Chapter 20: Intensity-Modulated Radiation Therapy
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What is Intensity-Modulated Radiation Therapy? Pg. 430
In the traditional external beam photon radiation therapy, most treatments are delivered with radiation beams that are of uniform intensity across the field (within the flatness specification limits). Occasionally, wedges or compensators are used to modify the intensity profile to offset contour irregularities and/or produce more uniform composite dose distributions such as in techniques using wedges. This process of changing beam intensity profiles to meet the goals of a composite plan is called intensity modulation.
The term intensity-modulated radiation therapy (IMRT) refers to a radiation therapy technique in which nonuniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution.
How is the plan optimization specified? Pg. 430
The treatment criteria for plan optimization are specified by the planner and the optimal fluence profiles for a given set of beam directions are determined through "Inverse Planning".
For clinical implementation of IMRT, what is the least amount of systems needed and what are they? Pg. 430
The clinical implementation of IMRT requires at least two systems:
- 1.) A treatment-planning computer system that can calculate nonuniform fluence maps for multiple beams directed from different directions to maximize dose to the target volume while minimizing dose to the critical normal structures.
- 2.) A system of delivering the nonuniform fluences as planned.
What is the principle of IMRT? Pg. 430
The principle of IMRT is to treat a patient from a number of different directions (or continuous arcs) with beams of nonuniform fluences, which have been optimized to deliver a high dose to the target volume and an acceptably low dose to the surrounding normal structures.
What are the two broad categories in which computer methods that have been devised to calculate optimum intensity profiles, methods that are based on inverse planning, can be placed into? Pg. 430
- 1.) Analytic Methods.
- 2.) Iterative Methods.
What is the basics of analytic methods used to calculate optimum intensity profiles? Pg. 430
Analytic methods involve mathematical techniques in which the desired dose distribution is inverted by using a back projection algorithm. In effect, this is a reverse of a computed tomography (CT) reconstruction algorithm in which two-dimensional images are reconstructed from one-dimensional intensity functions.
What is one significant problem with the analytical methods of calculating the optimum intensity profiles? Pg. 431
One problem with analytical methods is that, unlike CT reconstruction, exact analytical solutions do not exist for determining incident fluences that would produce the desired dose distribution without allowing negative beam weights. The problem can be circumvented by setting negative weights to zero but not without penalty in terms of unwanted deviations from the desired goal. So some algorithms have been devised to involve both analytical and iterative procedures.
What is the fundamental measure of optimal dose calculations with the iterative methods? Pg. 431
The fundamental measure is the cost function. Optimization techniques have been devised in which beamlet weights for a given number of beams are iteratively adjusted to minimize the value of a cost function, which quantitatively represents deviation from the desired goal.
With the iterative method, how is the fundamental measure used? Pg. 431
The cost function is a least square function where there is a difference (subtraction) between the desired dose and the experimental dose. The goal is to reduce the cost function below a certain threshold that is satisfactory based on the consensus reached by the physicists.
Is the patient input data needed for inverse-planning the same that is needed for forward-planning? Pg. 431
Yes. Three-dimensional image data, image registration, and segmentation are all required when planning for IMRT. For each target (planning target volume [PTV]), the user enters the plan criteria: maximum dose, minimum dose, and a dose volume histogram.
Once an acceptable IMRT plan has been generated by the inverse-planning, optimum dose calculations, what is done with that information? Pg. 431
After an acceptable IMRT plan has been generated, the intensity profiles (or fluence maps) for each beam are electronically transmitted to the treatment accelerator fitted with appropriate hardware and software to deliver the planned intensity-modulated beams.
How are x-ray beams normally generated by radiation therapy accelerators? Pg. 432
Radiation therapy accelerators normally generate x-ray beams that are flattened (made uniform by the use of flattening filters) and collimated by four moveable jaws to produce rectangular fields.
To produce intensity-modulated fluence profiles, precalculated by a treatment plan, the accelerator must be equipped with a system that can change the given beam profile into a profile of arbitrary shape. Many classes of intensity-modulated systems have been devised. List some that comes to mind. Pg. 432
- 1.) compensators.
- 2.) wedges.
- 3.) transmission blocks.
- 4.) dynamic jaws.
- 5.) moving bar.
- 6.) multi-leaf collimators (MLCs)
- 7.) tomotherapy collimators.
- 8.) scanned elementary beams of variable intensity.
Which systems used to modulate the intensity of an x-ray beam are done manually and are time consuming? Pg. 432
Compensators, wedges, and transmission blocks are manual techniques that are time consuming, inefficient, and don't belong to the modern class of IMRT systems.
For linear accelerators, what seems to be the most practical device for delivering IMBs? Pg. 432
For linear accelerators it seems that the computer-controlled MLC is the most practical device for delivering IMBs.
How are computed-controlled MLCs useful? Pg. 432
A computer-controlled multileaf collimator is not only useful in shaping beam apertures for conventional radiotherapy, but it can also be programmed to deliver IMRT. This has been done in three different ways.
Describe the process of a "step-and-shoot" IMRT delivery method. Pg. 432
The patient is treated by multiple fields and each field is subdivided into a set of subfields irradiated with uniform beam intensity levels. The subfields are created by the MLC and delivered in a stack arrangement one at a time in sequence without operator intervention. The accelerator is turned off while the leaves move to create the next subfield. The composite of dose increments delivered to each subfield creates the intensity-modulated beam as planned by the treatment-planning system.
What is a mixed mode of IMB delivery called? Pg. 433
A mixed mode of IMB deliver is called "dynamic-step-and-shoot". In this method the radiation is "on" all the time, even when the leaves are moving from one static subfield position to the next. This technique has the advantage of blurring the incremental steps in the delivery of static subfields.
Explain how Dynamic Delivery (IMRT) works. Pg. 433
In this technique the corresponding (opposing) leaves sweep simultaneously and unidirectionally, each with a different velocity as a function of time. The period that the aperture between leaves remains open (dwell time) allows the delivery of variable intensity to different points in the field. This method has been called by several names: the "sliding window", "leaf-chasing", "camera-shutter", and "sweeping variable gap".
How fast can each leaf within an MLC move? Pg. 434
The leaves of a dynamic MLC are motor driven and are capable of moving with a speed of greater than 2 cm per second.
I think Kahn meant to say "...are capable of moving with a speed NO greater than 2 cm per second".
What is the dynamic MLC algorithm based on? Pg. 435
- 1.) If the gradient of the intensity profile is positive (increasing fluence), the leading leaf should move at the maximum speed and the trailing leaf should provide the required intensity modulation.
- 2.) If the spatial gradient of the intensity profile is negative (decreasing influence), the trailing leaf should move at the maximum speed and the leading leaf should provide the required intensity modulation.
What is IMAT (Intensity-modulated Arc Therapy)? Pg. 435
This method is similar to the step-and-shoot in that each field (positioned along the arc) is subdivided into subfields of uniform intensity, which are superimposed to produce the desired intensity modulation. However, the MLC moves dynamically to shape each subfield while the gantry is rotating and the beam is on all the time.
In IMAT, we know that if there are N levels of subfields, there are N!^2 different ways to position the leaves. Within all those ways, how are the specific positions chosen? Pg. 435
They're chosen based on which have adjacent setups which require minimum distance in repositioning for the next subfield.
What is Tomotherapy? Pg. 435
Tomotherapy is an IMRT technique in which the patient is treated slice by slice by intensity-modulated beams in a manner analogous to CT imaging. A special collimator is designed to generate the IMBs as the gantry rotates around the longitudinal axis of the patient. In one device the couch is indexed one to two slices at a time and in the other couch moves continuously as in a helical CT.
What is the Peacock system for tomotherapy? Pg. 436
The NOMOS collimator decvice is called the MIMiC and is used in conjunction with a treatment planning system, PEACOCKPLAN. The MIMiC and the PEACOCKPLAN together are known as the PEACOCK system.
What is of concern in MIMiC-based IMRT (Tomotherapy)? Pg. 436
Each bank within MIMiC can treat 1- or 2-cm-thick slices of tissue 20cm in diamaeter; because there are two such banks, a 2- or 4-cm slice of tissue can be treated at one time. For extending the length of treatment volume beyond 4cm, the couch is moves to treat the adjacent slices. This gives rise to field junctions, which is of concern in MIMiC-based IMRT.
What are some facts about the MIMiC used for IMRT Tomotherapy? Pg. 436
The MIMiC leaves are made of tungsten and are approximately 8 cm thick in the direction of the beam. The transmitted intensity through a leaf is approximately 1% for 10-MV x-rays. The leaf interfaces are multistepped to limit interleaf leakage to within 1%. Each leaf can be switched in 100 to 150 milliseconds, thus allowing a rapid change in beam apertures as the gantry rotates. Considering the number of possible field apertures at each gantry angle and the number of intensity steps that can be delivered at each gantry position, it is possible to create more than 10^13 beam configurations for each arc. Thus, the intensity modulation of beams can be finely controlled by the MIMiC technology.
What is one significant, potential problem with MIMiC-based IMRT (tomotherapy)? Pg. 436
A potential problem with MIMiC-based IMRT is the possibility of mismatch between adjacent slice pairs needed to treat a long target volume. Carol et al. have studied the problem and shown that perfectly matched slices gave rise to 2% to 3% dose inhomogeneity across the junction. However, even a 2-mm error in indexing the couch resulted in dose inhomogeneity of the order of 40%. NOMOS solved this problem by designing accurate table indexing and patient fixation devices.
For IMRT tomotherapy, what is Crane and Talon? Pg. 437
Crane: Because of the potential field-matching problems in the use of MIMiC for treating adjacent slice pairs along the length of the patient, it is imperative to move the couch with extreme accuracy. A special indexing table called the CRANE has been designed by NOMOS, which is capable of moving the couch longitudinally with a 300-lb weight to distances of 0.1 to 0.2 mm. With such accuracy it is possible to reduce the junctional dose inhomogeneity to within plus or minus 3 percent.
Talon: Because of the stringent matchline requirements, NOMOS supplies an invasive head fixation system called the TALON. The device is attachable to the CT or the treatment unit couch and fixes the head position by the insertion of two bone screws into the inner table of the skull. Once the bone screws have been inserted, the TALON can be removed or reattached quickly as needed. An evacuated headrest can also be used to assist in repositioning at each treatment session.
What is the difference between helical tomotherapy and MIMiC-based IMRT, and what is one significant advantage that helical tomotherapy has over the other? Pg. 437.
The major difference between the MIMiC-based tomotherapy and helical tomotherapy is that in the former case the patient couch is stationary while the gantry rotates to treat each slice pair at a time and in the latter case the patient is translated continuously along with the gantry rotation. The field matching problems are thus minimized in the helical tomotherapy.
When commissioning an IMRT system (e.g., step-and-shoot, sliding window, IMAT, tomotherapy), who has the responsibility of finally approving its clinical application? Pg. 439
The final approval for clinical application is the responsibility of the physicist and the physician in charge.
What are the five tests that are used to check the mechanical accuracy of the DMLC, which is fundamental to the accurate deliver of IMBs? Pg. 439
- 1.) Stability of leaf speed.
- 2.) Dose profile across adjacent leaves.
- 3.) Leaf acceleration and deceleration.
- 4.) Positional accuracy of leaves.
- 5.) Routine mechanical check.
What IMRT technique were the dosimetric checks made for and what are they? Pg. 442
A series of dosimetric checks have been recommended by LoSassao et al. specifically for the "sliding window" technique. These include measurements of MLC transmission, transmission through leaf ends, head scatter, and dose distribution in selected intensity-modulated fields.
For IMRT MLCs, why do they have rounded leaf ends? Pg. 442
Several manufacturers offer MLCs with rounded leaf ends. This is done to maintain a constant geometric penumbra at different leaf positions in the beam.
How is the leakage between adjacent MLC leaves minimized? Pg. 443
The leakage between the adjacent leaves is minimized by designing the leaves so that their sides partially overlap; that is, one side of the leaf protrudes outward ("tongue") and the other recesses inward ("groove") so that the central parts of the adjacent leaves fit like a jigsaw puzzle. This overlap of the leaves reduce the extent of radiation leakage through interleaf gaps, which are necessary for leaf motion relative to each other. This so called "tongue and groove" effect gives rise to higher radiation leakage than that through the middle body of the leaves but less than what it would be if the leaf sides were designed plane-faced. As was shown, interleaf transmission with the "tongue and groove" is between 2.5% and 2.7%.
When does the collimator scatter factor Sc depend on the collimator jaws or the MLCs? Pg. 443
If the MLC in the linac head is installed closer to the patient surface than the collimator jaws (as in Varian accelerators), the Sc factor depends predominately on the jaw opening and not on the MLC opening.
In the use of static MLC in voncentional radiotherapy, Sc for a given jaw opening is affected very little by the MLC setting for fields larger than 4 x 4cm. However, as the MLC aperture is reduced to much smaller openings, the Sc factor could drop significantly (e.g., by 5% for a 1 x 1-cm field). The reduction is caused by the MLC aperture approaching the geometric penumbra (radiation source has a finite size).
On the other hand, if the MLC is located above the collimator jaws, the head scatter would be affected more by the MLC setting than the jaw opening.
In either case, the treatment-planning algorithm must account for the Sc factors depending on the MLC geometry and the IMRT technique used.
After the IMRT technique has been commissioned, why is it essential to set up a quality assurance (QA) program? Pg. 444
It is essential to set up a quality assurance program because you want to maintain original accuracies, tolerances, and specifications of the system.
What are the two most commonly used methods to calculate the dose distribution when performing IMRT techniques? Pg. 445
Because the size and shape of the beam apertures are greatly variable and field dimensions of 1 cm or less may be frequently required to provide intensity modulation, the most commonly used methods of dose calculation in IMRT are
- 1.) pencil beam.
- 2.) convolution superposition.
*Monte Carlo techniques are also under development but are considered futuristic at this time because of their limitation on computation speed.
When calculating dose distributions for IMRT techniques, what is one major concern when having to calculate the monitor units following the completion of the dose calculations? Pg. 445
Because of the difficult, nearly impossible task of manually calculating the Monitor Units, the MUs are calculated simultaneously to the dose calculations within the TPS. That causes a problem because of the long time principle of calculating the MUs independent of the TPS. So as of right now a separate, independent software is needed to deliver IMRT in accordance to that longstanding principle of calculating/checking the MUs independent of the TPS.
Why are the pencil beam and convolution-superposition algorithms chosen to be used for IMRT TPS? Pg. 445
In IMRT, because beam intensity within the field is modulated (beam profile is no longer uniform), pencil beam or convolution-superposition algorithms are the methods of choice for dose calculations.
What treatments can IMRT be used for and what is its basic difference with conventional radiotherapy (3D-CRT)? Pg. 448
IMRT can be used for any treatment for which external beam radiation therapy is an appropriate choice.
The basic difference between conventional radiotherapy and IMRT is that the latter provides an extra degree of freedom, that is, intensity modulation, in achieving dose conformity. Especially targets of concave shape surrounding sensitive structures can be treated conformly with steep dose gradients outside the target boundaries-a task that is almost impossible to accomplish with conventional techniques.
Is IMRT limited by target size or location? Pg. 448
IMRT is not limited by target size or location.
What are two differences between brachytherapy and IMRT? Pg. 450
IMRT is comparable to brachytherapy in dose conformity but it is a different modality radiobiologically. So the choice between IMRT and brachytherapy should be based not only on the technical or dosimetric considerations, but also on the raiobiologic properties of brachytherapy versus external beam. For example, treatment of prostate with seed implants has a different rationale than for IMRT although dose conformity is comparable in terms of dose falloff beyond the prostate volume. Radiobiology of the two modalities is obviously different because of differences in dose homogeneity and dose rate or fractionation (e.g., continuous vs. fractionated dose delivery).
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