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U.S.Patent No. 7,945,022
 
3DVH transforms the field of per‑patient dose QA by generating clinically‑relevant and intuitive analyses of complex IMRT and VMAT plans. With proven accuracy, 3DVH estimates the 3D dose to the patient‑specific geometry. Powered by the patented "Planned Dose Perturbation (PDP™) algorithm, 3DVH uses existing QA measurements. Clinicians can abandon abstract and unpredictive "passing rate" metrics and instead use the same methods to QA treatment plans as those which were used to create them.
 
Benefits
  • Transform 2D measurements to 3D dose volume for advanced analysis
  • Perform 3D dose and DVH QA analysis on patient - not phantom - geometry
  • Supports coplanar and non-coplanar beams
  • Identify TPS and beam delivery errors
  • Intuitive and familiar presentation of dose and DVH with statistics per anatomical structure
  • Fast results with automated tools - Quick Stat Templates, Quick Dose Profiles, DICOM
    compliant workflow
  • No forward dose calculation into the patient CT
  • No commissioning
  • Uses existing measurements and devices
  • With optional Respiratory MotionSim™ module, analyze the dosimetric impact of a moving target
 
The Solution: Patient Relevant Dose QA
Software Interface
 
Panel
  1. Structures
    Isolate structures and analyze using Quick Stats. Analyze only what is necessary.
  2. Critical Structure Statistics
    View estimated patient dose in highlighted regions as well as statistics for any structure. Customize desired values in the Quick Stats panel.
  3. Panel
  4. Dose Display
    Isolate what is important, whether it be Reference, Comparison or Difference and assign custom colors and opacities to each
  5. Parameters
    Enter the analysis parameters and get a quick comparison of the results.

Software
 
False Negative — Conventional analysis indicates no finding of error, but 3DVH® shows substantial target underdose
 
SNC Patient™ software with 2%/2mm criteria showing 95.3% passing rate
SNC Patient software with 3%/3mm criteria showing 99.2% passing rate
SNC Patient™ software with 2%/2mm criteria showing 95.3% passing rate

Evaluating the same plan, 3DVH shows substantial target underdose
SNC Patient™ software with 2%/2mm criteria showing 95.3% passing rate

 

False Positive — Conventional analysis indicates a finding of error, but 3DVH® shows no clinically meaningful error in planned delivery
 
SNC Patient software with 2%/2mm criteria showing 85.0% passing rate
SNC Patient software with 3%/3mm criteria showing 79.7% passing rate
SNC Patient™ software with 2%/2mm criteria showing 95.3% passing rate

Evaluating the same plan, 3DVH shows delivery consistent with plan
SNC Patient™ software with 2%/2mm criteria showing 95.3% passing rate
 
3DVH® Features
 
ArcCHECK®
EPIDose™
MapCHECK™
Create an estimated 3D or 4D patient dose volume for
QA comparison
  • Use real QA measurements to estimate 3D or 4D dose in patient
  • Compare DVH, slice, and absolute point dose of estimated 3DVH vs. TPS
  • Use DVH for targets and critical structures, instead of 2D gamma passing rates
  • Automatically corrects for depth, SSD, and
    patient geometry
1
In depth, structure-based (ROI) analysis
  • Customize the display of statistics by ROI
  • Report the 3D gamma metrics on a per structure basis
  • Isolate and analyze only what is necessary
  • Create Quick Stat templates for standardization
    and reuse
  • Specify within each template structures and constraints-per-structure; designed to support unlimited configurations
Powerful tools in an intuitive user interface
  • Display Reference Dose, Comparison Dose, or
    Dose Difference
  • Assign custom colors and opacities for clarity and ease-of-use
  • Ability to view Beams-Eye-View for IMRT treatments
Easily correct for daily output fluctuation
  • Daily output correction propagates to the estimated patient dose volume
  • Select from multiple default chamber geometries
QA Plan Reporting
  • Create reports of QA results in PDF format
  • Filter report contents to suit clinic requirements
PDP for Non‑coplanar Beams
  • Compute an estimated patient dose via the ArcCHECK PDP algorithm for a treatment delivery involving non‑coplanar arcs
4D Workspace Tools and Graphics
  • Look at gantry angle and cumulative or differential dose over time
  • View cumulative and instantaneous ArcCHECK dose and correlate to gantry angle and MLC position
  • Compare MLC measured positions to
    predicted positions
VirtualGel™
  • Rapid reconstruction of a high-density, high-resolution 3D phantom dose
  • Directly compare ArcCHECK 3D dose volume computed by 3DVH to a TPS-generated dose, without performing the PDP process
  • View dose profile along the x, y, or z axis through the origin and along any column of diodes
  • Facilitates detection of beam model errors during commissioning
  • All the benefits of a gel without the extra handling and processing
2
Respiratory MotionSim™
  • Quantify and visualize the effects of organ motion on the estimated patient dose distribution
  • Uses proven PDP™ measurement-guided reconstruction method to estimate 4D dynamic dose
  • Evaluate both 3D Dose and DVH changes caused by motion to determine if motion management is necessary, and to QA motion management plans
  • Run simulations virtually without the need for additional hardware
  • Separately licensed module
  • See Respiratory MotionSim datasheet for
    more information

 

1 Must use a cavity plug with a solid PMMA insert
2 Requires the non‑DICOM ArcCHECK movie file
 
Specifications
 
Compatibility
  • Hardware: ArcCHECK®, MapCHECK® 2
  • Software: SNC Patient™, EPIDose™
  • Rotational Therapy: RapidArc®, VMAT
  • Static Gantry: IMRT
  • Treatment Planning Systems: Pinnacle®, Eclipse®, and most TPS systems that can export DICOM data (TomoTherapy® coming soon)
  • FFF & non-FFF Deliveries
System Requirements
Operating system
Windows XP, Windows 7
CPU (GHz)
Recommended 2.4 or better,
multi‑core (2 or more cores)
RAM
Recommended 4GB or more
Hard drive space
Recommended 5GB or more
 
Publications
 
Recommended Reading
 
 
 
 
ArcCHECK® for SRS/SBRT
 
Gamma Pass Rate Papers
 
Additional Publications
 
Case Study
 
The Problem: 2D QA is not strongly correlated to actual errors
 
Actual Errors versus 2D Gamma Passing Rate
Perfect correlation of 2D QA Gamma Passing Rate (%) and Actual Error in DVH Metric is desired.
 
Actual Errors versus 2D Gamma Passing Rate
False negatives: where 2D QA Gamma Passing Rate meets criteria yet actual errors fail criteria.

False positives: where 2D QA Gamma Passing Rate fails criteria yet actual errors pass criteria.
 
2D: A Lack of Correlation
Commonly accepted 2D QA Gamma Passing Rate lacks significant correlation to actual errors in a patient geometry. While 2D QA techniques are useful for QA, 2D QA Gamma Passing Rates are not reliable as an estimator of actual error in a treatment plan.

2D: A Lack of Correlation

"Per-beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors," B. Nelms et al., Med. Phys. 2011 Oct;38(2):1037‑1044

 
3D Volumetric with 3DVH: Significant Correlation
When QA measured errors, large or small, are used to create an estimated 3D dose volume in a patient geometry for QA comparison, significant correlation is achieved.

3D Volumetric with 3DVH: Significant Correlation

"Moving from gamma passing rates to patient DVH-based QA metrics in pretreatment dose QA," Zhen H. et al., Med. Phys. 2011 Oct;38(10):5477-89

 
Frequently Asked Questions

Explain the Planned Dose Perturbation (PDP™) process in layman terms. How does PDP work?

Very simply, PDP uses measured diode data and compares it to the expected treatment plan data. The differences between measured and expected doses are used to create an error map of hot and cold spots. This error map is used to perturb the 3D treatment plan to produce a new 3D dose reconstruction that displays the estimated dose to the patient. The perturbation can be thought of as a back-projection of the error (voxel-by-voxel) through the patient dose, changing each dose voxel according to measured dose difference. By doing this both the TPS and the functioning of the linac are audited.

Does 3DVH take into consideration heterogeneity? Does it use the CT of the patient?

The heterogeneity of the patient is taken into consideration because the original treatment plan doses vary based on tissue heterogeneities. To put it another way - dose deposition is directly related to tissue density, therefore if the dose deposition is known, the tissue density is inherently known. By perturbing the original doses (which are based on the heterogeneities of the patient), the heterogeneity is taken into account from the beginning.

What if my CT to Electron Density Curve is incorrect, and therefore my TPS is incorrectly calculating the dose through heterogeneous tissues? Will 3DVH catch this error?

3DVH would not catch this error, but there are already several other ways to catch CT to Electron Density Curve errors already in place in most hospitals. Monthly CT QA (per TG-66) and Annual QA should catch any CT to Electron Density errors. Because these errors are easily caught using a phantom with various known densities, this is not a problem that would require a full forward calculation model to address.

More insidious heterogeneity errors can be imagined if the TPS algorithm is simply not good enough to accurately calculate around heterogeneous tissue interfaces. Again, this is a general problem (not patient specific) and should be discovered when commissioning a new TPS algorithm. A physicist could easily detect these problems by using any heterogeneous phantom as a patient and doing an end-to-end test where multiple points are measured around heterogeneities. This is a TPS commissioning task, and should not need to be repeated for every patient QA.

The "cost" of incorporating heterogeneity into 3DVH would be to perform a full forward calculation, which is an approach other companies have taken. Sun Nuclear chose instead to base QA on measured errors because this is the most independent way to perform patient-specific QA. A secondary forward dose calculation introduces unknown sources of error by introducing a new calculation algorithm. If there were a difference between the QA and the TPS, the question rightly becomes, which do you trust? 3DVH is based on measured data that can be confirmed using ion chamber measurements. Numerous peer-reviewed and published papers substantiate the accuracy of 3DVH. See the Publications page for references.

How is 3DVH capable of moving from the calculation of dose in a homogeneous phantom to dose differences in a heterogeneous patient?

3DVH is able to do this final step of dose translation by mapping the errors from a full resolution 3D phantom dose to 3D patient dose using the coordinates from each. Using the isocenter as the reference, the error ratio of the 3D phantom dose is then applied to the 3D patient dose. Because of published research we can confidently say that the results prove that this approach produces very accurate results. See the Publications page for references.

How are you deriving delivered dose in the patient based on measured differences in phantom? I would think you would need to do a forward calculation on the patient CT using the measured differences to arrive at an accurate delivered dose.

Sun Nuclear carefully considered various approaches possible for determining dose in a patient volume before developing 3DVH and decided that the correct approach was to use measured data and the original treatment plan rather than re-creating a new forward calculating algorithm. Forward calculating algorithms have the inherent problem in that the user is never sure which algorithm to trust. And since the TPS system has been validated and continually improved for years, the argument can be made that TPS should receive preference in this argument. By using the TPS data as a starting point and then perturbing the treatment plan based solely on measured errors, 3DVH allows the user to compare a measurement-guided 3D dose reconstruction to the expected treatment plan dose. This is done without adding another algorithm as an extra variable and possible source of error.

What is the DVH bin size?

DVH bin size refers to the size of each dose bin used in accumulating the DVH statistics (i.e. binning dose points into the histograms). 3DVH assigns dose bins using the maximum dose of each plan in order to use the highest number of bins for a given plan, as follows:

For global max < 1 Gy, bin size = 0.00001 Gy;
If global max >1 and < 10 Gy, bin size = 0.001 Gy;
If global max >10 Gy and < 100 Gy, bin size = 0.01 Gy
If global max > 100 Gy, bin size = 0.1 Gy.
For small structures a supersample is pursued (ie. Trigeminal)


The number of bins is, at minimum, 1000 and up to 10,000, assuring high resolution of DVH bin width and Quick Stat computation.

When 3DVH is performing the planned dose perturbation, is the dose difference for each voxel applied evenly across the patent dose?

No. The magnitude of the dose error is used in addition to the depth and patient surface characteristics to adjust the dose across the patient volume. These corrections are derived from the basics of the Compton Effect. A detailed discussion of this process can be found in the Zhen et al. paper. See Publications page for full reference., assuring high resolution of DVH bin width and Quick Stat computation.

I don't believe that 3DVH gives me any additional relevant information or further analysis of my measurements. Why should I buy it?

3DVH gives the most relevant type of analysis - clinical DVH data based on measurements. For the first time the physicist and doctor know where the hot and cold spots are falling in the patient's anatomy - and a hot spot falling in the Cord is obviously much different from a hot spot falling in a GTV! One may require a treatment plan change; the second may actually improve the treatment. This difference could not be perceived with simple pass rate criteria such as 3%/3mm with 95% passing. Recent publications have shown that gamma pass rates have very poor correlation to clinically important goals (i.e. coverage of the PTV, minimizing dose to the Cord). Having the ability to reconstruct the patient DVH from measured data is a great step forward in determining if the treatment plan will be effective for a specific patient.

How accurate is 3DVH calculation? How can you prove it if you cannot measure inside the patient?

3DVH has been proven to be very accurate; please see the Publications page for references to numerous studies with results published and peer-reviewed. Researchers used several approaches to test the algorithm. One approach, most similar to a patient measurement, is to take measurements with film and numerous chambers in a heterogeneous phantom, using these to verify that the 3DVH results were equivalent to the measured results. This approach was used in the Opp et al. paper.

A second approach is to know the correct answer in advance and test if 3DVH is able to reproduce the answer. This is done by introducing an error in the treatment plan, but delivering the treatment correctly (as initially planned). If 3DVH correctly reconstructs the original treatment plan (despite its perturbation calculations beginning with the erroneous treatment plan), then one can state with confidence that 3DVH is accurately reconstructing dose based on measured data. This was the approach used in the University of Wisconsin papers on 3DVH.

How can 3DVH give accurate results for my specific linac if you do not take measurements from it?

The short answer is that 3DVH is a measurement-guided algorithm, not a forward dose calculation, which would require a full characterization of your machine including small differences such as transmission differences, PDD differences, TPS differences, and so on.

What is done instead is to acquire datasets from multiple similar machines (i.e. Varian, 120 millennium MLC, and 6MV beam) and create a "Golden Machine" model which represents the mean of several representative machines with the same Vendor, MLC-type, and Energy. Even if your machine is different from our model, we will detect those differences from the measured data at multiple depths acquired with the ArcCHECK device during a standard VMAT/IMRT QA measurement. We have 1386 diodes at various depths that are measuring dose throughout your plan delivery. These measurements feed our calculation and if your machine varies from our "Golden Machine" data, that's fine - we will perturb the dose to match the measurement. This is the essence of Planned Dose Perturbation algorithm - we perturb the planned dose using the measurement.

Finally, the reason we create Machine, MLC, and Energy-specific models is that we want to perturb the dose as little as possible, so we want to begin with a model that's very similar to your beam. If you were to purposely use the wrong model, the 3DVH software would still correct your plan by perturbing the dose to match the measurements, but the end result would appear noisier since the algorithm would have to work much harder and make significant changes to the starting point "Golden Machine" data.

Now that I have this information (3DVH Patient based QA results), what do I do with it? Does the doctor now have to review every IMRT QA?

Physicists and doctors will have to discuss this in order to determine the correct answer for their clinic, much like doctors/physicists had to do when IGRT was first implemented.

The following is an example of the kinds of clinical implementation protocols that could be followed: The physician could determine thresholds of DVH differences for PTV, Cord, and other OARs. If the threshold (i.e. 3% difference in PTV at 95%Rx; 200cGy difference in the Cord Max dose) was exceeded in the physicists review of the 3DVH results, the doctor could be asked review the DVH and make the clinical decision whether to proceed. At that point, either the plan could be revisited (or perhaps just scaled if all organs were cold/hot) and the 3DVH QA re-run. Throughout this process physics would be responsible for overall practice improvement if systematic errors were discovered. Please Note: These are all clinical decisions that must be agreed upon within the clinic; this suggestion is given only as an example, not as a guide.

Recently a clinical site shared with SNC that they use the Organ-specific gamma results for their passing criteria; i.e. the PTV/CTV/GTV must pass by 95%. The OARs are evaluated similarly, except that if all of the errors are "cold" then they are disregarded. This is just another example of how a protocol could be described, and not intended as a recommendation from Sun Nuclear.

3DVH sounds like it's opening Pandora's Box. What would we do if we found errors of concern in the delivered patient dose?

Clinical practice improvement is the goal we all strive for, and 3DVH is a tool to achieve that goal. Many clinics have used 3DVH and found hidden systematic errors that they were then able to correct for all patients, an incredible benefit. As with IGRT, sometimes not knowing was easier, but patients greatly benefited once we could shift them correctly prior to treatment. Finding systematic or patient-specific errors by analyzing the delivery of the dose in patient geometry BEFORE the patient is treated is a tremendous step forward in radiation therapy.

A few practical suggestions if an error is found. If the plan is an IMRT plan, review the isodose lines and fluence errors in the BEV window. Several common modeling errors can be inferred by analyzing the comparison between the TPS curve and the 3DVH curve. Differences in the penumbras may point to using a chamber that was too large for commissioning. Errors in only the peaks and valleys may point to a modeling error caused by over processing scans. Tongue and Groove errors can also readily be detected in the BEV window as cold stripes. For VMAT plans, similar conclusions can be drawn from the "Measure/Dose Profile" option available in the 3D Dose window. For patient-specific errors, you may want to consider reducing the modulation of the beams. These are all clinical judgments that should be made by Radiation Oncology clinicians, but our customer support team can be of assistance in ruling out common mistakes in 3DVH setup.

I'm used to exporting 2D planar dose. How do I export 3D dose files for the patient and phantom?

This varies by TPS, but the user should export the following DICOM RT files:
For the Patient Plan - RT Plan, RT Dose, RT Struct, CT Images
For the Phantom Plan - RT Dose, RT Plan (RT Plan data is only used to find the isocenter of the phantom; it's unnecessary after initial setup of 3DVH.)


Eclipse - use FileExportWizard and select the appropriate files listed above. Use an appropriate DICOM Export Filter and save the files.

Pinnacle - use FileExportDICOM and select the appropriate files listed above, along with "sum of selected prescriptions"

Monaco/XiO - use FileExport DICOM and select the appropriate files listed above.

For more detailed instructions, please use the "User's Guide - TPS Data for ArcCHECK" document.

I'm interested in the Merge feature of SNC Patient™ software. To what degree will this affect the resolution/spacing? Will it significantly aid SRS/SBRT case QA integrity?

The merge feature allows the user to double the density of measurements by shifting the ArcCHECK 0.5cm and 2.7 degrees. If you purchased the ArcCHECK prior to the Patient 6.2 release, contact Support Operations and ask them to mail you an overlay that can be placed on the ArcCHECK to guide the Merge measurement shifts. The merge feature will enable SRS/SBRT users to get better density for ArcCHECK pass rate analysis. The density of the diodes will be doubled; the beam's eye view density ranging from sub-millimeter to 5mm spacing. Because of this increased density, plans down to 5mm field size should be measurable. Please note that precise setup of the ArcCHECK is very important with such small measurements; a small tilt or rotation can unintentionally produce a large magnitude error unintentionally. The increased density will not be usable by 3DVH until a future release.

How much longer will it take me to do my IMRT QA if I start using 3DVH?

Measurement time on the linac is not increased at all to use 3DVH. The measurements performed with ArcCHECK and MapCHECK2 are exactly the same whether or not 3DVH is used. The 3DVH perturbation calculation takes approximately 1-2 minutes per arc. For IMRT plans the calculation is usually less than 1-2 minutes for all beams. Once adopted, 3DVH should save you time because the metrics provided by 3DVH are far more intuitive, sensitive, and specific than passing rate metrics. Passing rate metrics, whether they pass or fail, tell the physicist/doctor little about the clinical fitness of a specific treatment plan, which can leave a dosimetry team guessing what needs to be altered when plans fail passing rate criteria. The continual practice improvement that comes with adopting 3DVH will assist the entire treatment team in efficiently producing excellent treatment plans.

I don't have an ArcCHECK. Why can't I use 3DVH for Rotational VMAT QA using the MapCHECK 2 or EPIDose measurements?

ArcCHECK was specifically designed for VMAT QA because the complexity of the treatment (specifically the dynamic gantry rotation and speed) called for a device that was three dimensional. This allows the physicist to get large field size, density, and range of depth readings throughout an arc beam. ArcCHECK has a surface that is always normal to the beam, and also allows for two depths of measurement - entrance and exit dose. This is the ideal arrangement for VMAT QA.

The reason 3DVH cannot perform a calculation for a VMAT plan when using a 2D array is that there is simply not enough data for accuracy. The 2D array will be positioned in one of two ways - either sitting on the couch with the VMAT beam delivered 360 degrees around it, or attached to the linac head so that the 2D array is always perpendicular to the beam. Either arrangement poses problems - the first one loses a lot of data density because the array is not always normal to the beam. At angles such as 90 or 270 degrees, the 2D array approaches becoming a 1D array and very few diodes are collecting data. Additionally, because there's no entrance and exit dose (as in the AC), 3DVH can't determine the gantry angle the data is coming from through the Virtual Inclinometer. The second scenario (2D array attached to the gantry) by definition cannot verify the gantry angle and speed; the gantry angle/speed must be assumed to be correct to produce a 3D reconstruction. With the array attached to the linac (or using the EPID), 3DVH has no way of knowing were the data is coming from geometrically. Some vendors use a mechanical inclinometer, but SNC feels this is too user and surface dependent, and would not produce results that were trustworthy enough to be used in the 3DVH calculations.

How can the physicist utilize the Virtual Inclinometer functionality?

The Virtual Inclinometer's primary use is to provide the required gantry angle data necessary for the 3DVH reconstruction, so this is its most obvious use.

The Virtual Inclinometer is such an accurate and useful tool that Sun Nuclear wanted to make it available to the user for ancillary uses. There are two suggested uses for the Virtual Inclinometer functionality, though inventive physicists can think of many more!

VMAT Monthly QA. The physicist can perform monthly VMAT QA by using the same complex VMAT plan each month. Use the Virtual Inclinometer to confirm gantry vs. time remains constant. (i.e. the first month the user can record the gantry angle at Time = 20, 40, and 60 seconds. The following months confirm that the gantry reading is remaining constant at those time intervals.)

Static Gantry Angle Monthly QA. In place of the standard gantry angle test - using a level and recording the gantry angle at 0, 90, 180, and 270 - the physicist can make a treatment plan with four static open fields. Using the Virtual Inclinometer tool, the actual gantry angle could be recorded versus the nominal gantry angle. The accuracy of the Virtual Inclinometer tool is <1 degrees.

Lastly, SNC Patient™ software makes use of the inclinometer to offer gantry QA tools inside the Machine QA tool. We can now perform a Star Shot with static or dynamic fields, and report back the offset of the isocenter per gantry angle. The software will also test the constancy of gantry speed.

Is it correct to say that MLC movement in the 4D Workspace matches the treatment plan, rather than the measurement? How would this benefit a physicist?

Correct. The MLC patterns come directly from the treatment plan. When analyzing a 3DVH result that has unexpected errors, seeing the leaf patterns of the plan can sometimes offer a clue as to whether the plan was over-modulated or not. If not over-modulated, the physicist can move on to other causes, such as modeling issues that would be more visible in the BEV tab.

The main practical use of the 4D Workspace is for the physicist to quickly and easily be able review the dynamics of the plan they are analyzing. The physicist can quickly check modulation patterns, the number of Monitor Units per beam/plan, the number of Control Points, and the Gantry motion vs. MLC leaf motion. Physicists can also review the AC entry/exit dose (in conjunction with the MLC patterns) as a function of time for qualitative analysis of delivery dynamics.

This 4D Workspace is also used in Respiratory MotionSim™ for 4D simulation of tumor motion.

Is it correct to say that MLC movement in the 4D Workspace matches the treatment plan, rather than the measurement? How would this benefit a physicist?

Correct. The MLC patterns come directly from the treatment plan. When analyzing a 3DVH result that has unexpected errors, seeing the leaf patterns of the plan can sometimes offer a clue as to whether the plan was over-modulated or not. If not over-modulated, the physicist can move on to other causes, such as modeling issues that would be more visible in the BEV tab.

The main practical use of the 4D Workspace is for the physicist to quickly and easily be able review the dynamics of the plan they are analyzing. The physicist can quickly check modulation patterns, the number of Monitor Units per beam/plan, the number of Control Points, and the Gantry motion vs. MLC leaf motion. Physicists can also review the AC entry/exit dose (in conjunction with the MLC patterns) as a function of time for qualitative analysis of delivery dynamics.

This 4D Workspace is also used in Respiratory MotionSim™ for 4D simulation of tumor motion.

What am I supposed to do with the Control Point Analysis? How does knowing the details of a sub-arc help the physicist affect the plan?

Control Point Analysis helps the physicist determine where the error is coming from. VMAT QA, when taken as a whole arc is really composite dose QA, which means errors can blur into one another and get hidden. When Control Point Analysis is used, the VMAT QA becomes analogous to Beam by Beam IMRT QA, so that the physicist can easily see beams/sub-arcs that aren't being delivered accurately. For example, if the Couch isn't modeled correctly, the posterior part of the plan will likely have errors. If the MLC's are drifting from their intended position due to gravity, the portions of the plan delivered near gantry angles 90 or 270 will have erroneous readings. Control Point Analysis, very simply, is taking a composite QA back to a sub-arc by sub-arc QA so that the physicist can more easily pick up on sub-arc specific (directional) errors.
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