Chapter 11 Kahn: The Physics of Radiotherapy
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When are single photon beams used in radiation treatment?
Only in cases when the tumor is superficial.
Once aware that the tumor is superficial, what are the three conditions in which treating that tumor is acceptable?
- 1.) The dose distribution within the tumor is reasonably uniform (plus or minus 5%).
- 2.) No tissues in the path of the beam is not excessive (e.g. 110% of the prescribed dose.)
- 3.) Healthy tissue, or normal critical structures, not receive a dose above the tolerance threshold.
What two conditions render the use of a single megavoltage beam?
- 1.) Using a combination of beams is technically difficult.
- 2.) When a combination of beams results in unnecessary or excessive irradiation to normal tissues.
Since single beams are mainly used when dealing with superficial tumors, what beam quality is used?
Superficial x-ray beam.
When dealing with most tumors, why is it beneficial to use more than one radiation beam to treat the tumors?
Using combination beams renders optimal dose distributions within target and surrounding normal tissue.
What are the advantages of using parallel opposed radiation beams?
- 1.) Simplicity and reproducibility of setup.
- 2.) Homogeneous dose distribution to tumor.
- 3.) Reduction in probability of geometric miss when compared to angled beams.
What are the disadvantages of using parallel opposed radiation beams?
The one disadvantage is the delivery of excessive dose to normal and healthy tissue adjacent to the tumor and in the beam's path.
How can a composite isodose distribution of two parallel opposed radiation beams be acquired?
It can be acquired by simply adding the depth dose contribution of each field together, and then adding the points of equivalent dose together.
Prior to acquiring the composite isodose distribution of parallel opposed beams, how are the isodose distribution for each beam normalized for SSD and SAD (isocenter)?
For SSD setup, the dose is normalized at the Dmax while for SAD (isocenter) it is normalized at the isocenter. Example is page 186.
Since homogeneous dose, or uniform dose distribution, is an advantage of parallel opposed fields, it still has its circumstances. What is the uniformity of dose distribution dependent on in treatment?
- 1.) Patient thickness.
- 2.) Beam energy (beam quality).
- 3.) Beam flatness.
When dealing with parallel opposed beams and the study of a homogeneous dose distribution, what are two causes of an increase of the central axis maximum dose near the surface relative to the midpoint (midpoint is position of normalization)?
- 1.) An increase in patient thickness.
- 2.) A decrease in beam energy.
What is the "tissue lateral effect" when dealing with parallel opposed beams and homogeneous dose distribution?
It is when the patient thickness increases or when the radiation beam energy decreases, hence causing an increase in the central axis maximum dose near the surface with respect to the midpoint.
When treating with multiple beams, the question arises whether one should treat one field per day or all fields per day. For parallel opposed beams, what causes greater biological damage to normal subcutaneous tissue?
It has been shown that treating with one field per day produces greater biological damage rather than two fields per day, despite the fact that the total dose is the same.
With parallel opposed beams, in addition to treating one field per day causing greater biological damage hence treating with both fields per day, what else complicates this treatment?
Khan's explanation confusing: "Apparently, the biological effect in the normal tissue is greater if it receives alternating high- and low-dose fractions compared with the equal but medium-size dose fractions resulting from treating both fields daily. This is called the "the edge effect" or "the tissue lateral damage".
What is a way to compare composite parallel dose distributions for different-quality beams?
Calculate the "integral dose".
What is the "integral dose"?
The integral dose is a measure of the total energy absorbed in the treated volume.
When does the integral increase or decrease when the beam energy is changed?
When the beam energy is increased, the integral dose decreases, and when the beam energy decreases, the integral dose increases.
What is a disadvantage, or negative result, if the integral dose increases?
If the integral dose increases, the probability of damage to normal tissue also increases. Despite the helpful information provided by the integral dose, it is seldom used.
Some useful strategies in delivering the maximum dose to the tumor in a uniform fashion and minimizing the dose to adjunct healthy tissue is...
- a.) using fields of appropriate size.
- b.) increasing the number of fields or portals.
- c.) selecting appropriate beam directions.
- d.) adjusting beam weights (dose contribution from individual fields).
- e.) using appropriate beam energy.
- f.) using beam modifiers such as wedge filters and compensations.
A significant disadvantage of parallel opposed beams is a significant dose to surrounding normal tissue. How can you solve this problem?
You can solve this problem by using three or more fields. Thus, by using multiple fields, the ratio of the tumor dose to the normal tissue dose is increased.
Despite the advantage of using multiple fields to reduce the dose to surrounding healthy tissue, there are still some technical and clinical limitations. What are they?
Some examples are, certain beam angles are prohibited because of the presence of critical organs in those directions. Also, the setup accuracy of a treatment may be better with parallel opposed than with the multiple angled beam arrangement. It is, therefore, important to realize that the acceptability of a treatment plan depends not only on the dose distribution on paper, but also on the practical feasibility, setup accuracy, and reproducibility of the treatment technique.
What is an isodose chart?
An isodose chart for a given beam consists of a family of isodose curves usually drawn at equal increments of percent depth dose, representing the variation in dose as a function of depth and transverse distance from the central axis.
Where is the point of normalization within an SSD and SAD isodose chart?
For the SSD chart, the point of normalization is either at the point of maximum dose on the central axis or at a fixed distance along the central axis in the irradiated medium. For the SAD chart, the point of normalization is at the axis of rotation and is typically beyond the maximum dose point.
What are four general properties of photon dose distributions?
- 1.) The dose at any depth is greatest on the central axis of the beam and gradually decreases toward the edges of the beam, with the exception of some linac x-ray beams, which exhibit areas of high dose or "horns" near the surface in the periphery of the field.
- 2.) Near the edges of the beam (the penumbra region), the dose rate decreases rapidly as a function of the lateral distance from the beam axis.
- 3.) Near the beam edge, falloff of the beam is caused not only by the geometric penumbra, but also by the reduced side scatter. Therefore, the geometric penumbra is not the best measure, rather it is the physical penumbra.
- 4.) Outsides the geometric limits of the beam and the penumbra, the dose variation is the result of side scatter from the field and both leakage and scatter from the collimator system.
What is "physical penumbra"?
Physical penumbra is defined as the lateral distance between two specified isodose curves at a specified depth (e.g., lateral distance between 90% and 20% isodose lines at the depth of Dmax.
What is a "beam profile"?
The beam profile is a plot where it shows the dose variation across the field at a specified depth.
How is the field size defined with respect to isodose charts?
The field size is defined as the lateral distance between the 50% isodose lines at a reference depth. This definition is practically achieved by a procedure called the "beam alignment" in which the field-defining light is made to coincide with the 50% isodose lines of the radiation beam projected on a plane perpendicular to the beam axis and at the standard SSD or source to axis distance (SAD).
What tools can be used to measure isodose charts?
- 1.) ion chambers.
- 2.) solid-state detectors.
- 3.) radiographic films.
Which method of measuring isodose charts is most reliable and why?
Ion chambers because of its relatively flat energy response and precision.
What is the medium (phantom) of choice for ionometric measurements for isodose charts?
Water. The ion chamber can be made waterproof by a thin plastic sleeve that covers the chamber as well as the portion of the cable immersed in the water.
What are a few important points regarding the measurements of isodose charts using ionization chambers?
- 1.) The ion chamber used for iosodse measurements should be sufficiently small to render it capable of making measurements in regions of high-dose gradients, such as near the edges of the beam.
- 2.) It is recommended that the sensitive volume of the chamber be less than 15 mm long and have an inside diameter of 5 mm or less.
- 3.) Because the photon beam spectrum changes with position in the phantom owing to scatter, the energy response of the chamber should be as flat as possible. This can be checked by obtaining the exposure calibration of the chamber for orthovoltage and Co-60 beams. A variation of approximately 5% in response throughout this energy range is acceptable.
What is the general method of measuring isodose curves?
There are two ion chambers, a probe and a monitor, and they're both automatically guided. The probe measures the dose rate at different points within the water phantom and the monitor measures the dose rate at a fixed point. The ratio of the probe to monitor is the percentage of the point (probe) to reference point (monitor) hence giving percent depth dose. The ratio renders the percentage independent of the linacs output.
Should isodose charts from companies be trusted?
No, you should independently measure the isodose charts to verify its validity. A deviation of 2% or less in local dose is acceptable up to depths of 20 cm.
What are the parameters that affect the single-beam isodose distribution?
- 1.) beam quality.
- 2.) source size (s).
- 3.) beam collimation.
- 4.) field size.
- 5.) SSD.
- 6.) SDD (source to diaphragm distance).
What causes greater lateral scatter which causes isodose curves outside the field to bulge out?
Lower-energy beams. In other words, the absorbed dose in the medium outside the primary beam is greater for low-energy beams than for those of higher energy.
Is the physical penumbra dependent on beam quality?
We know that the shape of the isodose curve, on the lateral ends, are affected by s, SSD, and SDD. What is the name of the property affected by those three parameters which in result affects isodose curves?
The geometric penumbra. In addition, the SSD affects the percent depth dose and therefore the depth of the isodose curves.
What does the term "collimation" actually refer to (definition)?
The term collimation is used here to designate not only the collimator blocks that give shape and size to the beam, but also the flattening filter and other absorbers or scatterers in the beam between the target? (it could be source) and the patient.
What beam quality requires the use of the flattening filter?
Megavoltage x-ray beams. The flattening filter, has the greatest influence in determining the shape of the isodose curve.
What is the function of the "flattening filter"?
The function of the flattening filter is to make the beam intensity distribution relatively uniform across the field. Therefore, the filter is thickest in the middle and tapers off toward the edges.
With a flattening filter, where is the greater average beam energy? Is it near the periphery of it or in the center?
In general, the average energy of the beam is somewhat lower for the peripheral areas compared with the central part of the beam. This change in quality across the beam causes the flatness to change with depth. However, the change in flatness with depth is caused by not only the selective hardening of the beam across the field, but also the changes in the distribution of radiation scatter as the depth increases.
Is the field size one of the most important parameters in treatment planning, specifically dealing with isodose curves?
Yes. Adequate dosimetric coverage of the tumor requires a determination of appropriate field size.This determination must always be made dosimetrically rather than geometrically. In other words, a certain isodose curve enclosing the treatment volume should be the guide in choosing a field size rather than the geometric dimensions of the field.
What is the smallest field size that requires caution when doing treatment planning?
6 cm is when great caution should be exercised since fields equal to that or smaller have a large percentage of the field lying within the penumbra region.
What special filter (absorber) is placed in the path of a beam to modify its isodose distribution by causing a progressive decrease in the intensity across the beam, resulting a tilt of the isodose curves from their normal positions?
A wedge filter. The isodose curves are tilted toward the thin end, and the degree of tilt depends on the slope of the wedge filter.
Are wedge filters, as shown in most figures, perfectly sloped?
No. In actual wedge filter design, the sloping surface is made either straight or sigmoid in shape; the latter design is used to produce straighter isodose curves.
What design of wedge filters produce straighter, tilted isodose curves?
The sigmoid shaped wedge filter.
What are wedge filters (absorbers) usually made out of?
The wedge is usually made of a dense material, such as lead or steel, and is mounted on a transparent plastic tray, which can be inserted in the beam at a specified distance from the source.
What is the minimum distance from the skin surface is the wedge filter placed and why?
The distance is arranged such that the wedge tray is always at a distance of at least 15 cm from the skin surface, so as to avoid destroying the skin-sparing effect of the megavoltage beam.
What is the wedge isodose angle ( or simply wedge angle)?
The wedge angle refers to the "angle through which an isodose curve is titled at the central ray of a beam at a specified depth".
Important notes regarding the "wedge isodose angle".
There is no general agreement as to the choice of reference depth. Some choose depth as a function of field size, while others define wedge angle as the angle between the 50% isodose curve and the normal to the central axis. The latter choice, however, becomes impractical when higher-energy beams are used. For example, the central axis depth of the 50% isodose curve for a 10-MV beam lies at about 18 cm for a 10 X 10 cm field and 100 - cm SSD. This depth is too large in the context of most wedge filter applications. VERY IMPORTANT: The wedge filters are mostly used for treating superficial tumors, for example, not more than 10 cm deep. Therefore, the current recommendation is to use a single reference depth of 10 cm for wedge angle specification.
What are wedge filters mostly used for?
They are mostly used for treating superficial tumors, for example, not more than 10 cm deep.
Do wedge filters decrease the linacs output?
Yes, and this must be taken into account in treatment calculations.
What is the "wedge transmission factor"?
The wedge transmission factor (or simply wedge factor), is defined as the ratio of doses with and without the wedge, at a point in phantom along the central axis of the beam. This factor should be measured in phantom at a suitable depth beyond the depth of maximum dose (e.g., 10cm). This factor includes the fact that the wedge filter decreases the output of the linac.
Wedge filters are of two main types. What are they and explain their properties.
The first is called "individualized wedge system". IT requires a separate wedge for each beam width, optimally designed to minimize the loss of beam output. A mechanism is provided to align the thin end of the wedge with the border of the light field. The second system uses a "universal wedge" which includes one wedge that serves for all beam widths. Such a filter is fixed centrally in the beam, while the field can be opened to any size.
How does the wedge filter affect the beam quality of the radiation beam?
Two ways: the first is the wedge filter, mostly made from lead or steel, attenuates the lower energy photons within the x-ray spectrum hence hardens the beam. The second way, which is not as significant, is beam-softening due to the compton interactions between the photons and the material.
Wedge filters do affect the beam quality two different ways, but do these beam quality alterations affect other correction factors such as equivalent square, BSF, TAR, TMR, etc.?
No it doesn't have an impact. Most remain approximately equal when dealing with depths less than 10 cm, and a way to reduce this error is to measure the wedge transmission factor at a reference depth close to the point of interest.
Aron and Scapicchio created a technique to design the wedge filters. Briefly explain exactly how they perform that.
Following the drawing of the lines which explains the isodose distributions for wedged and non-wedged fields, the wedge to unwedged ratio is derived. The largest ratio is then normalized and then the relative transmission ratio is derived.
The isocenter is the point of intersection of the collimator axis and the gantry axis of rotation.
What are the benefits of isocentric treatment techniques?
The main benefit is not having to adjust the patient with respect to the x-ray source in order to accurately calculate the percent depth dose. Since SSD = SAD - d, SSD can be calculated for each beam (different angles) and have the percent depth dose calculated in accordance to that. The other benefit is being able to use the TMR (or TPR) in calculating the dose when changing gantry angles.
What is rotation therapy and is it significantly better than stationary therapy?
Rotation therapy is a special case of the isocentric techique in which the beam moves continuously about the patient, or the patient is rotated while the beam is held fixed. And no, rotation therapy has insignificant advantage over the use of multiple stationary beams.
When should rotation therapy not be used?
It shouldn't be used when "intricate blocking" is required.
What circumstances would rotation therapy (isocentric technique) be of significant help?
Rotation therapy is best suited for small, deep-seated tumors. If the tumor is confined within a region extending not more than halfway from the center of the contour cross section, rotation therapy may be a proper choice.
What three circumstances would render rotation therapy a bad choice of treatment techique?
- 1.) the volume to be irradiated is too large.
- 2.) the external surface differs markedly from a cylinder.
- 3.) the tumor is too far off center.
When performing rotation therapy, what is the uniqueness/difference between partial arc motions and a complete 360 degree rotation?
When that the maximum dose for the 360 degree rotation occurs at the isocenter, while for the partial arcs it is displaced toward the irradiated sector.
In rotation therapy, something called "past pointing" is used to correctly choose the isocenter point. What is "past pointing" ?
Due to the maximum dose point of partial arc therapy moving closer to the irradiated sector, past pointing means to choose the isocenter center to be beyond the tumor area rather than on the tumor itself.
Using wedge filters, how would you transform a non-uniform dose plateau to a uniform dose plateau when using two beams at a hinge angle, and a specific wedge isodose angle?
You would choose theta which is equal to 90-degrees - (half-hinge-angle). This technique is used when dealing with relatively superficial tumors, extending from the surface to a depth of several centimeters.
The wedge angle equation, theta = 90 - (hinge/2) may not yield an optimum plan for a given patient contour. Due to patient contours typically being irregular or curved in shape, non-uniform distribution occurs again. How can this problem be solved?
This problem can be solved using "compensators" (discussed in Chp. 12) which make the skin surface effectively flat and perpendicular to each beam. An alternative approach is to modify the wedge angle (using a different wedge angle than the eqn in the question) so that a part of the wedge angle cacts as a compensator and the rest as a true wedge filter.
When are wedge-pair techniques used and what is the common acceptable maximum dose?
Wedge-pair techniques are normally used for treating small, superficial tumor volumes anywhere from 0 to 7cm deep. And a high-dose region (hot spot) of up to +10% within the treatment volume is usually acceptable. These hot spots occur under the thin ends of the wedges and their magnitude increases with field size and wedge angle.
What is the gross tumor volume (GTV)?
The GTV is the gross demonstrable extent and location of the tumor. It may consist of primary tumor, metastatic lymphadenopathy, or other metastases. Delineation of GTV is possible of the tumor is visible, palpable, or demonstrable through imaging.
What situation causes the incapability of defining the GTV?
When it is surgically removed.
What is the clinical target volume (CTV)?
The CTV consists of the demonstrated tumor(s) if present and any other tissue with presumed tumor. It represents, therefore, the true extent and location of the tumor. Delineation of the CTV assumes that there are no tumor cells outside this volume. The CTV must receive adequate dose to achieve the therapeutic aim.
What is the internal target volume (ITV) and which ICRU report sets viable definitions?
It is ICRU report no. 62 and it recommends that an internal margin (IM) be added to the CTV to compensate for "internal physiologic movements" and variation in size, shape, and position of the CTV during therapy in relation to an internal reference point and its corresponding coordinate system. In other words, the CTV plus the IM renders the ITV.
What is the planning target volume (PTV)?
The planning target volume (PTV) is the ITV plus a setup margin (SM). In other words, it is the CTV plus the IM plus the SM. The sizes of the margins and are based off subjective decisions by the treatment planners.
What is the planning organ at risk volume (POARV)?
The health adjacent organs need to be treated with equivalent attention as the tumor. Once the organ is outlined, a margin needs to be added to the OARV in order make sure that it doesn't receive an intolerable dose.
What is the treated volume?
Additional margins must be added to the PTV due to the limitations of the specific treatment technique chosen to treat the patient. So the minimum target dose should be represented by an isodose curve surrounding the PTV. That minimum isodose curve surround the PTV is called the "treated volume".
What is the "irradiated volume"?
The volume of tissue receiving a significant dose (e.g., >= 50% of the specific target dose) is called the irradiated volume. The irradiate volume is larger than the treated volume and depends on the treatment technique used.
What is the maximum target dose and what is an important point within its definition?
The maximum dose is defined at the highest dose in the target area. The important point is that in order to define the area within the irradiated volume as the maximum dose, it has to cover a minimum area of 2 cm2. If the high doses cover less than that minimum area, it can be ignored. I think it can be ignored due to its insignificant biological effect.
What is the minimum target dose?
The minimum target dose is the lowest absorbed dose in the target area.
What is the media target dose?
The media target dose is simply the value between the maximum and the minimum absorbed dose values within the target.
What is the modal target dose?
The modal target dose is the absorbed dose that occurs most frequently within the target area.
What is a "hot spot"?
A hot spot is an area outside the target that receives a higher dose than the specified target dose. Like the maximum target dose, a hot spot is considered clinically meaningful only if it covers an area of at least 2 cm2.
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