Helical tomotherapy quality assurance

Quantitative assessment of helical tomotherapy plans complexity

PURPOSE

An unnecessary amount of complexity in radiotherapy plans affects the efficiency of the treatments, increasing the uncertainty of dose deposition and its susceptibility to anatomical changes or setup errors. To date, tools for quantitatively assessing the complexity of tomotherapy plans are still limited. In this study, new metrics were developed to characterize different aspects of helical tomotherapy (HT) plans, and their actual effectiveness was investigated.

METHODS

The complexity of 464 HT plans delivered on a Radixact platform was evaluated. A new set of metrics was devised to assess beam geometry, leaf opening time (LOT) variability, and modulation over space and time. Sixty-five complexity metrics were extracted from the dataset using the newly in-house developed software library TCoMX: 29 metrics already proposed in the literature and 36 newly developed metrics. Their reciprocal relation is discussed. Their effectiveness was evaluated through correlation analyses with patient-specific quality assurance (PSQA) results.

RESULTS

An inverse linear relation was found between the average number of closed leaves and the average number of MLC openings and closures as well as between the choice of the modulation factor and the discontinuity of the field, suggesting some intrinsic link between the LOT distribution and the geometrical complexity of the MLC openings. The newly proposed metrics were at least as correlated as the existing ones to the PSQA results. Metrics describing the geometrical complexity of the MLC openings showed the strongest connection to the PSQA results. Significant correlations were found between at least one of the new metrics and the γ index passing rate P R γ % ( 3 % G , 2 mm ) $P{R}_{\gamma}\%(3\%G,2\textit{mm})$ for six out of seven groups of plans considered.

CONCLUSION

The new metrics proposed were shown to be effective to characterize more comprehensively the complexity of HT plans. A software library for their automatic extraction is described and made available.

Comparison of conformal radiotherapy, intensity-modulated radiotherapy and tomotherapy irradiation techniques in prostate cancers

Aim

The aim of this study is to compare three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT) and tomotherapy techniques used in the treatment of prostate cancer with target and critical organ doses to be included.

Materials and Methods

The target dose was studied with 4-and 6-field 3D-CRT, 7-field IMRT and tomotherapy techniques used to treat ten patients for prostate cancer and the dose volume histograms of critical organs were analyzed. The same target volumes, critical organs doses prescribed and treatment times for the three techniques were compared. Total dose of 76 Gy was given using 6 MV and 18 MV for 3D-CRT, 6 MV for IMRT and tomotherapy techniques.

Results

When we compare the three techniques, for rectum V35(p:0·001), V65(p:0·001), D50(p:0·020) and D25(p:0·002), for bladder V50(p:0·027), V65(p:0·006), V100(p:0·006) and for femoral head, the V50(p:0·001) dose was found to be significantly different and more favourable in the tomotherapy technique. Significant differences were found with IMRT planning in 50% of bladder volume (p:0·002). There is no significant difference between the three techniques for doses of 100% volume of rectum and 25% of volume of bladder. The minimum dose that healthy tissue received which was outside the tumour volume was investigated.

Findings

Doses to critical organs were lower using the tomotherapy technique. However, the minimum doses that healthy tissue received were higher for the tomotherapy technique. When the beam on times were compared for all three techniques, a significant difference was found in favor of tomotherapy.

FMEA of manual and automated methods for commissioning a radiotherapy treatment planning system

PURPOSE

To evaluate the level of risk involved in treatment planning system (TPS) commissioning using a manual test procedure, and to compare the associated process-based risk to that of an automated commissioning process (ACP) by performing an in-depth failure modes and effects analysis (FMEA).

METHODS

The authors collaborated to determine the potential failure modes of the TPS commissioning process using (a) approaches involving manual data measurement, modeling, and validation tests and (b) an automated process utilizing application programming interface (API) scripting, preloaded, and premodeled standard radiation beam data, digital heterogeneous phantom, and an automated commissioning test suite (ACTS). The severity (S), occurrence (O), and detectability (D) were scored for each failure mode and the risk priority numbers (RPN) were derived based on TG-100 scale. Failure modes were then analyzed and ranked based on RPN. The total number of failure modes, RPN scores and the top 10 failure modes with highest risk were described and cross-compared between the two approaches. RPN reduction analysis is also presented and used as another quantifiable metric to evaluate the proposed approach.

RESULTS

The FMEA of a MTP resulted in 47 failure modes with an RPN_ave of 161 and S_ave of 6.7. The highest risk process of "Measurement Equipment Selection" resulted in an RPN_max of 640. The FMEA of an ACP resulted in 36 failure modes with an RPN_ave of 73 and S_ave of 6.7. The highest risk process of "EPID Calibration" resulted in an RPN_max of 576.

CONCLUSIONS

An FMEA of treatment planning commissioning tests using automation and standardization via API scripting, preloaded, and pre-modeled standard beam data, and digital phantoms suggests that errors and risks may be reduced through the use of an ACP.

Effects of changing modulation and pitch parameters on tomotherapy delivery quality assurance plans

This study was aimed at investigating delivery quality assurance (DQA) discrepancies observed for helical tomotherapy plans. A selection of tomotherapy plans that initially failed the DQA process was chosen for this investigation. These plans failed the fluence analysis as assessed using gamma criteria (3%, 3 mm) with radiographic film. Each of these plans was modified (keeping the planning constraints the same), beamlets rebatched and reoptimized. By increasing and decreasing the modulation factor, the fluence in a circumferential plane as measured with a diode array was assessed. A subset of these plans was investigated using varied pitch values. Metrics for each plan that were examined were point doses, fluences, leaf opening times, planned leaf sinograms, and uniformity indices. In order to ensure that the treatment constraints remained the same, the dose-volume histograms (DVHs) of all the modulated plans were compared to the original plan. It was observed that a large increase in the modulation factor did not significantly improve DVH uniformity, but reduced the gamma analysis pass rate. This also increased the treatment delivery time by slowing down the gantry rotation speed which then increases the maximum to mean non-zero leaf open time ratio. Increasing and decreasing the pitch value did not substantially change treatment time, but the delivery accuracy was adversely affected. This may be due to many other factors, such as the complexity of the treatment plan and site. Patient sites included in this study were head and neck, right breast, prostate, abdomen, adrenal, and brain. The impact of leaf timing inaccuracies on plans was greater with higher modulation factors. Point-dose measurements were seen to be less susceptible to changes in pitch and modulation factors. The initial modulation factor used by the optimizer, such that the TPS generated 'actual' modulation factor within the range of 1.4 to 2.5, resulted in an improved deliverable plan.

A method for testing the performance and the accuracy of the binary MLC used in helical tomotherapy

During a helical tomotherapy a binary MLC is used for fluence modulation. The 64 pneumatically driven leaves of the MLC are either completely open or closed. The fast and frequent leaf movements result in a high demand of accuracy and stability of the MLC. This article is based on the analytical investigation of the accuracy and the stability of the MLC. Different patterns of MLC movements were generated to investigate the characteristics of the MLC. One of the considered aspects contains the friction between the leaves. The influence of variations of the compressed air on the MLC was also explored. The integrated MVCT detector of the tomotherapy system deposits the treatment data in a matrix. The detector is triggered with the linear accelerator, which is pulsed by 300Hz. The data matrix is available after the treatment. An IDL (Interactive Data Language) routine was programmed in order to analyse the matrix. The points of time, at which the leaves open (POT), and the period, in which the leaves stay open (LOT), were measured and compared with the desired values. That procedure has been repeated several times a week for approximately 6 months to investigate the stability of the MLC. Relative deviations of the LOT from -0.4% to -5.4% were measured. The friction between the leaves had no significant influence on the LOT. The available compressed air, that is used to move the leaves, depends on the number of moving leaves and also on the previous movements of the MLC. Variations of the compressed air resulted in deviations of the LOT from -1.8% to -3.7%. The measured POT deviates from the programmed POT up to -18.4ms±0.7ms. This maximal deviation correlates with a shift of the gantry angle of 0.52̊ which is negligible. The MLC has shown a stable behaviour over the 6 months. A separate consideration of the leaves showed no higher standard deviation of the LOT than ±0.7ms during the investigated time. The variation between the different leaves is much higher than the deviations of LOT caused by friction and changes of compressed air. The deviations of the LOT vary between -2.6ms and -11.0ms. The developed method is feasible in order to recognize a deterioration of the MLC performance.

A quality assurance tool for helical tomotherapy using a step-wedge phantom and the on-board MVCT detector

The purpose of this study was to develop and evaluate filmless quality assurance (QA) tools for helical tomotherapy by using the signals from the on-board megavoltage computed tomography (MVCT) detector and applying a dedicated step-wedge phantom. The step-wedge phantom is a 15 cm long step-like aluminum block positioned on the couch. The phantom was moved through the slit beam and MVCT detector signals were analyzed. Two QA procedures were developed, with gantry fixed at 0°: 1) step-wedge procedure: to check beam energy consistency, field width, laser alignment with respect to the virtual isocenter, couch movement, and couch velocity; and 2) completion procedure: to check the accuracy of a field abutment made by the tomotherapy system after a treatment interruption. The procedures were designed as constancy tool and were validated by measurement of deliberately induced variations and comparison with a reference method. Two Hi-Art II machines were monitored over a period of three years using the step-wedge procedures. The data acquisition takes 5 minutes. The analysis is fully automated and results are available directly after acquisition. Couch speed deviations up to 2% were induced. The mean absolute difference between expected and measured couch speed was 0.2% ± 0.2% (1 standard deviation SD). Field width was varied around the 10 mm nominal size, between 9.7 and 11.1 mm, in steps of 0.2 mm. Mean difference between the step-wedge analysis and the reference method was < 0.01 mm ± 0.03 mm (1 SD). Laser (mis)alignment relative to a reference situation was detected with 0.3 mm precision (1SD). The step-wedge profile was fitted to a PDD in water. The PDD ratio D20/D10, measured at depths of 20 cm and 10 cm, was used to check beam energy consistency. Beam energy variations were induced. Mean difference between step-wedge and water PDD ratios was 0.2% ± 0.3% (1SD). The completion procedure was able to reveal abutment mismatches with a mean error of -0.6 mm ± 0.2 mm (1SD). The trending data over a period of three years showed a mean deviation of 0.4% ± 0.1% (1 SD) in couch speed. The spread in field width was 0.15 mm (1 SD). The sagittal and transverse lasers showed a variation of 0.5 mm (1 SD). Beam energy varied 1.0% (1 SD). A mean abutment mismatch was found of -0.4 mm ± 0.2 mm (1 SD) between interrupted treatments. The on-board MVCT detector, in combination with the step-wedge phantom, is a suitable tool for a QA program for helical tomotherapy. The method allowed frequent monitoring of machine behavior for the past three years.

Independent quality assurance of a helical tomotherapy machine using the dose magnifying glass

PURPOSE

Helical tomotherapy is a complex delivery technique, integrating CT image guidance and intensity modulated radiotherapy in a single system. The integration of the CT detector ring on the gantry not only allows patient position verification but is also often used to perform various QA procedures. This convenience lacks the rigor of a machine-independent QA process.

METHODS

In this article, a Si strip detector, known as the Dose Magnifying Glass (DMG), was used to perform machine-independent QA measurements of the multileaf collimator alignment, leaf open time threshold, and leaf fluence output factor (LFOF).

RESULTS

The DMG measurements showed good agreements with EDR2 film for the MLC alignment test while the CT detector agrees well with DMG measurements for leaf open time threshold and LFOF measurements. The leaf open time threshold was found to be approximately 20 ms. The LFOF measured with the DMG agreed within error with the CT detector measured LFOF.

CONCLUSIONS

The DMG with its 0.2 mm spatial resolution coupled to TERA ASIC allowed real-time high temporal resolution measurements of the tomotherapy leaf movement. In conclusion, DMG was shown to be a suitable tool for machine-independent QA of a tomotherapy unit.

Treatment planning to improve delivery accuracy and patient throughput in helical tomotherapy

PURPOSE

To investigate delivery quality assurance (DQA) discrepancies observed for a subset of helical tomotherapy patients.

METHODS AND MATERIALS

Six tomotherapy patient plans were selected for analysis. Three had passing DQA ion chamber (IC) measurements, whereas 3 had measurements deviating from the expected dose by more than 3.0%. All plans used similar parameters, including: 2.5 cm field-width, 15-s gantry period, and pitch values ranging from 0.143 to 0.215. Preliminary analysis suggested discrepancies were associated with plans having predominantly small leaf open times (LOTs). To test this, patients with failing DQA measurements were replanned using an increased pitch of 0.287. New DQA plans were generated and IC measurements performed. Exit fluence data were also collected during DQA delivery for dose reconstruction purposes.

RESULTS

Sinogram analysis showed increases in mean LOTs ranging from 29.8% to 83.1% for the increased pitch replans. IC measurements for these plans showed a reduction in dose discrepancies, bringing all measurements within +/-3.0%. The replans were also more efficient to deliver, resulting in reduced treatment times. Dose reconstruction results were in excellent agreement with IC measurements, illustrating the impact of leaf-timing inaccuracies on plans having predominantly small LOTs.

CONCLUSIONS

The impact of leaf-timing inaccuracies on plans with small mean LOTs can be considerable. These inaccuracies result from deviations in multileaf collimator latency from the linear approximation used by the treatment planning system and can be important for plans having a 15-s gantry period. The ability to reduce this effect while improving delivery efficiency by increasing the pitch is demonstrated.

Quality assurance needs for modern image-based radiotherapy: recommendations from 2007 interorganizational symposium on "quality assurance of radiation therapy: challenges of advanced technology"

This report summarizes the consensus findings and recommendations emerging from 2007 Symposium, "Quality Assurance of Radiation Therapy: Challenges of Advanced Technology." The Symposium was held in Dallas February 20-22, 2007. The 3-day program, which was sponsored jointly by the American Society for Therapeutic Radiology and Oncology (ASTRO), American Association of Physicists in Medicine (AAPM), and National Cancer Institute (NCI), included >40 invited speakers from the radiation oncology and industrial engineering/human factor communities and attracted nearly 350 attendees, mostly medical physicists. A summary of the major findings follows. The current process of developing consensus recommendations for prescriptive quality assurance (QA) tests remains valid for many of the devices and software systems used in modern radiotherapy (RT), although for some technologies, QA guidance is incomplete or out of date. The current approach to QA does not seem feasible for image-based planning, image-guided therapies, or computer-controlled therapy. In these areas, additional scientific investigation and innovative approaches are needed to manage risk and mitigate errors, including a better balance between mitigating the risk of catastrophic error and maintaining treatment quality, complimenting the current device-centered QA perspective by a more process-centered approach, and broadening community participation in QA guidance formulation and implementation. Industrial engineers and human factor experts can make significant contributions toward advancing a broader, more process-oriented, risk-based formulation of RT QA. Healthcare administrators need to appropriately increase personnel and ancillary equipment resources, as well as capital resources, when new advanced technology RT modalities are implemented. The pace of formalizing clinical physics training must rapidly increase to provide an adequately trained physics workforce for advanced technology RT. The specific recommendations of the Symposium included the following. First, the AAPM, in cooperation with other advisory bodies, should undertake a systematic program to update conventional QA guidance using available risk-assessment methods. Second, the AAPM advanced technology RT Task Groups should better balance clinical process vs. device operation aspects--encouraging greater levels of multidisciplinary participation such as industrial engineering consultants and use-risk assessment and process-flow techniques. Third, ASTRO should form a multidisciplinary subcommittee, consisting of physician, physicist, vendor, and industrial engineering representatives, to better address modern RT quality management and QA needs. Finally, government and private entities committed to improved healthcare quality and safety should support research directed toward addressing QA problems in image-guided therapies.