3DVH FAQs. What is PDP questions

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1 3DVH FAQs What is PDP questions 1. Explain the PDP in layman terms. How does PDP work? a. Very simply, PDP uses measured diode data, and compares it to the expected treatment plan data. The differences between Measured and Expected doses 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 delivered 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 we audit both the TPS and the functioning of the Linac. 2. Does 3DVH take into consideration inhomogeneities? Does it use the CT of the patient? a. The heterogeneity of the patient is inherently 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 we know the dose deposition, we inherently know the tissue density. By perturbing the original doses (which are based on the heterogeneities of the patient), the heterogeneity is taken into account from the beginning. 3. 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? a. 3DVH would not catch this error, but there are already several ways to catch CT to Electron Density curve errors 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. b. 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. c. The cost of incorporating heterogeneity into 3DVH would be performing a full forward calculation, which is an approach other companies have taken. We chose instead to base our QA on measured

2 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 is a difference between the QA and the TPS, the question rightly becomes, who do you trust? Because 3DVH is based on measured data, allows confirmation through ion chamber measurements), and has numerous papers published on its accuracy, the results carry more weight. 4. How is 3DVH capable of moving from the calculation of dose in a homogeneous phantom to dose differences in a heterogeneous patient? a. 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. b. These papers are the currently published accuracy studies on 3DVH, all of which show remarkable accuracy. i. Validation of measurement-guided 3D VMAT dose reconstruction on a heterogeneous anthropomorphic phantom Opp D, Nelms BE, Zhang G, Stevens C, and Feygelman V., JACMP (accepted, in copyediting) ii. Motion as a perturbation: Measurement-guided dose estimates to moving patient voxels during modulated arc deliveries* Feygelman V, Stambaugh C, Zhang G, Hunt D, Opp D, Wolf TK, Nelms BE., Med Phys Feb;40(2): (11 pp). *Editor s Choice iii. VMAT QA: Measurement-guided 4D dose reconstruction on a patient Vladimir Feygelman Moffitt Cancer Center, MedPhys July 2012, AC-PDP iv. 3D DVH-based metric analysis versus per-beam planar analysis in IMRT pretreatment verification. Carrasco P, Jornet N, Latorre A, Eudaldo T, Ruiz A, Ribas M., Med Phys Aug;39(8): v. Per-beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors. Nelms BE, Zhen H, Tomé WA. Med Phys Feb;38(2): vi. Moving from gamma passing rates to patient DVHbased QA metrics in pretreatment dose QA. Zhen H, Nelms BE, Tome WA., Med Phys Oct;38(10): 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. a. Sun Nuclear carefully considered the various approaches possible for determining Dose in a patient volume before developing 3DVH. We

3 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 their TPS system has been validated and continually improved for years, the argument can be made that TPS should receive preference in this argument. Sun Nuclear, by using the TPS data as a starting point and then perturbing the treatment plan based solely on measured errors, is allowing the user to compare a Measurement- Guided 3D dose reconstruction to the Expected treatment plan dose, without adding another algorithm as an extra variable/possible source of error. It is often the most elegant solution that is also the best one. 6. If I use MapCHECK 2 to measure dose errors, the detector density is not nearly as high as the 2-3 mm density of the TPS dose in phantom. How do you accurately produce a high density dose difference map using a low density measurement? a. It is true that we need a high/full density measure dose grid to adequately compare the measured dose to the expected treatment plan dose. Intelligent interpolation (or Smarterpolation ) is our solution. Using Smarterpolation (see white paper on website) we actually reconstruct a full density measured dose grid using the measured diode doses, along with the isodose lines of the original treatment plan. The isodose lines of the treatment plan inform the interpolation between measured dose points (rather than drawing a straight line between points), but please note that the measured dose is never altered in any way. The isodose data merely assists in filling the gaps between measured data points in an intelligent manner. We thereby maintain the fidelity of the measured data while reconstructing a full density error map. 7. What is the Voxel size used by 3DVH? Does it scale to the TPS voxel size? What do the differences in voxel size do to the accuracy of the results? a. 3DVH reconstructs at a very high-resolution internal dose grid resolution precisely so that we don t want to have to interpolate results and introduce uncertainty. The internal Dose Grid size for AC-PDP is < 2mm, and for MC-PDP it is 1mm. b. This high resolution dose grid enables us to compare the 3DVH and TPS results without having to interpolate the 3DVH results. The TPS data is left in its original state (pixel size) as received from the TPS. 8. What is the DVH bin size? a. 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).

4 3DVH assigns dose bins using the maximum dose of each plan in order to use the highest number of bins for a given plan. 3DVH uses the following bin sizes: i. for global max < 1 Gy, bin size = Gy; ii. 1 < global max < 10 Gy, bin size = Gy; iii. 10 Gy < global max < 100 Gy, bin size = 0.01 Gy iv. global max > 100 Gy, bin size = 0.1 Gy. b. From this, you can see that the number of bins is at minimum 1000 and up to 10,000, assuring high resolution of DVH bin width and Quick Stat computation. 9. When 3DVH is performing the planned dose perturbation, is the dose difference for each voxel applied evenly across the patent dose? a. 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. 10. I don t believe that 3DVH gives me any additional relevant information or further analysis of my measurements - why should I buy it? a. 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. 11. How accurate is 3DVH calculation? How can you prove it if you cannot measure inside the patient? a. Very accurate there are several publications that have analyzed the accuracy of 3DVH, all of which have had excellent results. To prove that the algorithm works there are several approaches. The approach that is most similar to a patient measurement would be 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 paper listed below.

5 i. Validation of measurement-guided 3D VMAT dose reconstruction on a heterogeneous anthropomorphic phantom Opp D, Nelms BE, Zhang G, Stevens C, and Feygelman V., JACMP (accepted, in copyediting) b. A second approach is to already know the correct answer and test if 3DVH is able to reproduce this 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 being begun with the erroneous treatment plan), then we 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. c. Papers on accuracy: i. Validation of measurement-guided 3D VMAT dose reconstruction on a heterogeneous anthropomorphic phantom Opp D, Nelms BE, Zhang G, Stevens C, and Feygelman V., JACMP (accepted, in copyediting) ii. Motion as a perturbation: Measurement-guided dose estimates to moving patient voxels during modulated arc deliveries* Feygelman V, Stambaugh C, Zhang G, Hunt D, Opp D, Wolf TK, Nelms BE., Med Phys Feb;40(2): (11 pp). *Editor s Choice iii. VMAT QA: Measurement-guided 4D dose reconstruction on a patient Vladimir Feygelman Moffitt Cancer Center, MedPhys July 2012, AC-PDP iv. 3D DVH-based metric analysis versus per-beam planar analysis in IMRT pretreatment verification. Carrasco P, Jornet N, Latorre A, Eudaldo T, Ruiz A, Ribas M., Med Phys Aug;39(8): v. Per-beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors. Nelms BE, Zhen H, Tomé WA. Med Phys Feb;38(2): vi. Moving from gamma passing rates to patient DVHbased QA metrics in pretreatment dose QA. Zhen H, Nelms BE, Tome WA., Med Phys Oct;38(10): How can 3DVH give accurate results for my specific linac if you do not take measurements from it? a. 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. The purpose of our model isn t a forward calculation; if it was, we would in fact need to capture the small differences from machine to machine such as transmission differences, PDD differences, TPS differences, and so on. b. What we do instead is to acquire datasets from multiple similar machines (i.e. Varian, 120 millennium MLC, and 6MV beam) and

6 create a Golden Machine model which represents the mean of several representative machines with the same Vendor, MLC-type, and Energy. c. Since we are Measurement-Guided, even if your machine is different from our model, we will detect those differences from the measured data at multiple depths that we acquire 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 morph to match the measurement. This is the essence of Planned Dose Perturbation algorithm we morph the planned dose using the measurement. d. Finally, the reason we do create Machine, MLC, and Energy-specific models is that we want to perturb or morph 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 morphing the dose to match the measurements, but the end result would appear more noisy since the algorithm would have to work much harder and make significant changes to the starting point Golden Machine data. Practical Use questions 13. 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? a. This is a clinical question that physicists and doctors will have to discuss in order to determine the correct answer for their clinic, much like doctors/physicists had to do when IGRT was first implemented. b. 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. c. One of our most recent clinical sites shared with us 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

7 is just another example of how a protocol could be described, and not intended as a recommendation from Sun Nuclear DVH sounds like it s opening Pandora s Box - what would we do if we found errors of concern in the delivered patient dose? a. More information is always a good thing, though it can be intimidating. 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, which is an incredible benefit! As with IGRT, sometimes not knowing was/is easier, but patients greatly benefited once we could shift them correctly prior to treatment. Patients will likewise greatly benefit from using 3DVH to determine what is actually being delivered to the patient prior to treatment finding systematic errors, or patientspecific errors by analyzing the delivery of the dose in patient anatomy BEFORE the patient is treated is a tremendous step forward in radiation therapy. b. 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 overprocessing 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. 15. I m used to exporting 2D planar dose how do I export 3D dose files for the patient and phantom? a. This varies by TPS, but the user should export the following Dicom RT files: i. For the Patient Plan RT Plan, RT Dose, RT Struct, CT Images ii. 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.) b. Eclipse use File Export Wizard and select the appropriate files listed above. Use an appropriate DICOM Export Filter and save the files. c. Pinnacle use File Export DICOM and select the appropriate files listed above, along with sum of selected prescriptions

8 d. Monaco/XiO use File Export DICOM and select the appropriate files listed above. e. For more detailed instructions, please use the User s Guide - TPS Data for ArcCHECK document. 16. I m interested in the new merge feature (Patient 6.2 release). To what degree will this affect the resolution/spacing? And will it significantly aid SRS/SBRT case QA integrity? a. 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. b. 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 produce a large magnitude error unintentionally. c. The increased density will not be usable by 3DVH until a future release. 17. How much longer will it take me to do my IMRT QA if I start using 3DVH? a. To use 3DVH, measurement time on the Linac is not increased at all. The measurements performed on an ArcCHECK or MapCHECK2 are exactly the same whether or not 3DVH is used. b. The 3DVH perturbation calculation takes approximately of 2-3 minutes for RapidArc plans with 2 Arcs. For IMRT plans the calculation is usually less than 1-2 minutes. c. 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 do 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. 18. I don t have an ArcCHECK - why can t I use 3DVH for Rotational VMAT QA using the MapCHECK 2 or EPIDose measurements? a. ArcCHECK was specifically designed for VMAT QA because the complexity of the treatment (specifically the dynamic Gantry rotation and speed) called out for a device that was 3 dimensional. This allows the physicist to get a large field size, density, and range of depth

9 readings throughout an Arc beam. ArcCHECK not only has a surface that is always normal to the beam, it also allows for two depths of measurement entrance and exit dose. This is the ideal arrangement for VMAT QA and we highly recommend the ArcCHECK over 2D arrays because of the loss of data with 2D arrays due to angular shadowing of the detectors. b. Based on the above, the reason we cannot perform a 3DVH calculation is that there is simply not enough data to do an accurate calculation on a VMAT plan when using a 2D array. 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. This means that at angles such as 90 or 270 degrees, there are very few diodes collecting data as the 2D array approaches becoming a 1D array. 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. Tools questions 19. How can the physicist utilize the Virtual Inclinometer functionality? a. 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. b. 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! c. VMAT Monthly QA - the physicist can perform their monthly VMAT QA by using the same, complex VMAT plan each month. Use the Virtual Inclinometer to confirm that the Gantry vs Time at several points 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.) d. 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,

10 and 270 the physicists 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. e. Lastly, the Patient software (as of version 6.2) 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. 20. For the 4D Workspace, the MLC movement is simply what it was in the Treatment Plan, and not what it actually is, correct? How would this workstation benefit a physicist? a. 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. b. 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 can review the Gantry motion vs. MLC leaf motion. c. 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. d. This 4D workspace will also be used in future applications (NOT YET RELEASED) for 4D simulation of tumor motion. 21. 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? a. Simply put, 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. Or 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.