HDR brachytherapy afterloader quality assurance optimization using monolithic silicon strip detectors

BACKGROUND

There currently exists no widespread high dose-rate (HDR) brachytherapy afterloader quality assurance (QA) tool for simultaneously assessing the afterloader's positional, temporal, transit velocity and air kerma strength accuracy.

PURPOSE

The purpose of this study was to develop a precise and rigorous technique for performing daily QA of HDR brachytherapy afterloaders, incorporating QA of: dwell position accuracy, dwell time accuracy, transit velocity consistency and relative air kerma strength (AKS) of an Ir-192 source.

METHOD

A Sharp ProGuide 240 mm catheter (Elekta Brachytherapy, Veenendaal, The Netherlands) was fixed 5 mm above a 256 channel epitaxial diode array 'dose magnifying glass' (DMG256) (Centre for Medical and Radiation Physics, University of Wollongong). Three dwell positions, each of 5.0 s dwell times, were spaced 13.0 mm apart along the array with the Flexitron HDR afterloader (Elekta Brachytherapy, Veenendaal, The Netherlands). The DMG256 was connected to a data acquisition system (DAQ) and a computer via USB2.0 link for live readout and post-processing. The outputted data files were analyzed using a Python script to provide positional and temporal localization of the Ir-192 source by tracking the centroid of the detected response. Measurements were repeated on a weekly basis, for a period of 5 weeks to determine the consistency of the measured parameters over an extended period.

RESULTS

Using the DMG256 for relative AKS measurements resulted in measured values within 0.6%-3.0% of the expected activity over a 7-week period. The sub-millisecond temporal accuracy of the device allowed for measurements of the transit velocity with an average of (10.88 ± 1.01) cm/s for 13 mm steps. The dwell position localization for 1, 2, 3, 5, and 10 mm steps had an accuracy between 0.1 and 0.3 mm (3σ), with a fixed temporal accuracy of 10 ms.

CONCLUSION

The DMG256 silicon strip detector allows for clinics to perform rigorous daily QA of HDR afterloader dwell position and dwell time accuracy with greater precision than the current standard methodology using closed circuit television and a stopwatch. Additionally, DMG256 unlocks the ability to perform measurements of transit velocity/time and relative AKS, which are not possible using current standard techniques.

On the evaluation of edgeless diode detectors for patient-specific QA in high-dose stereotactic radiosurgery

PURPOSE

In this work, the potential of an innovative "edgeless" silicon diode was evaluated as a response to the still unmet need of a reliable tool for plan dosimetry verification of very high dose, non-coplanar, patient-specific radiosurgery treatments. In order to prove the effectiveness of the proposed technology, we focused on radiosurgical treatments for functional disease like tremor or pain.

METHODS

The edgeless diodes response has been validated with respect to clinical practice standard detectors by reproducing the reference dosimetry data adopted for the Treatment Planning System. In order to evaluate the potential for radiosurgery patient-specific treatment plan verification, the anthropomorphic phantom Alderson RANDO has been adopted along with three edgeless sensors, one placed in the centre of the Planning Target Volume, one superiorly and one inferiorly.

RESULTS

The reference dosimetry data obtained from the edgeless detectors are within 2.6% for output factor, off-axis ratio and well within 2% for tissue phantom ratio when compared to PTW 60,018 diode. The edgeless detectors measure a dose discrepancy of approximately 3.6% from the mean value calculated by the TPS. Larger discrepancies are obtained in very steep gradient dose regions when the sensors are placed outside the PTV.

CONCLUSIONS

The angular independent edgeless diode is proposed as an innovative dosimeter for patient quality assurance of brain functional disorders and other radiosurgery treatments. The comparison of the diode measurements with TPS calculations confirms that edgeless diodes are suitable candidates for patient-specific dosimetric verification in very high dose ranges delivered by non-isocentric stereotactic radiosurgery modalities.

Quality assurance of VMAT on flattened and flattening filter-free accelerators using a high spatial resolution detector

PURPOSE

This study investigated the use of high spatial resolution solid-state detectors (DUO and Octa) combined with an inclinometer for machine-based quality assurance (QA) of Volumetric Modulated Arc Therapy (VMAT) with flattened and flattening filter-free beams.

METHOD

The proposed system was inserted in the accessory tray of the gantry head of a Varian 21iX Clinac and a Truebeam linear accelerator. Mutual dependence of the dose rate (DR) and gantry speed (GS) was assessed using the standard Varian customer acceptance plan (CAP). The multi-leaf collimator (MLC) leaf speed was evaluated under static gantry conditions in directions parallel and orthogonal to gravity as well as under dynamic gantry conditions. Measurements were compared to machine log files.

RESULTS

DR and GS as a function of gantry angle were reconstructed using the DUO/inclinometer and in agreement to within 1% with the machine log files in the sectors of constant DR and GS. The MLC leaf speeds agreed with the nominal speeds and those extracted from the machine log files to within 0.03 cm s-1 . The effect of gravity on the leaf motion was only observed when the leaves traveled faster than the nominal maximum velocity stated by the vendor. Under dynamic gantry conditions, MLC leaf speeds ranging between 0.33 and 1.42 cm s-1 were evaluated. Comparing the average MLC leaf speeds with the machine log files found differences between 0.9% and 5.7%, with the largest discrepancy occurring under conditions of fastest leaf velocity, lowest DR and lowest detector signal.

CONCLUSIONS

The investigation on the use of solid-state detectors in combination with an inclinometer has demonstrated the capability to provide efficient and independent verification of DR, GS, and MLC leaf speed during dynamic VMAT delivery. Good agreement with machine log files suggests the detector/inclinometer system is a useful tool for machine-specific VMAT QA.

Evaluation of the PTW microDiamond in edge-on orientation for dosimetry in small fields

Purpose

The PTW microDiamond has an enhanced spatial resolution when operated in an edge‐on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS‐483 code of practice for small field dosimetry. In this work the suitability of an edge‐on orientation and advantages over the recommended face‐on orientation will be presented.

Methods

The PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm² to a 0.5 × 0.5 cm² field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge‐on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms.

Results

The OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm² field size. The edge‐on orientation overresponds in the build‐up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge‐on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm² field. The PTW microDiamond in edge‐on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes.

Conclusions

The microDiamond was shown to be suitable for small field dosimetry when operated in edge‐on orientation. When edge‐on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.

Characterization of an organic semiconductor diode for dosimetry in radiotherapy

PURPOSE

The development of novel detectors for dosimetry in advanced radiotherapy modalities requires materials that have a water equivalent response to ionizing radiation such that characterization of radiation beams can be performed without the need for complex calibration procedures and correction factors. Organic semiconductors are potentially an ideal technology in fabricating devices for dosimetry due to tissue equivalence, mechanical flexibility, and relatively cheap manufacturing cost. The response of a commercial organic photodetector (OPD), coupled to a plastic scintillator, to ionizing radiation from a linear accelerator and orthovoltage x-ray tube has been characterized to assess its potential as a dosimeter for radiotherapy. The radiation hardness of the OPD has also been investigated to demonstrate its longevity for such applications.

METHODS

Radiation hardness measurements were achieved by observing the response of the OPD to the visible spectrum and 70 keV x rays after pre-exposure to 40 kGy of ionizing radiation. The response of a preirradiated OPD to 6-MV photons from a linear accelerator in reference conditions was compared to a nonirradiated OPD with respect to direct and indirect (RP400 plastic scintillator) detection mechanisms. Dose rate dependence of the OPD was measured by varying the surface-to-source distance between 90 and 300 cm. Energy dependence was characterized from 29.5 to 129 keV with an x-ray tube. The percentage depth dose (PDD) curves were measured from 0.5 to 20 cm and compared to an ionization chamber.

RESULTS

The OPD sensitivity to visible light showed substantial degradation of the broad 450 to 600 nm peak from the donor after irradiation to 40 kGy. After irradiation, the spectral shape has a dominant absorbance peak at 370 nm, as the acceptor better withstood radiation damage. Its response to x rays stabilized to 30% after 35 kGy, with a 0.5% difference between 770 Gy increments. The OPD exhibited reproducible detection of ionizing radiation when coupled with a scintillator. Indirect detection showed a linear response from 25 to 500 cGy and constant response to dose rates from 0.31 Gy/pulse to 3.4 × 10^-4  Gy/pulse. However, without the scintillator, response increased by 100% at low dose rates. Energy independence between 100 keV and 1.2 MeV advocates their use as a dosimeter without beam correction factors. A dependence on the scintillator thickness used during a comparison of the PDD to the ionizing chamber was identified. A 1-mm-thick scintillator coupled with the OPD demonstrated the best agreement of ± 3%.

CONCLUSIONS

The response of OPDs to ionizing radiation has been characterized, showing promising use as a dosimeter when coupled with a plastic scintillator. The mechanisms of charge transport and trapping within organic materials varies for visible and ionizing radiation, due to differing properties for direct and indirect detection mechanisms and observing a substantial decrease in sensitivity to the visible spectrum after 40 kGy. This study proved that OPDs produce a stable response to 6-MV photons, and with a deeper understanding of the charge transport mechanisms due to exposure to ionizing radiation, they are promising candidates as the first flexible, water equivalent, real-time dosimeter.

Initial testing of a pixelated silicon detector prototype in proton therapy

As technology continues to develop, external beam radiation therapy is being employed, with increased conformity, to treat smaller targets. As this occurs, the dosimetry methods and tools employed to quantify these fields for treatment also have to evolve to provide increased spatial resolution. The team at the University of Wollongong has developed a pixelated silicon detector prototype known as the dose magnifying glass (DMG) for real‐time small‐field metrology. This device has been tested in photon fields and IMRT. The purpose of this work was to conduct the initial performance tests with proton radiation, using beam energies and modulations typically associated with proton radiosurgery. Depth dose and lateral beam profiles were measured and compared with those collected using a PTW parallel‐plate ionization chamber, a PTW proton‐specific dosimetry diode, EBT3 Gafchromic film, and Monte Carlo simulations. Measurements of the depth dose profile yielded good agreement when compared with Monte Carlo, diode and ionization chamber. Bragg peak location was measured accurately by the DMG by scanning along the depth dose profile, and the relative response of the DMG at the center of modulation was within 2.5% of that for the PTW dosimetry diode for all energy and modulation combinations tested. Real‐time beam profile measurements of a 5 mm 127 MeV proton beam also yielded FWHM and FW90 within ±1 channel (0.1 mm) of the Monte Carlo and EBT3 film data across all depths tested. The DMG tested here proved to be a useful device at measuring depth dose profiles in proton therapy with a stable response across the entire proton spread‐out Bragg peak. In addition, the linear array of small sensitive volumes allowed for accurate point and high spatial resolution one‐dimensional profile measurements of small radiation fields in real time to be completed with minimal impact from partial volume averaging.

Evaluation of the dosimetric properties of a diode detector for small field proton radiosurgery

The small fields and sharp gradients typically encountered in proton radiosurgery require high spatial resolution dosimetric measurements, especially below 1–2 cm diameters. Radiochromic film provides high resolution, but requires postprocessing and special handling. Promising alternatives are diode detectors with small sensitive volumes (SV) that are capable of high resolution and real‐time dose acquisition. In this study we evaluated the PTW PR60020 proton dosimetry diode using radiation fields and beam energies relevant to radiosurgery applications. Energies of 127 and 157 MeV (9.7 to 15 cm range) and initial diameters of 8, 10, 12, and 20 mm were delivered using single‐stage scattering and four modulations (0, 15, 30, and 60 mm) to a water tank in our treatment room. Depth dose and beam profile data were compared with PTW Markus N23343 ionization chamber, EBT2 Gafchromic film, and Monte Carlo simulations. Transverse dose profiles were measured using the diode in "edge‐on" orientation or EBT2 film. Diode response was linear with respect to dose, uniform with dose rate, and showed an orientation‐dependent (i.e., beam parallel to, or perpendicular to, detector axis) response of less than 1%. Diode vs. Markus depth‐dose profiles, as well as Markus relative dose ratio vs. simulated dose‐weighted average lineal energy plots, suggest that any LET‐dependent diode response is negligible from particle entrance up to the very distal portion of the SOBP for the energies tested. Finally, while not possible with the ionization chamber due to partial volume effects, accurate diode depth‐dose measurements of 8, 10, and 12 mm diameter beams were obtained compared to Monte Carlo simulations. Because of the small SV that allows measurements without partial volume effects and the capability of submillimeter resolution (in edge‐on orientation) that is crucial for small fields and high‐dose gradients (e.g., penumbra, distal edge), as well as negligible LET dependence over nearly the full the SOBP, the PTW proton diode proved to be a useful high‐resolution, real‐time metrology device for small proton field radiation measurements such as would be encountered in radiosurgery applications.

PACS numbers: 87.56.‐v, 87.56.jf, 87.56.Fc

Proton beam scattering system optimization for clinical and research applications

PURPOSE

To develop and test a method for optimizing and constructing a dual scattering system in passively scattered proton therapy.

METHODS

A beam optics optimization algorithm was developed to optimize the thickness of the first scatterer (S1) and the profile (of both the high-Z material and Lexan) of the second scatterer (S2) to deliver a proton beam matching a given set of parameters, including field diameter, fluence, flatness, and symmetry. A new manufacturing process was also tested that allows the contoured second scattering foil to be created much more economically and quickly using Cerrobend casting. Two application-specific scattering systems were developed and tested using both experimental and Monte Carlo techniques to validate the optimization process described.

RESULTS

A scattering system was optimized and constructed to deliver large uniform irradiations of radiobiology samples at low dose rates. This system was successfully built and tested using film and ionization chambers. The system delivered a uniform radiation field of 50 cm diameter (to a dose of ± 7% of the central axis) while the depth dose profile could be tuned to match the specifications of the particular investigator using modulator wheels and range shifters. A second scattering system for intermediate field size (4 cm < diameter < 10 cm) stereotactic radiosurgery and radiation therapy (SRS and SRT) treatments was also developed and tested using GEANT4. This system improved beam efficiency by over 70% compared with existing scattering systems while maintaining field flatness and depth dose profile. In both cases the proton range uniformity across the radiation field was maintained, further indicating the accuracy of the energy loss formalism in the optimization algorithm.

CONCLUSIONS

The methods described allow for rapid prototyping of scattering foils to meet the demands of both research and clinical beam delivery applications in proton therapy.

Confirmation of Skin Doses Resulting from Bolus Effect of Intervening Alpha-cradle and Carbon Fiber Couch in Radiotherapy

In this study, we verified the treatment planning calculations of skin doses with the incorporation of the bolus effect due to the intervening alpha-cradle (AC) and carbon fiber couch (CFC) using radiochromic EBT2 films. A polystyrene phantom (25 × 25 × 15 cm3) with six EBT2 films separated by polystyrene slabs, at depths of 0, 0.1, 0.2, 0.5, 1, 1.4 cm, was positioned above an AC, which was ~1 cm thick. The phantom and AC assembly were CT scanned and the CT-images were transferred to the treatment planning system (TPS) for calculations in three scenarios: (A) ignoring AC and CFC, (B) accounting for AC only, (C) accounting for both AC and CFC. A single posterior 10 × 10 cm2 field, a pair of posterior-oblique 10 × 10 cm2 fields, and a posterior IMRT field (6 MV photons from a Varian Trilogy linac) were planned. For each radiation field configuration, the same MU were used in all three scenarios in the TPS. Each plan for scenario C was delivered to expose a stack of EBT2 films in the phantom through AC and CFC. In addition, in vivo EBT2 film measurement on a lung cancer patient immobilized with AC undergoing IMRT was also included in this study. Point doses and planar distributions generated from the TPS for the three scenarios were compared with the data from the EBT2 film measurements. For all the field arrangements, the EBT2 film data including the in vivo measurement agreed with the doses calculated for scenario (C), within the uncertainty of the EBT2 measurements (~4%). For the single posterior field (a pair of posterior-oblique fields), the TPS generated doses were lower than the EBT2 doses by 34%, 33%, 31%, 13% (34%, 31%, 31%, 11%) for scenario A and by 27%, 25%, 22%, 8% (25%, 21%, 21%, 6%) for scenario B at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. For the IMRT field, the 2D dose distributions at each depth calculated in scenario C agree with those measured data. When comparing the central axis doses for the IMRT field, we found the TPS generated doses for scenario A (B) were lower than the EBT2 data by 35%, 34%, 31%, 16% (29%, 26%, 23%, 10%) at the depths of 0, 0.1, 0.2, 0.5 cm, respectively. There were no significant differences for the depths of 1.0 and 1.4 cm for all the radiation fields studied. TPS calculation of doses in the skin layers accounting for AC and CFC was verified by EBT2 film data. Ignoring the presence of AC and/or CFC in TPS calculation would significantly underestimate the doses in the skin layers. For the clinicians, as more hypofractionated regimens and stereotactic regimens are being used, this information will be useful to avoid potential serious skin toxicities, and also assist in clinical decisions and report these doses accurately to relevant clinical trials/cooperative groups, such as RTOG.

Improvements in dose calculation accuracy for small off-axis targets in high dose per fraction tomotherapy

PURPOSE

A recent field safety notice from TomoTherapy detailed the underdosing of small, off-axis targets when receiving high doses per fraction. This is due to angular undersampling in the dose calculation gantry angles. This study evaluates a correction method to reduce the underdosing, to be implemented in the current version (v4.1) of the TomoTherapy treatment planning software.

METHODS

The correction method, termed "Super Sampling" involved the tripling of the number of gantry angles from which the dose is calculated during optimization and dose calculation. Radiochromic film was used to measure the dose to small targets at various off-axis distances receiving a minimum of 21 Gy in one fraction. Measurements were also performed for single small targets at the center of the Lucy phantom, using radiochromic film and the dose magnifying glass (DMG).

RESULTS

Without super sampling, the peak dose deficit increased from 0% to 18% for a 10 mm target and 0% to 30% for a 5 mm target as off-axis target distances increased from 0 to 16.5 cm. When super sampling was turned on, the dose deficit trend was removed and all peak doses were within 5% of the planned dose. For measurements in the Lucy phantom at 9.7 cm off-axis, the positional and dose magnitude accuracy using super sampling was verified using radiochromic film and the DMG.

CONCLUSIONS

A correction method implemented in the TomoTherapy treatment planning system which triples the angular sampling of the gantry angles used during optimization and dose calculation removes the underdosing for targets as small as 5 mm diameter, up to 16.5 cm off-axis receiving up to 21 Gy.