Relative dosimetrical verification in high dose rate brachytherapy using two-dimensional detector array IMatriXX

Commissioning considerations for the Bravos high-dose-rate afterloader: Towards improving treatment delivery accuracy

BACKGROUND AND PURPOSE

The upgrade of major equipment can be disruptive to clinical operations and introduce risk as policy and procedures need to adapt to new technical possibilities and constraints. We describe here the transition from GammaMedPlus-iX to Bravos in a busy brachytherapy clinic, involving four afterloaders across two sites.

MATERIAL AND METHODS

Our clinic employs three high-dose-rate remote afterloaders in four dedicated treatment vaults at the main site and a fourth afterloader at a regional location. Of more than 600 new HDR treatment plans performed annually, most are planned and treated intraoperatively. Most treatments are for prostate cancer, followed by GYN, intraoperative brachytherapy, GI, and other sites. Applicators used include vendor-provided applicators as well as third party applicators and in-house 3D-printed devices to provide interstitial, intracavitary, intraluminal, and surface treatments. All applicators were commissioned according to recommended guidelines. The choice of tolerances and the design of new procedures were informed by current guidelines and leveraged new HDR afterloader functionalities. A review of clinical operations in the 4 months postupgrade was conducted to evaluate the feasibility of new tolerances and the effectiveness of new procedures.

RESULTS

The procedures outlined improved and standardized afterloader QA and treatment protocols with clear actionable steps for staff to follow to ensure treatments are delivered as planned. Re-commissioning of applicators yielded results similar to those previously reported by other investigators. A review of initial treatment data revealed that in one case, due to the implementation of tight tolerances, obstruction near the tip of the channel was detected and corrected prior to treatment. It confirms that the implementation of the tolerances adopted is feasible and effective in flagging treatment deviations.

CONCLUSION

Enhanced procedures and QA processes were implemented successfully. We established clear actionable steps to follow by staff to ensure that treatments are delivered accurately.

Quantitative study on dose distribution of Freiburg flap for keloid high-dose-rate brachytherapy based on MatriXX

PURPOSE

To quantify the dose distribution effect of insufficient scattering conditions in keloid HDR brachytherapy with Freiburg fFlap (FF) applicator.

MATERIALS AND METHODS

A phantom composed of FF applicator, MatriXX and solid water slices was designed and scanned for treatment planning. Bolus with different thicknesses were covered to offer different scatter conditions. Planar dose distributions were measured by MatriXX. The maximum value (Max), average value (Avg) and γ passing rate (3 mm/3%) were evaluated by the software MyQA Platform.

RESULTS

The maximum and average doses measured by MatriXX were lower than the calculated values. The difference increased as field size decreased. The Max value, found at 0.86 cm level in the two tube case, was -20.0%, and the avg value was -11.9%. All the γ values were less than 95%. This difference gradually decreased with increasing bolus thickness and the γ values were significantly improved.

CONCLUSION

MatriXX could be used for dose verification of HDR brachytherapy with an FF applicator. When the FF applicator was applied for keloid, insufficient scattering conditions would cause an insufficient target dose. This difference could be reduced by covering the bolus with different thicknesses on the applicator. The smaller the field, the thicker the bolus required.

On the potential of 2D ion chamber arrays for high-dose rate remote afterloading brachytherapy quality assurance

Objective. To investigate the potential of 2D ion chamber arrays to serve as a standalone tool for the verification of source strength, positioning and dwell time, within the framework of¹⁹²Ir high-dose rate brachytherapy device quality assurance (QA).Approach.A commercially available ion chamber array was used. Fitting of a 2D Lorentzian peak function to experimental data from a multiple source dwell position irradiation on a frame-by-frame basis, facilitated tracking of the source center orthogonal projection on the array plane. For source air kerma strength verification, Monte Carlo simulation was employed to obtain a chamber array- and source-specific correction factor of calibration with a 6 MV photon beam. This factor converted the signal measured by each ion chamber element to air kerma in free space. A source positioning correction was also applied to lift potential geometry mismatch between experiment and Monte Carlo simulation.Main results.Spatial and temporal accuracy of source movement was verified within 0.5 mm and 0.02 s, respectively, in compliance with the test endpoints recommended by international professional societies. The source air kerma strength was verified experimentally within method uncertainties estimated as 1.44% (k = 1). The source positioning correction method employed did not introduce bias to experimental results of irradiations where source positioning was accurate. Development of a custom jig attachable to the chamber array for accurate and reproducible experimental set up would improve testing accuracy and obviate the need for source positioning correction in air kerma strength verification.Significance.Delivery of a single irradiation plan, optimized based on results of this work, to a 2D ion chamber array can be used for concurrent testing of source position, dwell time and air kerma strength, and the procedure can be expedited through automation. Chamber arrays merit further study in treatment planning QA and real time,in vivodose verification.

Automated multi-parameter high-dose-rate brachytherapy quality assurance via radioluminescence imaging

The purpose of this study is to leverage radioluminescence imaging for the development of an automated high-dose-rate (HDR) brachytherapy quality assurance (QA) system that enables simultaneous measurements of dwell position, dwell time, wire velocity, and relative source strength in a single test. The system consists of a radioluminescence phosphor sheet (a mixture of Gd2O2S:Tb and PDMS) positioned atop a HDR needle applicator, a complementary metal-oxide-semiconductor digital camera used to capture the emitted radioluminescence signals from the scintillator sheet, and an in-house graphical user interface for signal processing. The signal processing was used to extract source intensity, location, and elapsed time, yielding the final measurements on dwell position, dwell time, and wire velocity. The source strength relative to the well chamber calibration (in unit of Air-Kerma strength, Sk ) is measured by establishing a calibration curve that correlates Sk with the detector response. Validation experiments are performed using three customized treatment plans. With these plans, the dwell position and dwell time are verified for a range of 110.0 cm-117.5 cm and 2 s-16 s, respectively, and the linear correlation with Sk is demonstrated for the source strength varying between 28 348 U (cGy cm2 h-1) and 41 906 U. The wire velocity, i.e. the speed of the radioactive source averaged over the distance in between dwell positions, is calculated for various distances ranging from 5 mm to 50 mm. Results show that the mean deviations of the measured dwell position and dwell time are 0.1 mm (range from 0 to 0.2 mm) and 32.5 ms (range from 0 to 60.0 ms) with respect to the planned values, respectively, and the system response is highly linear with Sk ( R2 = 0.998). Moreover, the measured wire velocities are comparable to previously reported values. Benefitting from the compact hardware design and image processing algorithms, the system provides a practical, reliable, and comprehensive solution for HDR QA.

Imaging Cherenkov emission for quality assurance of high-dose-rate brachytherapy

With advances in high-dose-rate (HDR) brachytherapy, the importance of quality assurance (QA) is increasing to ensure safe delivery of the treatment by measuring dose distribution and positioning the source with much closer intervals for highly active sources. However, conventional QA is time-consuming, involving the use of several different measurement tools. Here, we developed simple QA method for HDR brachytherapy based on the imaging of Cherenkov emission and evaluated its performance. Light emission from pure water irradiated by an 192Ir γ-ray source was captured using a charge-coupled device camera. Monte Carlo calculations showed that the observed light was primarily Cherenkov emissions produced by Compton-scattered electrons from the γ-rays. The uncorrected Cherenkov light distribution, which was 5% on average except near the source (within 7 mm from the centre), agreed with the dose distribution calculated using the treatment planning system. The accuracy was attributed to isotropic radiation and short-range Compton electrons. The source positional interval, as measured from the light images, was comparable to the expected intervals, yielding spatial resolution similar to that permitted by conventional film measurements. The method should be highly suitable for quick and easy QA investigations of HDR brachytherapy as it allows simultaneous measurements of dose distribution, source strength, and source position using a single image.

End-to-end test for computed tomography-based high-dose-rate brachytherapy

PURPOSE

One of the important developments in brachytherapy in recent years has been the clinical implementation of complex modern technical procedures. Today, 3D-imaging has become the standard procedure and it is used for contouring and precise position determination and reconstruction of used brachytherapy applicators. Treatment planning is performed on the basis of these imaging methods, followed by data transfer to the afterloading device. Therefore, checking the entire treatment chain is of high importance. In this work, we describe an end-to-end test for computed tomography (CT)-based brachytherapy with an high-dose-rate (HDR) afterloading device, which fulfills the recommendation of the German radiation-protection-commission.

MATERIAL AND METHODS

The treatment chain consists of a SOMATOM S64 CT scanner (Siemens Medical), the treatment planning system (TPS) BrachyVision v.13.7 (VMS), which utilizes the calculation formalism TG-43 and the Acuros algorithm v. 1.5.0 (VMS) as well as GammaMedplus HDR afterloader (VMS) using an Ir-192 source. Measurement setups for common brachytherapy applicators are defined in a water phantom, and the required PMMA applicator holders are developed. These setups are scanned with the CT and the data is imported into the TPS. Computed TPS reference dose values for significant points located on the side of the applicator are compared with dose measurements performed with a PinPoint 3D chamber 31016 (PTW Freiburg).

RESULTS

The deviations for the end-to-end test between computed and measured values are shown to be ≤ 5%, when using an implant needle or vaginal cylinder. Furthermore, it can be demonstrated that the test procedure provides reproducible results, while repositioning the applicators without carrying out a new CT-scan.

CONCLUSIONS

The end-to-end test presented allows a practice-oriented realization for checking the whole treatment chain for HDR afterloading technique and CT-imaging. The presented phantom seems feasible for performing periodic system checks as well as to verify newly introduced brachytherapy techniques with sufficient accuracy.

Verification of high-dose-rate brachytherapy treatment planning dose distribution using liquid-filled ionization chamber array

Purpose

This study aims to investigate the dosimetric performance of a liquid-filled ionization chamber array in high-dose-rate (HDR) brachytherapy dosimetry. A comparative study was carried out with air-filled ionization chamber array and EBT3 Gafchromic films to demonstrate its suitability in brachytherapy.

Material and methods

The PTW OCTAVIUS detector 1000 SRS (IA 2.5-5 mm) is a liquid-filled ionization chamber array of area 11 x 11 cm² and chamber spacing of 2.5-5 mm, whereas the PTW OCTAVIUS detector 729 (IA 10 mm) is an air vented ionization chamber array of area 27 x 27 cm² and chamber spacing of 10 mm. EBT3 films were exposed to doses up to a maximum of 6 Gy and evaluated using multi-channel analysis. The detectors were evaluated using test plans to mimic a HDR intracavitary gynecological treatment. The plan was calculated and delivered with the applicator plane placed 20 mm from the detector plane. The acquired measurements were compared to the treatment plan. In addition to point dose measurement, profile/isodose, gamma analysis, and uncertainty analysis were performed. Detector sensitivity was evaluated by introducing simulated errors to the test plans.

Results

The mean point dose differences between measured and calculated plans were 0.2% ± 1.6%, 1.8% ± 1.0%, and 1.5% ± 0.81% for film, IA 10 mm, and IA 2.5-5 mm, respectively. The average percentage of passed gamma (global/local) values using 3%/3 mm criteria was above 99.8% for all three detectors on the original plan. For IA 2.5-5 mm, local gamma criteria of 2%/1 mm with a passing rate of at least 95% was found to be sensitive when simulated positional errors of 1 mm was introduced.

Conclusion

The dosimetric properties of IA 2.5-5 mm showed the applicability of liquid-filled ionization chamber array as a potential QA device for HDR brachytherapy treatment planning systems.

Quality Assurance Procedures based on Dosimetric, Gamma Analysis as a Fast Reliable Tool for Commissioning Brachytherapy Treatment Planning Systems

Background

Fast and easily repeatable methods for commissioning procedures for brachytherapy (BT) treatment planning systems (TPS) are needed. Radiochromic film dosimetry with gamma analysis is widely used in external beam quality assurance (QA) procedures and planar film dosimetry is also increasingly used for verification of the dose distribution in BT applications. Using the gamma analysis method for comparing calculated and measured dose data could be used for commissioning procedures of the newly developed TG-186 and MBDCA calculation algorithms. The aim of this study was dosimetric verification of the calculation algorithm used in TPS when the CT/MRI ring applicator is used.

Materials and methods

Ring applicators with 26 and 30 mm diameters and a 60 mm intra-uterine tube with 60° angle were used for verification. Gafchromic® EBT films were used as dosimetric media. Dose grids, corresponding to each plane (dosimetric film location), were exported from the TPS as a raw data. Gafchromic® films were digitized after irradiation. gamma analysis of the data were performed using the OMNI Pro I’mRT® system, as recommended by the AAPM TG-119 rapport criterion for gamma analysis of 3%, 3 mm and a level of 95%.

Results

For the 26 mm and 30 mm rings, the average gamma ranged, respectively, from 0.1 to 0.44 and from 0.1 to 0.27. In both cases, 99% of the measured points corresponded with the calculated data.

Conclusions

This analysis showed excellent agreement between the dose distribution calculated with the TPS and the doses measured by Gafchromic films. This finding confirms the viability of using film dosimetry in BT.

To determine the source dwell positions of HDR brachytherapy using 2D 729 ion chamber array

The purpose of this study was to determine the dwell position of a high-dose-rate (HDR) brachytherapy Ir-192 source using a PTW Seven29 2D detector array. A Nucletron Microselectron HDR device and 2D array ionisation chamber, equipped with 729 ionisation chambers uniformly arranged in a 27×27 matrix with an active array area of 27×27 cm2, were used for this study. Different dwell positions were assigned in the HDR machine. Rigid interstitial needles and a vaginal applicator were positioned on the 2D array, which was then exposed according to the programmed dwell positions. Subsequently, the positional accuracy of the source position was analysed. This process was repeated for different dwell positions. The results were analysed using an in-house-developed Excel programme. Different random dwell position checks as well as dwell position measurements were performed using a radiochromic film. The dwell positions measured by the 2D array were found to be in good agreement with those measured by the film. The standard deviations between the doses obtained from the different dwell positions were 0·191828, 0·329973, 0·370632 and 0·779939, whereas the corresponding standard deviations of the doses at the vaginal cylinder were 0·60303, 0·242808, 0·242808 and 0·065309. When the planned and measured dwell positions were plotted, a linear relationship was obtained.

The evaluation of a 2D diode array in “magic phantom” for use in high dose rate brachytherapy pretreatment quality assurance

PURPOSE

High dose rate (HDR) brachytherapy is a treatment method that is used increasingly worldwide. The development of a sound quality assurance program for the verification of treatment deliveries can be challenging due to the high source activity utilized and the need for precise measurements of dwell positions and times. This paper describes the application of a novel phantom, based on a 2D 11 × 11 diode array detection system, named “magic phantom” (MPh), to accurately measure plan dwell positions and times, compare them directly to the treatment plan, determine errors in treatment delivery, and calculate absorbed dose.

METHODS

The magic phantom system was CT scanned and a 20 catheter plan was generated to simulate a nonspecific treatment scenario. This plan was delivered to the MPh and, using a custom developed software suite, the dwell positions and times were measured and compared to the plan. The original plan was also modified, with changes not disclosed to the primary authors, and measured again using the device and software to determine the modifications. A new metric, the “position–time gamma index,” was developed to quantify the quality of a treatment delivery when compared to the treatment plan. The MPh was evaluated to determine the minimum measurable dwell time and step size. The incorporation of the TG-43U1 formalism directly into the software allows for dose calculations to be made based on the measured plan. The estimated dose distributions calculated by the software were compared to the treatment plan and to calibrated EBT3 film, using the 2D gamma analysis method.

RESULTS

For the original plan, the magic phantom system was capable of measuring all dwell points and dwell times and the majority were found to be within 0.93 mm and 0.25 s, respectively, from the plan. By measuring the altered plan and comparing it to the unmodified treatment plan, the use of the position–time gamma index showed that all modifications made could be readily detected. The MPh was able to measure dwell times down to 0.067 ± 0.001 s and planned dwell positions separated by 1 mm. The dose calculation carried out by the MPh software was found to be in agreement with values calculated by the treatment planning system within 0.75%. Using the 2D gamma index, the dose map of the MPh plane and measured EBT3 were found to have a pass rate of over 95% when compared to the original plan.

CONCLUSIONS

The application of this magic phantom quality assurance system to HDR brachytherapy has demonstrated promising ability to perform the verification of treatment plans, based upon the measured dwell positions and times. The introduction of the quantitative position–time gamma index allows for direct comparison of measured parameters against the plan and could be used prior to patient treatment to ensure accurate delivery.

Review of clinical brachytherapy uncertainties: analysis guidelines of GEC-ESTRO and the AAPM

Background and purpose

A substantial reduction of uncertainties in clinical brachytherapy should result in improved outcome in terms of increased local control and reduced side effects. Types of uncertainties have to be identified, grouped, and quantified.

Methods

A detailed literature review was performed to identify uncertainty components and their relative importance to the combined overall uncertainty.

Results

Very few components (e.g., source strength and afterloader timer) are independent of clinical disease site and location of administered dose. While the influence of medium on dose calculation can be substantial for low energy sources or non-deeply seated implants, the influence of medium is of minor importance for high-energy sources in the pelvic region. The level of uncertainties due to target, organ, applicator, and/or source movement in relation to the geometry assumed for treatment planning is highly dependent on fractionation and the level of image guided adaptive treatment. Most studies to date report the results in a manner that allows no direct reproduction and further comparison with other studies. Often, no distinction is made between variations, uncertainties, and errors or mistakes. The literature review facilitated the drafting of recommendations for uniform uncertainty reporting in clinical BT, which are also provided. The recommended comprehensive uncertainty investigations are key to obtain a general impression of uncertainties, and may help to identify elements of the brachytherapy treatment process that need improvement in terms of diminishing their dosimetric uncertainties. It is recommended to present data on the analyzed parameters (distance shifts, volume changes, source or applicator position, etc.), and also their influence on absorbed dose for clinically-relevant dose parameters (e.g., target parameters such as D₉₀ or OAR doses). Publications on brachytherapy should include a statement of total dose uncertainty for the entire treatment course, taking into account the fractionation schedule and level of image guidance for adaptation.

Conclusions

This report on brachytherapy clinical uncertainties represents a working project developed by the Brachytherapy Physics Quality Assurances System (BRAPHYQS) subcommittee to the Physics Committee within GEC-ESTRO. Further, this report has been reviewed and approved by the American Association of Physicists in Medicine.