2D Ion Chamber Array Research Articles

Carbon ion mono-energetic and spread-out Bragg peak measurements using nanocomposite Fricke gel dosimeters with LET-independent response

This study investigates the LET-dependence of dose measurements performed with a nanocomposite Fricke gel (NC-FG) upon irradiation with carbon ion beams to address the problem of quenching of gel dosimeters in high-LET particle therapy. The preparation of the NC-FG in a simple gel preparation device was performed for atmospheric and anoxic environmental conditions and two different container materials. Irradiation was performed with carbon ion beams at energies of 133.39MeV/u or 194.87MeV/u, complemented by a modulated 10 mm carbon ion spread-out Bragg peak (SOBP) irradiation. The R1 profiles obtained from the MRI readout were compared to the treatment planning system and a 2D-ionisation chamber array measurement performed for the mono-energetic and SOBP irradiations, respectively. For different dose levels, no difference in the gel response was obtained for the 3D-printed plastic containers, while the glass vials exhibited a linear dose response. Additional use of the glove box changes produced similar results. For both the mono-energetic and the SOBP irradiation, a linear dose response without quenching was observed. For depths ranging up to the Bragg peak position, the mono-energetic profiles deviated by less than ±15% from the planned dose distribution, excluding the lowest entrance surface dose (ESD). For the SOBP profiles, the deviation from the measured dose distribution was less than ±10% for depths between 20 mm and 33 mm. This study presents a LET-independent measurement of mono-energetic carbon ion Bragg peaks and, for the first time, a LET-independent measurement of a carbon ion SOBP with the NC-FG prepared in a simple gel preparation device under atmospheric conditions. The NC-FG provides a possible solution for 3D dosimetry in carbon ion radiotherapy.

Lung radiotherapy verification with a 3D printed thorax phantom and an ionisation chamber array

In this study, a 3D printing error inspired the development of a novel method for using a sagittally-sliced 3D printed thorax phantom to perform dosimetric verifications of lung radiotherapy treatment methods using a 2D ionization chamber array. A full-size thorax model was designed for 3D printing with multiple tissue densities including lung and bone and printed as a series of 2.4 cm sagittal slices using a Raise 3D Pro dual nozzle printer (Raise 3D Technologies Inc, Irvine, USA). An error introduced midway through printing resulted in one half of the phantom being printed at unrealistically high densities. A method was therefore devised whereby the entire phantom was used to plan two lung treatments, one conventionally fractionated and one hypo-fractionated, which were then verified via measurements using an Octavius 729 ionisation chamber array (PTW-Freiburg GmbH, Freiburg, Germany) in combination with several correctly-printed slices of the phantom. The measurements allowed dose distributions in planes through the target, adjacent to the target and at the location key of organs at risk to be verified, for both treatment plans. This method has the potential to be adapted for use with other phantoms and other dosimetry arrays to allow efficient evaluation of future treatment techniques.

Patient-Specific Quality Assurance in Pencil Beam Scanning by 2-Dimensional Array

PurposeThis study aimed to determine the characteristics of 2D ionization chamber array and the confidence limits of the gamma passing rate in pencil beam scanning proton therapy. Materials and Methods:The Varian ProBeam Compact spot-scanning system and the PTW OCTAVIUS 1500XDR array were used as a proton therapy system and detector, respectively. Our methods consisted of 2 parts: (1) the characteristics of the detector were tested and (2) patient-specific quality assurance was performed and evaluated by gamma analysis using dose-difference and distance-to-agreement criteria of 3% and 2 mm, respectively, with 123 treatment plans in head and neck, breast, chest, abdomen, and pelvic regions. Results:The PTW OCTAVIUS 1500XDR array had good reproducibility, uniformity, linearity, repetition rate, and monitor unit per spot within 0.1%, with accuracy, energy dependence, and measurement depth within 0.5%. The overall uncertainty of the PTW OCTAVIUS 1500XDR array was 2.49%. For field size and range shifter, using gamma analysis, the passing rate was 100%. The overall results of patient-specific quality assurance with the gamma evaluation were 98.9% ± 1.6% in 123 plans and confidence limit was 95.7%. Conclusion:The PTW OTAVIUS 1500XDR offered effective performance in pencil beam scanning proton therapy.

Experimental validation of a 4D dynamic dose calculation model for proton pencil beam scanning without spot time stamp considering free-breathing motion.

We developed a 4-dimensional dynamic dose (4DDD) calculation model for proton pencil beam scanning (PBS). This model incorporates the spill start time for all energies and uses the remaining irradiated spot time model instead of irradiated spot time logs. This study aimed to validate the calculation accuracy of a log file-based 4DDD model by comparing it with dose measurements performed under free-breathing conditions, thereby serving as an alternative approach to the conventional log file-based system. Three cubic verification plans were created using a heterogeneous block phantom; these plans were created using 10 phase 4D-CT datasets of the phantom. The CIRS dynamic platform was used to simulate motion with amplitudes of 2.5, 3.75, and 5.0mm. These plans consisted of eight- and two-layered rescanning techniques. The lateral profiles were measured using a 2D ionization chamber array (2D-array) and EBT3 Gafchromic films at four starting phases, including three sinusoidal curves (periods of 3, 4, and 6s) and a representative patient curve during actual treatment. 4DDDs were calculated using in-house scripting that assigned a time stamp to each spot and performed dose accumulation using deformable image registration. Furthermore, to evaluate the impact of parameter selection on our 4DDD model calculations, simulations were performed assuming a ±10% change in irradiation time stamp (0.8±0.08s) and spot scan speed. We evaluated the 2D gamma index and the absolute point doses between the calculated values and the measurements. The 2D-array measurements revealed that the gamma scores for the static plans (no motion) and 4DDD plans exceeded 97.5% and 93.9% at 3%/3mm, respectively. The average gamma score of the 4DDD plans was at least 96.1%. When using EBT3 films, the gamma scores of the 4DDD model exceeded 92.4% and 98.7% at 2%/2mm and 3%/3mm, respectively. Regarding the 4DDD point dose differences, more than 95% of the dose regions exhibited discrepancies within ±5.0% for 97.7% of the total points across all plans. The spot time assignment accuracy of our 4DDD model was acceptable even with ±10% sensitivity. However, the accuracy of the scan speed, when varied within ±10%sensitivity, was not acceptable (minimum gamma scores of 82.6% and maximum dose difference of 12.9%). Our 4DDD calculations under free-breathing conditions using amplitudes of less than 5.0mm were in good agreement with the measurements regardless of the starting phases, breathing curve patterns (between 3 and 6s periods), and varying numbers of layered rescanning. The proposed system allows us to evaluate actual irradiated doses in various breathing periods, amplitudes, and starting phases, even on PBS machines without the ability to record spot logs.

An evaluation of the use of DirectSPR images for proton planning in the RayStation treatment planning software.

An important source of uncertainty in proton therapy treatment planning is the assignment of stopping-power ratio (SPR) from CT data. A commercial product is now available that creates an SPR map directly from dual-energy CT (DECT). This paper investigates the use of this new product in proton treatment planning and compares the results to the current method of assigning SPR based on a single-energy CT (SECT). Two tissue surrogate phantoms were CT scanned using both techniques. The SPRs derived from single-energy CT and by DirectSPR™ were compared to measured values. SECT-based values agreed with measurements within 4% except for low density lung and high density bone, which differed by 13% and 8%, respectively. DirectSPR™ values were within 2% of measured values for all tissues studied. Both methods were also applied to scanned containers of three types of animal tissue, and the expected range of protons of two different energies was calculated in the treatment planning system and compared to the range measured using a multi-layer ion chamber. The average difference between range measurements and calculations based on SPR maps from dual- and single-energy CT, respectively, was 0.1mm (0.07%) versus 2.2mm (1.5%). Finally, a phantom was created using a layer of various tissue surrogate plugs on top of a 2D ion chamber array. Dose measurements on this array were compared to predictions using both single- and dual-energy CTs and SPR maps. While standard gamma pass rates for predictions based on DECT-derived SPR maps were slightly higher than those based on single-energy CT, the differences were generally modest for this measurement setup. This study showed that SPR maps created by the commercial product from dual-energy CT can successfully be used in RayStation to generate proton dose distributions and that these predictions agree well with measurements.

Validation of a digital method for patient-specific verification of VMAT treatment using a 2D ionisation detector array

VMAT treatment involves the delivery of complex beam parameters including the dynamic multi-leaf collimators (MLC) from a linac. The use of a detector array has become a standard method to verify the treatment accuracy before the treatment planned is delivered to the patient with some limitations. This study aims to evaluate the accuracy of a high-resolution, digital method using linac log data for VMAT treatment verification. The log data was obtained from a Versa HD (Elekta ltd., Crawley, UK) linac during VMAT treatment. All the dynamic treatment parameters including MLC position, and MLC speed, were recorded at the rate of 4 Hz in the log data. The treatment delivery was also measured simultaneously using an Octavius 1500 2D ionisation chamber array (PTW, Freiburg, Germany) during the log data measurements. The parameters were extracted and analysed using algorithms written in MATLAB (MathWorks, Natick, MA). The results demonstrated that the log data can track MLC leaves with an accuracy of 1 mm at speeds ranging from 3.04 to 3.40 cm/s. Evaluation of the fluence generated for the VMAT delivery using log data was shown to agree well with the planned dose distribution measured in the 2D detector array, with an average gamma pass rate of 97.5% at 3%/3 mm. Log data obtained a higher gamma pass rate of 98.5% compared to the 2D detector array of 97.5%. The digital linac log data provides the basis for an essential high-resolution real-time verification tool that may be used for routine VMAT verification.

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 of 192Ir 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 vivo dose verification.

Evaluating Proton Dose and Associated Range Uncertainty Using Daily Cone-Beam CT.

PurposeThis study aimed to quantitatively evaluate the range uncertainties that arise from daily cone-beam CT (CBCT) images for proton dose calculation compared to CT using a measurement-based technique.MethodsFor head and thorax phantoms, wedge-shaped intensity-modulated proton therapy (IMPT) treatment plans were created such that the gradient of the wedge intersected and was measured with a 2D ion chamber array. The measured 2D dose distributions were compared with 2D dose planes extracted from the dose distributions using the IMPT plan calculated on CT and CBCT. Treatment plans of a thymoma cancer patient treated with breath-hold (BH) IMPT were recalculated on 28 CBCTs and 9 CTs, and the resulting dose distributions were compared.ResultsThe range uncertainties for the head phantom were determined to be 1.2% with CBCT, compared to 0.5% for CT, whereas the range uncertainties for the thorax phantom were 2.1% with CBCT, compared to 0.8% for CT. The doses calculated on CBCT and CT were similar with similar anatomy changes. For the thymoma patient, the primary source of anatomy change was the BH uncertainty, which could be up to 8 mm in the superior–inferior (SI) direction.ConclusionWe developed a measurement-based range uncertainty evaluation method with high sensitivity and used it to validate the accuracy of CBCT-based range and dose calculation. Our study demonstrated that the CBCT-based dose calculation could be used for daily dose validation in selected proton patients.

A High-Throughput In Vitro Radiobiology Platform for Megavoltage Photon Linear Accelerator Studies

We designed and developed a multiwell tissue culture plate irradiation setup, and intensity modulated radiotherapy plans were generated for 96-, 24-, and 6-well tissue culture plates. We demonstrated concordance between planned and measured/imaged radiation dose profiles using radiochromic film, a 2D ion chamber array, and an electronic portal-imaging device. Cell viability, clonogenic potential, and γ-H2AX foci analyses showed no significant differences between intensity-modulated radiotherapy and open-field, homogeneous irradiations. This novel platform may help to expedite radiobiology experiments within a clinical environment and may be used for wide-ranging ex vivo radiobiology applications.

Evaluation of Patient-Specific Quality Assurance for Carbon Ion Radiotherapy Using Full Energy Scanning Method at QST Hospital

Purpose: Patient-specific QA (PSQA) measurements for carbon ion radiotherapy (CIRT) are critical components of processes designed to identify discrepancies between calculated and delivered doses. We report the results of PSQA conducted at the QST Hospital during the period from September 2017 to March 2018. Methods: We analyzed PSQA results for 1448 fields for 10 disease sites with various target volumes, target depths and number of energy layers. For the PSQA, all the planned beams were recalculated on a water phantom with treatment planning software. The recalculated dose distributions were compared with the measured distributions using a 2D ionization chamber array at three depths, including 95% of the area of the prescription dose. These recalculated dose distributions were evaluated using the 3%/3mm gamma index with a passing threshold of 90%. Results: The passing rates for prostate, head and neck, and bone and soft tissue were 96.8%, 99.3%, and 91.7%, respectively. Additionally, 94.7% of lung plans with low energy beams passed. Overall, the CIRT in the QST Hospital reached a high passing rate of more than 95%. Although the remaining 5% failed to pass, there was no dependence between measurement depth and disease sites in these failures. Conclusion: Using PSQA measurements, we confirmed consistency between the planned and delivered doses for CIRT using the full energy scanning method.

Impact of Dose Calculation Algorithm on Skin Dose for Breast Cancer Treated With Intensity Modulated Proton Therapy

<h3>Purpose/Objective(s)</h3> Historically, pencil beam algorithms (PBA), have been used for intensity-modulated proton therapy (IMPT) dose calculation. Recently, improvements in calculation speeds and a desire to generate more accurate plans have led to wider adoption of Monte Carlo (MC) algorithms for IMPT. Our goal was to ascertain the impact of both PBA and MC calculation algorithms on the skin dose in breast cancer treated with IMPT by means of measurements. <h3>Materials/Methods</h3> Twenty-one comprehensive chest wall patients were included in this study with treatment planning performed using a treatment planning system. All IMPT plans were generated with 1 or 2 en-face fields. Single- and multiple-field-optimization were used in 19 and 2 patients, respectively. The prescription doses were 50.4 Gy for 15 and 45 Gy for 6 patients, all at 1.8 Gy/fraction. The skin OAR was defined as a 3 mm thick rind anterior to the chest wall target volume. For 12 patients, the skin dose was optimized with the objectives of V95% < = 50%, V100% < = 5%, and D0.03cc < = 50.9 Gy. For the remaining 9 patients, the skin dose was optimized to control D0.03cc alone. The physical skin dose was measured with a parallel plate ionization chamber and a 2D ionization chamber array device at various depths in solid water ranging from 0.2 to 1.3 cm. The agreement of calculated to measured dose was evaluated by 2D gamma analysis (3%/3mm) and point dose comparison. <h3>Results</h3> The average gamma passing rates for PBA and MC plans were 94% and 97%, respectively. The point dose comparison showed that the measured skin doses were systematically lower than the calculated dose in PBA plans by an average of 3.1% [range: -6.76% to 0.03%], whereas the measured doses were systematically higher than the calculated dose in MC plans by an average of 1.3% [range: -0.58% to 3.51%]. For the patients who received 50.4 Gy, the skin mean dose and D1% were 48.1 [range: 45.8 to 49.6] and 50.8 [range: 49.0 to 52.6] Gy. For the patients who received 45 Gy, the skin mean dose and D1% were 45.0 [range: 44.2 to 46.2] and 46.8 [range: 46.1 to 48.0] Gy. <h3>Conclusion</h3> This study demonstrated that comparing with the PBA algorithm generated plans, the MC generated plans yield a higher gamma passing rate (97% for MC vs. 94% for PBA), and a narrower 95% of confidence interval of skin dose (4.1 for MC vs. 6.8 for PBA). Measurements showed that MC computed plans systematically under-estimate the skin dose by an average of 1.3%, in contrast to PBA computed plans which systematically over-estimate the skin dose by an average of 3.1%. The clinical implications are that for individual patient, PBA could overestimate skin dose considerably more than MC. Therefore, cautions for skin dose should to be taken when PBA is replaced by MC in clinical practice, and it is recommended that appropriate optimization objectives be used during treatment planning to control skin dose.

Evaluation of the ability of three commercially available dosimeters to detect systematic delivery errors in step-and-shoot IMRT plans.

There is limited data on error detectability for step-and-shoot intensity modulated radiotherapy (sIMRT) plans, despite significant work on dynamic methods. However, sIMRT treatments have an ongoing role in clinical practice. This study aimed to evaluate variations in the sensitivity of three patient-specific quality assurance (QA) devices to systematic delivery errors in sIMRT plans. Four clinical sIMRT plans (prostate and head and neck) were edited to introduce errors in: Multi-Leaf Collimator (MLC) position (increasing field size, leaf pairs offset (1-3 mm) in opposite directions; and field shift, all leaves offset (1-3 mm) in one direction); collimator rotation (1-3 degrees) and gantry rotation (0.5-2 degrees). The total dose for each plan was measured using an ArcCHECK diode array. Each field, excluding those with gantry offsets, was also measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. 132 plans (858 fields) were delivered, producing 572 measured dose distributions. Measured doses were compared to calculated doses for the no-error plan using Gamma analysis with 3%/3 mm, 3%/2 mm, and 2%/2 mm criteria (1716 analyses). Generally, pass rates decreased with increasing errors and/or stricter gamma criteria. Pass rate variations with detector and plan type were also observed. For a 3%/3 mm gamma criteria, none of the devices could reliably detect 1 mm MLC position errors or 1 degree collimator rotation errors. This work has highlighted the need to adapt QA based on treatment plan type and the need for detector specific assessment criteria to detect clinically significant errors.

Commissioning of a three-dimensional arc-based technique for total body irradiation.

The purpose of this study is to describe the commissioning of a novel three‐dimensional arc‐based technique for total body irradiation (TBI) treatments. The development and implementation of this technique allowed our institution to transition from a bilateral two‐dimensional (2D) technique to a methodology based on volumetric dose calculation. The methodology described in this work is a derivation from the MATBI technique, with the static fields being replaced by four contiguous arc‐fields for each anterior and posterior incidence. The reduced number of fields we employed makes it possible to reach a satisfactory dose uniformity through manual optimization in a straightforward process. We use the Eclipse anisotropic analytical algorithm (AAA) algorithm, commissioned with preconfigured beam data for a 6 MV photon beam, at standard SSD (100 cm). A thorough evaluation of the accuracy of the AAA algorithm at an extended distance (approximately 200 cm) was carried out. For the evaluation, we compared measured and calculated percentage depth–dose and profiles that included open‐field, penumbra, and out‐of‐field regions. The analysis was performed for both static and arc fields, taking into consideration unshielded fields and also in the presence of lung shielding blocks. End‐to‐end tests were carried out for our institutional template plan by two means: with a 2D ion chamber array detector in solid phantom and using Gafchromic films in an anthropomorphic phantom. The results obtained in this work demonstrate that the Eclipse AAA algorithm commissioned for standard treatments can be safely used with our TBI planning technique. Moreover, this technique proved to be a highly efficient path to replace conventional treatment techniques, providing a homogeneous dose distribution, dosimetric robustness, and shorter treatment times. In addition, as inherited from the MATBI technique, our methodology can be implemented in small treatment rooms, with no need for ancillary equipment.

Triple channel analysis of Gafchromic EBT3 irradiated with clinical carbon-ion beams.

Self-developing radiochromic film is widely used in radiotherapy QA procedures. To compensate for typical film inhomogeneities, the triple channel analysis method is commonly used for photon-irradiated film. We investigated the applicability of this method for GafchromicTMEBT3 (Ashland) film irradiated with a clinically used carbon-ion beam. Calibration curves were taken from EBT3 film specimens irradiated with monoenergetic carbon-ion beams of different doses. Measurements of the lateral field shape and homogeneity were performed in the middle of a passively modulated spread-out Bragg peak and compared to simultaneous characterization by means of a 2D ionization chamber array. Additional measurements to investigate the applicability of EBT3 for quality assurance (QA) measurement in carbon-ion beams were performed. The triple-channel analysis reduced the relative standard deviation of the doses in a uniform carbon ion field by 30% (from 1.9% to 1.3%) and reduced the maximum deviation by almost a factor of 3 (from 28.6% to 9.8%), demonstrating the elimination of film artifacts. The corrected film signal showed considerably improved image quality and quantitative agreement with the ionization chamber data, thus providing a clear rationale for the usage of the triple channel analysis in carbon-beam QA.

Comprehensive Patient-Specific Intensity-Modulated Radiation Therapy Quality Assurance Comparing Mobius3D/FX to Conventional Methods of Evaluation.

PurposeTo determine the appropriateness of implementing Mobius3D/FX (Varian Medical Systems, Inc., Palo Alto, CA, USA) as not only a pretreatment secondary check but as an alternative to measurement-based patient-specific intensity-modulated radiation therapy (IMRT) quality assurance (QA).MethodsMobius3D/FX was commissioned and stock beam models were tweaked so that an independent recalculated 3D dose distribution can be obtained. Then, 50 patient-specific treatment plans for various indications were delivered across a 2D ion chamber array, radiochromic film setup, and electronic portal imager and analyzed with MobiusFX and gamma analysis. The concordance of plans scored as passing between MobiusFX and the conventional methods of QA was determined.ResultsAll analyzed treatment plans passed with a gamma passing rate >90% across all conventional QA methods, most commonly using a 3%/3mm gamma criterion except for film measurements where a 5%/3mm criterion was applied. There was good agreement and concordance between MobiusFX and conventional methods when using a 3%/3mm criteria for MobiusFX, whereas a 2%/2mm criteria appeared too stringent as it failed treatment plans deemed clinically acceptable using conventional methods.ConclusionsUsing a 50-sample subset of clinically delivered treatment plans this non-inferiority-type comparison shows Mobius3D/FX based on log file analysis to be a suitable alternative to conventional QA methods when utilizing the 3%/3mm gamma criterion. Methods based on log file analysis can provide an opportunity for resource sparing, improving the efficiency, and workflow for evaluating IMRT treatment plans.

Validation and practical implementation of seated position radiotherapy in a commercial TPS for proton therapy.

This work aims to validate new 6D couch features and their implementation for seated radiotherapy in RayStation (RS) treatment planning system (TPS). In RS TPS, new 6D couch features are (i) chair support device, (ii) patient treatment option of "Sitting: face towards the front of the chair", and (iii) patient support pitch and roll capabilities. The validation of pitch and roll was performed by comparing TPS generated DRRs with planar x-rays. Dosimetric tests through measurement by 2D ion chamber array were performed for beams created with varied scanning and treatment orientation and 6D couch rotations. For the implementation of 6D couch features for treatments in a seated position, the TPS and oncology information system (Mosaiq) settings are described for a commercial chair. An end-to-end test using an anthropomorphic phantom was performed to test the complete workflow from simulation to treatment delivery. The 6D couch features were found to have a consistent implementation that met IEC 61712 standard. The DRRs were found to have an acceptable agreement with planar x-rays based on visual inspection. For dose map comparison between measured and calculated, the gamma index analysis for all the beams was >95% at a 3% dose-difference and 3mm distance-to-agreement tolerances. For an end-to end-testing, the phantom was successfully set up at isocenter in the seated position and treatment was delivered. Chair-based treatments in a seated position can be implemented in RayStation through the use of newly released 6D couch features.

Beam Characteristics of the First Clinical 360°-rotational Compact Scanning Pencil Beam Proton Treatment System and Comparisons against a Multi-Room System

This paper is to present the dosimetric characteristics of the first clinical CBCT guided 360˚-rotational compact/single-room scanning pencil beam proton treatment system (SRPT) and comparisons against a recent beam data from a multi-room proton therapy system (MRPT) by the same vendor. The proton beam commissioning was conducted on the first clinical, newly-designed compact ProBeam 360˚ for its scanning pencil beams for protons ranging from 70MeV to 220MeV. The integrated depth dose curves (IDDs) were acquired in water with AP monogenic proton beams using a PTW Bragg peak chamber. The dose output factors, measured by a Marcus chamber at 15 mm transmission beam portion using single layer 10x10 cm2 proton plans, were then incorporated to the iDD's along with a factor of 1.1 for RBE offset. The beam spot sigma sizes were then derived from single Gaussian fitting for the in-air measured spot profiles. After beam modeling and validation for a treatment planning system, the proton dosimetric characteristics were compared against the recent data of MRPT. In contract to MRPT data, the Bragg peaks of SRPT have shown moderately extended proton beam range straggling in the peak region accompanied by a marginally elevated proximal transmission beam portion. This phenomenon displayed more noticeably in the mid-ranged or even high energies. Consequently, IDDs of SRPT showed about 0.41 to 0.94 mm broader Bragg peak widths (Rb80 - Ra80) in energies of 140 MeV or greater, peaked at 180 MeV. About 0.65-0.79 mm slightly extended distal falloffs (Rb20-80) were noticeable for energies between 140 MeV and 170 MeV, consistent with the findings of minimally raised energy sigma in AcurosPT Monte Carlo model. An independent check for the dose and energy accuracy of a spread-out Bragg peak plan on a simple phantom by the IROC showed 1.00 accuracy with 1.00 ±5% allowance. Additional comprehensive end-to-end independent check using a more realistic prostate phantom offered by IROC was reported a 96% accuracy and the passing score at a set passing criterion of 85%. The result over 108 patient specific individual field QAs with Octavius 1500 2D ionization chamber array showed 100% passing rate using a Gamma index of 3%/3mm or tighter from fields with or without range shifters. The high QA passing rates suggest a high confidence on the accuracy and quality of the commissioning for the newly designed SRPT. The commissioning results of the new-designed SRPT showed slightly widened proton Bragg peaks, elevated beam transition region and extended peak distal falloffs, particularly in mid-high energies, suggest minimally broader spectra for selected proton energies in comparing against MRPT data. This did not show a meaningful impact on the quality and accuracy of proton therapy program, indicated by high scores from independent dosimetry end-to-end checks and over 108 patient’s specific beam QA’s.

Derivative-based gamma index: a novel methodology for stringent patient-specific quality assurance in the stereotactic treatment planning of liver cancer

Objective: The development of a stringent derivative-based gamma (DBG) index for patient-specific QA in stereotactic radiotherapy treatment planning (SRTP) to account for the spatial change in dose. Methods: Twenty-five patients of liver SBRT were selected retrospectively for this study. Deliberately, two different kinds of treatment planning approaches were used for each patient. Firstly, the treatment plans were generated using a conventional treatment planning (CTP) approach in which the target was covered with a homogeneous dose along with the nominal dose fall-off around the treatment field. Subsequently, the other treatment plans were generated using an SRTP approach with the intent of heterogeneous dose within the target region along with a steeper dose gradient outside the treatment field as much as possible. For both kinds of treatment plans, two dimensional (2D) conventional gamma (CG) and DBG analysis were performed using the 2D ion chamber array and radiochromic film. Results: Difference in the DBG index was statistically significant whereas, for CG analysis, the difference in CG index was insignificant for both types of treatment plans (CTP and SRTP). A significant positive correlation was observed between the difference in the DBG index and the difference in HI for high gamma criteria. Conclusion: The DBG evaluation is found to be more rigorous, and sensitive to the only SRTP. The proposed method could be opted-in the routine clinical practice in addition to CG. Advances in knowledge: DBG is more sensitive to detect the spatial change of dose, especially in high dose gradient regions.

Quality assurance of IMRT treatment plans for a 1.5 T MR-linac using a 2D ionization chamber array and a static solid phantom

MR-guided radiotherapy requires novel quality assurance (QA) methods for intensity-modulated radiotherapy treatment plans (TPs). Here, an optimized method for TPs for a 1.5 T MR-linac was developed and implemented clinically.A static solid phantom and an MR-compatible 2D ionization chamber array were used. The array’s response with respect to the incident beam gantry angles was characterized for four different orientations of the array relative to the beam. A lookup table was created identifying the optimum orientation for each gantry angle. For the QA of clinical MR-linac TPs, beams were grouped according to their gantry angles and measured with up to four setups. The method was applied to n = 106 clinical TPs of 54 patients for different tumour entities. Reference plans and plans created in the online adaptive workflow were analysed, using a local 3%/3 mm gamma criterion for dose values larger than 30% of the maximum. Pass rates were averaged over all beam groups.The array’s response strongly depends on the beam incidence angle. Optimum angles typically range from −10° to 80° around the phantom setup angle. Consequently, plan verification required up to four setups. For clinical MR-linac TPs, the overall median pass rate was 98.5% (range 88.6%–100%). Pass rates depended on the tumour entity. Median pass rates were for liver metastases stereotactic body radiotherapy 99.2%, prostate cancer 99%, pancreatic cancer 98.9%, lymph node metastases 98.7%, partial breast irradiation (PBI) 98%, head-and-neck cancer 97.7%, rectal cancer 94% and others 96.6%. 85% of plans were accepted straightaway, with pass rates above 95%. A single plan with a pass rate below 90% was subsequently verified with a modified method. Off-axis target volumes, e.g. PBI, were verified successfully using a lateral shift of the phantom.The method is suitable to verify reference and online adapted TPs for a 1.5 T MR-linac, including plans for off-axis target volumes.

Commissioning of Clinac IX Trilogy Linear Accelerator for Stereotactic Radiosurgery

Comprehensive quality assurance (QA) and beam data acquisition has been performed for commissioning of Varian Trilogy linear accelerator for stereotactic radiosurgery (SRS). Mechanical, electrical and radiological tests were performed to check linac performance as per American Association of Physicists in Medicine (AAPM) Task Group (TG) 142. The geometric isocentre accuracy, congruence check and calibration of kV and MV imager were performed using the Isocal QA device. The off axis ratio profiles, depth dose data and beam output factors of 6 MV SRS beam for the field sizes ranging from 2 × 2 cm2 to 15 × 15 cm2 were measured using SFD diode detector and cc13 semiflex ion chamber. The multileaf collimator (MLC) characterization was done in the treatment planning system (TPS) using the parameters measured with the CC13 semiflex ionisation chamber in water phantom. Dose measured with 2D ion chamber array was compared to that calculated in TPS. The maximum change in MV and kV imager isocentre was within ±0.03 cm. The variation between SFD measured and ion chamber measured beam data is in positive correlation for fields sizes down up to 2 × 2 cm2. Variation in output factors is ±0.56 ± 0.05 %. The measured beam data is imported and calculated in the Eclipse TPS and is found within 1% gamma agreement index (GAI) with the Analytical Anisotropic Algorithm (AAA) model data. GAI for TPS verification QA is 97.8%, 98.5% and 98.3% respectively for fix jaw 2 × 2 cm2, 5 × 5 cm2 and rapid arc SRS plan. Successful modelling of the photon beam for high dose rate SRS mode is achieved for clinical treatment of patients.