J Smilowitz

A feasible method for clinical delivery verification and dose reconstruction in tomotherapy

Delivery verification is the process in which the energy fluence delivered during a treatment is verified. This verified energy fluence can be used in conjunction with an image in the treatment position to reconstruct the full three-dimensional dose deposited. A method for delivery verification that utilizes a measured database of detector signal is described in this work. This database is a function of two parameters, radiological path-length and detector-to-phantom distance, both of which are computed from a CT image taken at the time of delivery. Such a database was generated and used to perform delivery verification and dose reconstruction. Two experiments were conducted: a simulated prostate delivery on an inhomogeneous abdominal phantom, and a nasopharyngeal delivery on a dog cadaver. For both cases, it was found that the verified fluence and dose results using the database approach agreed very well with those using previously developed and proven techniques. Delivery verification with a measured database and CT image at the time of treatment is an accurate procedure for tomotherapy. The database eliminates the need for any patient-specific, pre- or post-treatment measurements. Moreover, such an approach creates an opportunity for accurate, real-time delivery verification and dose reconstruction given fast image reconstruction and dose computation tools.

AAPM Medical Physics Practice Guideline 5.a.: Commissioning and QA of Treatment Planning Dose Calculations - Megavoltage Photon and Electron Beams

The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.The American Association of Physicists in Medicine (AAPM) is a nonprofit professional society whose primary purposes are to advance the science, education and professional practice of medical physics. The AAPM has more than 8,000 members and is the principal organization of medical physicists in the United States. The AAPM will periodically define new practice guidelines for medical physics practice to help advance the science of medical physics and to improve the quality of service to patients throughout the United States. Existing medical physics practice guidelines will be reviewed for the purpose of revision or renewal, as appropriate, on their fifth anniversary or sooner. Each medical physics practice guideline represents a policy statement by the AAPM, has undergone a thorough consensus process in which it has been subjected to extensive review, and requires the approval of the Professional Council. The medical physics practice guidelines recognize that the safe and effective use of diagnostic and therapeutic radiology requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guidelines and technical standards by those entities not providing these services is not authorized. The following terms are used in the AAPM practice guidelines: Must and Must Not: Used to indicate that adherence to the recommendation is considered necessary to conform to this practice guideline. Should and Should Not: Used to indicate a prudent practice to which exceptions may occasionally be made in appropriate circumstances.

State of dose prescription and compliance to international standard (ICRU-83) in intensity modulated radiation therapy among academic institutions

PURPOSE The purpose of this study was to evaluate dose prescription and recording compliance to international standard (International Commission on Radiation Units & Measurements [ICRU]-83) in patients treated with intensity modulated radiation therapy (IMRT) among academic institutions. METHODS AND MATERIALS Ten institutions participated in this study to collect IMRT data to evaluate compliance to ICRU-83. Under institutional review board clearance, data from 5094 patients-including treatment site, technique, planner, physician, prescribed dose, target volume, monitor units, planning system, and dose calculation algorithm-were collected anonymously. The dose-volume histogram of each patient, as well as dose points, doses delivered to 100% (D100), 98% (D98), 95% (D95), 50% (D50), and 2% (D2), of sites was collected and sent to a central location for analysis. Homogeneity index (HI) as a measure of the steepness of target and is a measure of the shape of the dose-volume histogram was calculated for every patient and analyzed. RESULTS In general, ICRU recommendations for naming the target, reporting dose prescription, and achieving desired levels of dose to target were relatively poor. The nomenclature for the target in the dose prescription had large variations, having every permutation of name and number contrary to ICRU recommendations. There was statistically significant variability in D95, D50, and HI among institutions, tumor site, and technique with P values < .01. Nearly 95% of patients had D50 higher than 100% (103.5 ± 6.9) of prescribed dose and varied among institutions. On the other hand, D95 was close to 100% (97.1 ± 9.4) of prescribed dose. Liver and lung sites had a higher D50 compared with other sites. Pelvic sites had a lower variability indicated by HI (0.13 ± 1.21). Variability in D50 is 101.2 ± 8.5, 103.4 ± 6.8, 103.4 ± 8.2, and 109.5 ± 11.5 for IMRT, tomotherapy, volume modulated arc therapy, and stereotactic body radiation therapy with IMRT, respectively. CONCLUSIONS Nearly 95% of patient treatments deviated from the ICRU-83 recommended D50 prescription dose delivery. This variability is significant (P < .01) in terms of treatment site, technique, and institution. To reduce dosimetric and associated radiation outcome variability, dose prescription in every clinical trial should be unified with international guidelines.

Monte Carlo simulations of patient dose perturbations in rotational-type radiotherapy due to a transverse magnetic field: A tomotherapy investigation

PURPOSE Several groups are exploring the integration of magnetic resonance (MR) image guidance with radiotherapy to reduce tumor position uncertainty during photon radiotherapy. The therapeutic gain from reducing tumor position uncertainty using intrafraction MR imaging during radiotherapy could be partially offset if the negative effects of magnetic field-induced dose perturbations are not appreciated or accounted for. The authors hypothesize that a more rotationally symmetric modality such as helical tomotherapy will permit a systematic mediation of these dose perturbations. This investigation offers a unique look at the dose perturbations due to homogeneous transverse magnetic field during the delivery of Tomotherapy(®) Treatment System plans under varying degrees of rotational beamlet symmetry. METHODS The authors accurately reproduced treatment plan beamlet and patient configurations using the Monte Carlo code geant4. This code has a thoroughly benchmarked electromagnetic particle transport physics package well-suited for the radiotherapy energy regime. The three approved clinical treatment plans for this study were for a prostate, head and neck, and lung treatment. The dose heterogeneity index metric was used to quantify the effect of the dose perturbations to the target volumes. RESULTS The authors demonstrate the ability to reproduce the clinical dose-volume histograms (DVH) to within 4% dose agreement at each DVH point for the target volumes and most planning structures, and therefore, are able to confidently examine the effects of transverse magnetic fields on the plans. The authors investigated field strengths of 0.35, 0.7, 1, 1.5, and 3 T. Changes to the dose heterogeneity index of 0.1% were seen in the prostate and head and neck case, reflecting negligible dose perturbations to the target volumes, a change from 5.5% to 20.1% was observed with the lung case. CONCLUSIONS This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

Database Energy Fluence Verification and the Importance of On-Board CT Imaging in Dose Reconstruction

Helical tomotherapy is a radiation therapy technique that utilizes a rotating fan beam of intensity-modulated radiation to deliver dose. The integration of the accelerator, multi-leaf collimator (MLC), and a computed tomography (CT) detector on a single ring gantry allows for dose reconstruction, in which the full 3D dose delivered to the patient is reconstructed1. The incident energy fluence computed using the detected signal2 and a CT image in the treatment position are the inputs for any model-based dose computation algorithm such as convolution/superposition (C/S) or Monte Carlo (MC).

Tomographic Verification of Tomotherapy -- Before, During, and After Treatment

Many improvements in conformal radiotherapy have allowed for increased precision in treatment delivery, and the goal of tomotherapy is to carry these improvements further. However, the ability to deliver highly conformal radiation therapy treatments is only useful to the extent that the patient’s position and anatomy are known. Certainly, it is important to set-up the patient correctly, but ideally, one should also verify that a patient’s internal organs are correctly positioned and that they have not changed in size or shape. This verification can be achieved through the integration of tomographic imaging capabilities. Tomotherapy systems will feature such tomographic verification, initally through megavoltage CT (MVCT), and potentially with on-board kilovoltage CT (kVCT) in future implementations. In addition to being important to treatments, tomographic images are also necessary for dose reconstruction to verify the treatment delivery [1,2].

Implementation of the validation testing in MPPG 5.a “Commissioning and QA of treatment planning dose calculations–megavoltage photon and electron beams”

Abstract The AAPM Medical Physics Practice Guideline (MPPG) 5.a provides concise guidance on the commissioning and QA of beam modeling and dose calculation in radiotherapy treatment planning systems. This work discusses the implementation of the validation testing recommended in MPPG 5.a at two institutions. The two institutions worked collaboratively to create a common set of treatment fields and analysis tools to deliver and analyze the validation tests. This included the development of a novel, open‐source software tool to compare scanning water tank measurements to 3D DICOM‐RT Dose distributions. Dose calculation algorithms in both Pinnacle and Eclipse were tested with MPPG 5.a to validate the modeling of Varian TrueBeam linear accelerators. The validation process resulted in more than 200 water tank scans and more than 50 point measurements per institution, each of which was compared to a dose calculation from the institutions treatment planning system (TPS). Overall, the validation testing recommended in MPPG 5.a took approximately 79 person‐hours for a machine with four photon and five electron energies for a single TPS. Of the 79 person‐hours, 26 person‐hours required time on the machine, and the remainder involved preparation and analysis. The basic photon, electron, and heterogeneity correction tests were evaluated with the tolerances in MPPG 5.a, and the tolerances were met for all tests. The MPPG 5.a evaluation criteria were used to assess the small field and IMRT/VMAT validation tests. Both institutions found the use of MPPG 5.a to be a valuable resource during the commissioning process. The validation testing in MPPG 5.a showed the strengths and limitations of the TPS models. In addition, the data collected during the validation testing is useful for routine QA of the TPS, validation of software upgrades, and commissioning of new algorithms.

TH-EF-BRB-06: Intensity Modulated Imaging?: Clinical Workflow for Fluence Field Modulated CT On a TomoTherapy System

Purpose: This work presents a new approach that uses the multi leaf collimator present on TomoTherapy devices to modulate the imaging beam. This allows for targeted imaging in which only regions of clinical importance for IGRT (patient positioning confirmation) receive a high level of image quality. This allows the total imaging dose to decrease, especially for healthy tissue. Methods: A clinical TomoTherapy machine was programmed to deliver imaging dose such that small pre-defined volumes of interest (VOI) received relatively higher levels of image quality with respect to surrounding regions. Four different size ROIs were placed at varying distances from isocenter. The noise and mean CT number were compared between VOI and un-modulated scans. Dose distributions were generated using a treatment planning dose calculator. A clinically feasible workflow was developed to implement this technique that used treatment planning contours to define VOI positions. Results: The VOI-FFMCT technique produced an image noise within 5% of an unmodulated scan for all size VOIs. The VOI technique required a total imaging dose of 0.61, 0.69, 0.60, and 0.50 times the “full dose” acquisition dose for VOI sizes/locations of 10/13/10/6 cm in diameter and located 0/2/5/6 cm from isocenter respectively. Mean CT numbers for the VOI scans were within 1% of the unmodulated case. Conclusion: This approach, which combines state-of-the-art radiation therapy fluence control with a novel imaging approach has been implemented on a clinical system with no hardware or software changes. A clinically feasible method for implementing this technique was developed which requires no additional user input relative to the current procedures in our clinic. The method allows physicians to choose between: (1) better image quality at no dose penalty or (2) equal image quality while reducing dose levels relative to today’s standard of care. Research grants from GE HealthCare and TomoTherapy.

Long‐term dosimetric stability of multiple TomoTherapy delivery systems

&NA; The dosimetric stability of six TomoTherapy units was analyzed to investigate changes in performance over time and with system upgrades. Energy and output were tracked using monitor chamber signal, onboard megavoltage computed tomography (MVCT) detector profile, and external ion chamber measurements. The systems (and monitoring periods) include three Hi‐Art (67, 61, and 65 mos.), two TomoHDA (31 and 26 mos.), and one Radixact unit (11 mos.), representing approximately 10 years of clinical use. The four newest systems use the Dose Control Stability (DCS) system and Fixed Target Linear Accelerator (linac) (FTL). The output stability is reported as deviation from reference monitor chamber signal for all systems and/or from an external chamber signal. The energy stability was monitored using relative (center versus off‐axis) MVCT detector signal (beam profile) and/or the ratio of chamber measurements at 2 depths. The clinical TomoHDA data were used to benchmark the Radixact stability, which has the same FTL but runs at a higher dose rate. The output based on monitor chamber data of all systems is very stable. The standard deviation of daily output on the non‐DCS systems was 0.94–1.52%. As expected, the DCS systems had improved standard deviation: 0.004–0.06%. The beam energy was also very stable for all units. The standard deviation in profile flatness was 0.23–0.62% for rotating target systems and 0.04–0.09% for FTL. Ion chamber output and PDD ratios supported these results. The output stability on the Radixact system during extended treatment delivery (20, 30, and 40 min) was comparable to a clinical TomoHDA system. For each system, results are consistent between different measurement tools and techniques, proving not only the dosimetric stability, but also these quality parameters can be confirmed with various metrics. The replacement history over extended time periods of the major dosimetric components of the different delivery systems (target, linac, and magnetron) is also reported.

SU-F-P-11: Long Term Dosimetric Stability of 6 TomoTherapy Systems

PURPOSE The dosimetric stability of six TomoTherapy units was analyzed to investigate changes in performance over time and with system upgrades. METHODS Energy and output were tracked using monitor chamber signal, onboard MVCT detector signal and external ion chamber measurements. The systems (and monitoring periods) include 3 Hi-Art (67, 61 and 65 mos.), 2 HDA (29 and 25 mos.) and one research unit (7 mo.). Dose Control Stability system (DCS) was installed on 4 systems. Output stability is reported as deviation from reference monitor chamber signal for all systems, and from an external chamber for 4 systems. Energy stability was monitored using the relative (center versus off-axis) MVCT detector signal and/or the ratio of chamber measurements at 2 depths. The results from the clinical systems were used to benchmark the stability of the research unit, which has the same linear accelerator but runs at a higher dose rate. RESULTS The output based on monitor chamber data of all six systems is very stable. Non- DCS had a standard deviation of 1.7% and 1.8%. As expected, DCS systems had improved standard deviation: 0.003-0.05%. The energy was also very stable for all units. The standard deviation in exit detector flatness was 0.02-0.3%. Ion chamber output and 20/10 cm ratios supported these results. The stability for the research system, as monitored with a variety of metrics, is on par with the existing systems. CONCLUSION The output and energy of six TomoTherapy units over a total of almost 10 years is quite stable. For each system, the results are consistent between the different measurement tools and techniques, proving not only the dosimetric stability, but that these quality parameters can be confirmed with various metrics. A research unit operating at a higher dose rate performed as well as the clinical treatment units. University of Wisconsin and Accuray Inc. (vendor of TomoTherapy systems) have a research agreement which supplies funds for research to the University. This project was partially supporting with these funds.