy?
Binary multileaf collimators are used in helical tomotherapy to modulate the intensity as the beam as the patient passes thru the plane of the beam. A binary multivane collimator is composed of tungsten and the leaves may be completely open or closed - there is no middle ground. Compensators are used for static IMRT.
Why are transmission targets used to produce megavoltage x-rays from linear accelerators?
Bremsstrahlung photons are forward directed at megavoltage energies. At electron energies below 100 keV, x-rays are emitted isotopically. As the energy of the electrons increases, the x-ray emission becomes more forward peaked. This is why transmission-type targets are used in megavoltage x-ray tubes
For an open, flattening filter free beam, at what location within the photon beam is the dose rate the highest?
Central beam axis Photons that are generated in the linac target are primarily forward directed at MV energies. This results in a photon fluence that is a maximum on the beam central axis then decreases laterally. In flattening filter free beams, there is no flattening filter to preferentially attenuate the central axis photon fluence. The resulting photon fluence, and therefore dose rate, is highest on the beam central axis.
Machine output monitor chambers are not used in cobalt 60 units because:
Cobalt 60 has a predictable output
What imaging modality does CyberKnife use for target localization?
CyberKnife uses real-time orthogonal x-ray imaging.
Match the linac section with the component or function: Thyrotron tube
DC power section
MLC positioning is critical in this mode
Dynamic mode (where MLCs are moving while beam is on) because small positioning errors can cause large discrepancies in dose within the field. The most critical parameter for IMRT is the space between opposite MLC leaves, called the gap
Portal dosimetry using EPIDs refers to
EPID images can be utilized for IMRT QA, dose profile measurement, MLC alignment verification and fluence distribution map. Although EPIDs are most commonly used for generating portal images to verify patient position, this is not what is generally referred to as portal dosimetry.
What commonly used component of the photon beam delivery system is NOT essential for linac based IMRT delivery?
Flattening filter An IMRT plan modulates the beam intensity to create a non-uniform beam. Therefore a flattening filter, whose purpose is to create uniformly flat beam intensities, is not needed. Flattening filters are still used in some IMRT plans, but they are not required.
The finite size of Cobalt-60 source gives rise to what is called the ?
Geometric penumbra 1. If the SSD, d, or s increase the geometric penumbra increases 2. If the SDD increases the geometric penumbra decreases
what mechanisms can be used to enlarge an electron beam after exiting the bending magnet?
Given the small size of an electron beam after it exits the bending magnet, a mechanism is required to enlarge the beam. - single foil system - Dual foil system - Scanning electromagnet system (electron beam contains much less photon contamination due to the absence of the scattering foil. This scanning electromagnet system results in a sharper fall-off between the 90% and 20% depth dose.)
what must be changed for different energy electron beams on a linac?
Higher-energy electron beams require higher magnetic fields and, therefore, higher magnet current. Electron beams do not have targets or flattening filters.
All of the following are expected during flattening filter free (FFF) mode
In a FFF beam, the dose rate is increased, the x-ray spectrum is softened, dₘₐₓ is shifted to the surface, there is decreased scatter and the beam is not flat.
MLCs are made of _____.
MLCs are made of tungsten. Cerrobend is composed of 50% bismuth, 26.7% lead, 13.3% tin, and 10.0% cadmium.
microwave energy necessary to accelerate electrons in a linac comes from two devices
Magnetron tube (microwave generator used with energies of less than or equal to 6 MV; Elekta) Klystron tube (microwave amplifier; Varian)
A monitor chamber
Measures actual dose delivered to pt. Mandatory as linacs unable to deliver constant dose/time due to pulsed nature of unit. Consists of 2 parallel plates with mult outputs. Made of mica or kapton. 2 independent measurements taken at any given time. Has termination circuit in case both chambers fail.
In any finite radiation beam, there is a rapid fall-off of dose near the edges of the radiation field called
Physical penumbra; there are four parts - transmission penumbra (fixed by making collimator edge match beam divergence - side-scatter (more pronounced for kilovoltage units, less so for MV units) - Lateral electron disequilibrium (seen with higher energy beam than lower energy. e.g 25 MV electron have higher range of distance travelled laterally than 6 MV, and also have higher probability of pair production which contributes to the lateral scatter) - Geometric penumbra
The beam-on time calculation for a Gamma Knife treatment must account for a _____% per month decrease in dose rate.
The Gamma Knife uses ⁶⁰Co sources. ⁶⁰Co has a half-life of 5.26 years. This corresponds to a decay rate of approximately 1% per month.
The primary advantage of protons over x-rays for therapy is _____.
The absence of exit dose allows proton beams to spare organs at risk downstream.
The current for a 6 MeV electron beam is _____ than the current for a 6 MV photon beam.
The accelerated beam current is much lower in electron mode, as there is no loss in efficiency as seen in the photon mode due to the presence of the target and flattening filter. the typical peak beam current in a linac is roughly 600 mA for 6 MV x-rays and 150 mA for high-energy electrons
What is the fundamental characteristic of a sliding-window IMRT?
The beam is on while the MLCs are moving in a sliding-window or dynamic MLC IMRT. The physical compensator is slid into position is a valid IMRT technique, it is not characteristic of a sliding-window IMRT.
Kilovoltage x-ray machines usually have filters to reduce the dose resulting from very low energy x-rays. MV linacs, however, do not need filters primarily because of what reason?
The flattening filter effectively acts as the low energy x-ray filter
On a linac with a normal dose rate of 300 MU/min, a treatment requires a monitor unit setting of 150 MU. This means that:
The monitor chamber collects charge as the beam passes through it and terminates that beam after a charge equivalent to 150 MU has been collected
Select the path of photon travel after creation in the target to the patient surface:
The photon path after being created in the target is: primary collimator ⇒ flattening filter ⇒ monitor chambers ⇒ jaws.
What is the main physical mechanism by which photons are produced in a Gamma Knife?
The photons are produced during the de-excitation of a ⁶⁰Ni nucleus that is the result of radioactive beta decay of a ⁶⁰Co nucleus.
What is the best reason for removing a flattening filter inside a linear accelerator head for certain delivery modes?
The reason to remove the flattening filter is to obtain high-dose-rate beams, and thus, shorter delivery times especially for the treatment of SBRT and SRS cases. The flattening filter is not applicable to electron beams.
The principle reason that the penumbra of a 4 MV linac is better than the penumbra of a cobalt 60 teletherapy unit is because:
The source size is smaller It's all about the source size. For cobalt, it's a 1-2 cm, for linac, 1-2 mm.
For the same accelerating potential, a flattening filter free (FFF) beam has a(n) _____ dose rate and a(n) _____ average energy compared to a beam with a flattening filter.
When the flattening filter is removed, there is less attenuation and, therefore, a higher dose rate. However the beam is less hardened, meaning that the average energy is lower.
What is the approximate relative dose in the beam due to x-ray contamination when from an electron scattering foil?
X-ray contamination from electron scattering foils can be up to 5% for 20 MeV beams.
The sharpest beam edge penumbra for a linac MV x-ray beam is achieved when the field edge is defined by:
a custom designed lead or metal alloy field block because they are both closer to the patient and are molded so that the edges of the block perfectly match geometric beam divergence. They therefore produce the sharpest geometric penumbras and therefore the sharpest field edges.
Pencil beam scanning protons have _____ than double scattering protons
a lower entrance dose Typically pencil beam scanning has lower entrance dose since the plan can be inverse planned for proximal dose. Double scattering beams are optimized for distal dose. RBE is about the same as double scattering (1.1) and, typically, the penumbra is less sharp than double scattering because of the absence of an aperture near the patient. Apertures are slowly being introduced into PBS
Which of the following is indicated by the term passive scattering in proton beam therapy?
a series of scattering foils in the beam line sequentially flatten and widen the beam Passive scattering proton machines were by far the most commonly available type of proton machines prior to the recent increase of proton machines with active scattering over the last 20 years.
Cyclotron
a type of particle accelerator, that accelerates charged particles along a spiral path for heavy particle radiotherapy e.g used to produce protons, neutrons (from proton bombardment) energy range (60-250 MeV for protons) and (120-400 MeV/u for carbon ions). can only produced continuous radiation, not pulsed like linacs
There are three types of leakage through MLC leaves
a. Midleaf leakage (1.5 - 3%) b. Interleaf leakage (2.0 - 5.0%) c. Intraleaf leakage -random MLC leaf position errors up to 2 mm, no significant errors in dose -systematic MLC leaf position errors of 1 mm, significant errors in dose occur
Modern LINAC structures can operate in one of three microwave frequency regions
a. S-band (2-4 GHz) b. C-band (4-8 GHz) c. X-band (8-12 GHz) - mostly for robotic/intraop units most common linacs operate at 3 GHz, in the S-band corresponding to a wavelength of 10 cm
the removal of the flattening filter causes the following changes to the photon beam:
a. increase the dose rate (roughly 2-6 times higher) b. X-ray spectrum is softened c. Shift of Dmax towards surface d. non-flat beam profile e. decreased scattered radiation from linac head (flattening filter is the largest source of scatter)
The average leakage dose from the MLC relative to the primary dose is approximately _____%.
1 - 2 There is more leakage through MLCs than through LINAC X-Y jaws, which typically transmit only 0.1% of the primary beam. Also, MLC leakage dose is different through leaf tongues, grooves, and central portion.
A klystron serves to:
add power to microwaves. A klystron takes as input low-power microwaves and amplifies them before directing them through a waveguide and into the accelerator.
MLC leaf transmission on modern linear accelerators is approximately _____
1 - 2% The amount of MLC leaf transmission depends on the material composition of the MLC and the thickness of the leaf. MLC leakage can be minimized by having the jaw track along with the MLC.
three methods commonly used to change the energy in a linac
1. adjusting the microwave power 2. adjusting the electron current going through the accelerating structure 3. using energy switch to short out the last half of the accelerating structure to reduce the amount of acceleration that the electrons experience
IMRT can create highly conformal plans, sparing organs at risk. However, potential problems associated with IMRT include all of the following
1. steep dose gradients at the edges of PTVs or OARs, making setup accuracy more critical 2. the potential for the optimization algorithim to place hot spots outside the PTV 3. higher MUs, increasing leakage dose outside the treatment field
A ⁶⁰Co teletherapy unit emits photons with _____ energy/energies in MeV.
1.17 and 1.33
The depth of dₘₐₓ for a 6 MV photon beam is approximately:
1.5 cm dₘₐₓ = 0.46 x MeV^0.67
A typical Co-60 source for teletherapy contains approximately the following level of activity:
10,000 Ci Cobalt-60 pellets, cylinders, and/or discs are stored within a 1- to 2-cm sealed steel container containing approximately 10,000 Ci of activity.
The proton beam RBE at the end of the spread out Bragg Peak is _____ the RBE at the center of the spread out Bragg Peak.
always higher than End-of-range effects may be responsible for higher RBE effects at the end of the spread out Bragg Peak. Some proton centers attempt to mitigate these effects by not having all of the beams stop on the same critical structure.
Intraoperative radiation can be delivered using the following modalities:
an interstitial x-ray tube, remote afterloader, high-energy electron beam from a linac or an external x-ray tube.
tomotherapy units
are a cross between a linac and a CT unit - Tomotherapy treats via the helical delivery of IMRT. no flattening filter used pitch of 0.86 is best for beam matching, keeps dose variation to less than 3% Max field size on Halcyon VMAT is 28 x 28 cm - 6 MV FFF, no collimators and have 2 MLC leaf banks with one bank shifted by 0.5 cm relative to the other bank.
For the same nominal 6 MV x-ray beam energy, which of the following beams has the most penetrating depth dose? - beam with flattening filter plus dynamic wedge - beam with flattening filter plus hard wedge - beam with a flattening filter but no wedge - FFF beam plus hard wedge
beam with flattening filter plus hard wedge Both the flattening filter and the hard wedge are thick, high Z metals which selectively attenuate low energy x-rays more than high-energy x-rays, resulting in a higher average beam energy.
Electron cone
because electrons are easily scattered in air, close collimation with cone near the patient's surface is necessary to accurately define the radiation field
Synchrotons
called synchrotons due to the synchronization of energy and RF frequency. consist of circular ring of magnets that force the charged particles to move in a fixed circular path called the accelerator ring produced pulsed radiation
A treatment unit that operates at 100 kV _____.
can be used to treat non-melanoma skin cancers At 100 kV the unit is a superficial unit.
The MOST important quality assurance test of a LINAC kilovoltage (kV) cone beam CT (CBCT) system used for IGRT is:
coincidence between the MV and kV imaging isocenters. If the MV and kV isocenters are not coincident the patient may be positioned incorrectly without this being apparent.
In commercial amorphous-silicon flat panel imaging systems, which of the following is the primary means of detection in the pixilated array layer?
conversion of visible light into electrical charge X-rays are converted into electrons, usually in a thin metal plate. The electrons create light in a phosphor or scintillator, and the light produces electrical charge in the amorphous silicon panel array.
In amorphous silicon electronic portal imaging devices (EPIDs), the main purpose of the combination metal/phosphor screen is to:
convert incident x-rays into visible light X-rays produce electrons in the metal/copper plate, and the electrons produce light in the phosphor screen.
EPIDs (Electronic portal imaging devices)
create a digital image of the exit beam after it has gone through the patient. In fluorescent screen portal imaging, the exit beam strikes a screen that contains a fluorescent material (Gd2O2S). The image from the screen is transferred via a video camera, displayed on a display screen, and subsequently stored on a computer.
DRR
digitally reconstructed radiograph portal imaging via EPIDs, when coupled with digital radiography taken during CT simulation, provides a rapid powerful method of treatment verification
In contrast to photons, the flatness of an electron beam is defined at
dₘₐₓ, compared to at 10 cm in water for photon since electrons aren't as penetrative as photons
Betatrons
electrons are accelerated to full speed after several thousand revolutions and are deflected out of the doughnut, where they strike a target to produce photons, or used directly for electrons *It's placed in an alternating magnetic field *has low-dose rate *small field sizes
Neutron contamination is greatest in a(n) _____ beam
high beam energy High-energy x-ray beams are contaminated with neutrons. The neutron contamination increases rapidly as the energy of the beam is increased from 10 to 20 MV, and then remains approximately constant above this.
What properties of tungsten make it a useful target material in MV linacs?
high melting point and high atomic number Efficiency of x-ray production increases with increasing atomic number. X-ray production also produces a significant amount of heat, which might damage the target. The melting point of tungsten is 3,422°C and its atomic number is 74.
If a flattening filter designed for 15 MV x-rays is accidently used with a 6 MV beam, the horns at dₘₐₓ will be too _____ and the dose rate per pulse will be too _____
high, low As the x-ray energy beam increases, its fluence distribution will be more forward peaked. Therefore, higher-energy beams require thicker flattening filters in order to sufficiently flatten the beam. This means that the dose rate per pulse along the central axis will decrease if a low energy beam is using a higher energy beam's flattening filter. Further, due to too much attenuation along the central axis, the magnitude of the horns at dₘₐₓ will increase.
The skin dose from a range-modulated proton field as compared to single MV photon field is:
higher Typical entrance dose of range-modulated protons is ~ 70% to 90% compared to ~ 60% or less for photons.
The advantage of an electronic portal imager over a diode array for IMRT QA is _____.
increased resolution The main advantage of using an electronic portal imager (EPID) for IMRT QA is its higher resolution. Either tool can utilize dose difference and distance-to~agreement measurements. Both the EPID and diode array are capable of performing absolute dosimetry, and even if not, the ability to perform relative dosimetry wouldn't necessarily constitute an advantage.
As the field size increases the PDD _____, and as the energy increases the PDD _____.
increases; increases
A magnetron in a megavoltage linear accelerator is used to:
is a microwave power source used in megavoltage linear accelerator, radar, and microwave ovens.
Compared to static-field IMRT, VMAT _____.
is a rotational therapy technique Its main advantage is that the treatment time will be shorter, resulting in a decreased opportunity for intrafactional motion. The dose rate is typically less than or equal to that of static-field IMRT.
Spread out Bragg Peak
is defined by its modulation width, measured as the width between the distal and proximal 90% dose values relative to the maximum dose
Grid in a linac
is placed between the anode and cathode to regulate the quantity of electrons at any given time depending on the voltage that is switched to on the grid, range from zero (-150 V) to the max (~ +180 V)
What's the tongue and groove effect during IMRT treatments
it's caused when the tongue of one MLC leaf extends beyond the groove of adjacent leaf; this result in a thin light band (an underdose of approx 15%) For this reason, it's always good practice to verify the fields prior to patient treatment.
The maximum photon energy in an x-ray beam is determined by the:
kV across the tube Increasing the current (amps) will ↑ the number of photons produced and ↑ heat. An increase in the voltage will ↑ x-ray energy and penetration.
Rounded leaf ends are a feature of most MLCs because they _____ .
keep the geometric penumbra relatively constant for different leaf positions Having a circular, rather than straight, MLC end makes the geometric penumbra relatively constant at different leaf positions, but there will be a greater leaf end transmission when leaves are abutted.
Compared to an electron, a proton has a _____ mass and deposits a _____ percentage of its energy at the end of its range.
larger; larger Protons have a mass almost 2000 times the mass of an electron and, therefore, travel in nearly a straight line as they ionize, whereas electrons travel in a more tortuous route. The result is that protons deliver most of their energy to matter at the end of their range, the Bragg peak, whereas electrons do not.
The advantage of scanned electron beam over scattering foil include
lower Bremsstrahlung and sharper falloff between 90% and 20% depth dose
The Bragg peak for a proton therapy beam is typically "spread out" by passing the proton beam through an energy or range modulator. Spreading out of the Bragg peak also results in:
lower average energy dose rate and increased skin dose While the Bragg peak dose is decreased by spreading it out over depth, the skin dose remains approximately the same, the ratio of skin dose to Bragg dose increases.
At typical proton energies (~160 MeV),what percent of protons undergo non-elastic nuclear reactions with the nucleus of the foil material
20% And it causes the proton energy to be drastically reduced and secondary protons, neutrons, and nuclear fragments are produced there's about 1 mSv/cGy of neutron dose in passive foil proton beams Active scanning can greatly lower the neutron dose compared to passive foil system but are more sensitive to motion
the purpose of the primary collimator is to
prevent unnecessary radiation from escaping from the head of the unit After beam has been monitored, the beam size emerges as a 40 cm circle due to the primary collimator
A monitor unit is:
proportional to the dose delivered at a reference point A monitor unit (MU) represents the amount of charge collected by the monitor ionization chamber (located in the path of the beam, in the head), when a specific dose has been delivered at a reference point in the phantom, The beam terminates after the set number of MUs have been delivered. As the dose rate on a linac is not absolutely constant, dose cannot be monitored by time alone.
Which of the following is true regarding proton RBE?
proton RBE is higher in the distal portion of the Bragg peak Most proton treatment planning assumes a uniform RBE of 1.1, even thought it is known to be incorrect. Proton RBE varies with energy (i.e., depth) and is probably higher than 1.1 in the distal region of the Bragg peak.
Monoenergetic proton and electron therapy beams both have a finite range in tissue, but the Bragg peak for protons is sharper (i.e., narrower) than for electrons because:
protons are heavier than electrons and therefore scatter less The heavier the particle, the less "beam straggling" due to scattering and, therefore, the less range uncertainty.
Gantries for proton therapy accelerators are much larger than for electron linacs because:
protons are much more massive than electrons. The magnitude of the deflecting force exerted on a charged particle traveling perpendicular to a magnetic field is qvB, where q is the charge, v is the speed and B is the strength of the magnetic field. These quantities are all roughly the same for protons and electrons in treatment machine gantries (except that the charge is the opposite sign). The mass of a proton however is 2000 times larger than an electron and therefore it follows a curved trajectory that is much larger.
_____ is MOST important in the construction of a water-equivalent phantom for use in a MV photon beam.
same electron density The dominant mode of interaction of MV beams in water is Compton scattering, which is related to the electron density of the material.
The output of an x-ray machine is approximately proportional to the:
square of tube voltage at a given tube current
The main advantage of modern linac based EPIDs, compared to conventional portal images taken with radiographic film, are:
the images are in digital format and thus instantly viewable, can be digitally enhanced, are easily archived to a PACS, require no film processing, and do not require the radiation therapist to enter the treatment room.
Bending magnets are required in linacs for energies greater than 6 MeV because:
the length of the accelerator cavity is too long to be mounted in the direction of the beam axis The length of the accelerate waveguide increases as the photon energy increases. When it becomes greater than about 30 cm, it is mounted horizontally, and the electron beam is turned through 90° or 270° to point down at the isocenter. Linacs with dual photon energies and electron beams also require bending magnets for energy selection.
Tertiary Multileaf collimation
typical transmission factors have been found to be about 1.5 - 3% In static MLC, the average transmission is 2% compared to alloy blocks, which is 3.5%
Secondary Collimator
used to set field size, these are adjustable collimators that are designed to reduce transmission of the beam to less than 2% of the central axis dose - in other words, they define the field size.
what's the size of an accelerated electrons traveling along the accelerator structure in a linc
~ 2 mm
Microtrons
• Microtron combines the principle of linear accelerator and cyclotron. main adv includes ability to produce electrons for multiple tx rooms, smaller energy spread of the electron beam, smaller size of unit; simple method to select beam energy The main disadv is cost and reliability
The highest measured bremsstrahlung dose for a 6-MeV electron beam is located at the:
3-cm deep, central axis. Bremsstrahlung is dependent on high-Z material irradiated by the electron beam. The scattering foils are thickest at the central axis. Five percent maximum is an industry standard.
What is the geometric penumbra at dₘₐₓ for a ⁶⁰Co unit with an SSD of 80 cm and a SDD of 28 cm, if the source size is 1.8 cm.
3.4 cm
What thickness of lead is required to shield a 9-MeV electron beam?
4.5 mm This requires knowledge of the practical range of electrons in lead, the simple rule of thumb is that the practical range of an electron is ~E(MeV)/2 cm. However, since the density of lead is about 10 times greater than that in tissue, the practical range of an electron in lead is approximately a factor of 10 lower: ~E(MeV)/20 cm.
The length of the accelerating structure is _____ cm for a 6 MV unit and _____ cm for a 18 MV unit.
40; 150 The length of the accelerating structure is 40 cm for a 6 MV unit and 150 cm for a 18 MV unit. One can cause large reductions in the energy by shorting out the last half of the accelerating structure to reduce the amount of acceleration that the electrons experience when they travel down the linear tube.
