Software simulation of tumour motion dose effects during flattened and unflattened ITV-based VMAT lung SBRT

PurposeRestricted studies comparing different dose rate parameters are available while ITV-based VMAT lung SBRT planning leads to perform the analysis of the most suitable parameters of the external beams used. The special emphasis was placed on the impact of dose rate on dose distribution variations in target volumes due to interplay effects.MethodsFour VMAT plans were calculated for 15 lung tumours using 6 MV photon beam quality (flattening filter FF vs. flattening filter free FFF beams) and maximum dose rate of 600 MU/min, 1000 MU/min and 1400 MU/min. Three kinds of motion simulations were performed finally giving 180 plans with perturbed dose distributions.Results6FFF-1400 MUs/min plans were characterized by the shortest beam on time (1.8 ± 0.2 min). Analysing the performed motion simulation results, the mean dose (Dmean) is not a sensitive parameter to related interplay effects. Looking for local maximum and local minimum doses, some discrepancies were found, but their significance was presented for individual patients, not for the whole cohort. The same was observed for other verified dose metrics.ConclusionsGenerally, the evaluation of VMAT robustness between FF and FFF concepts against interplay effect showed a negligible effect of simulated motion influence on tumour coverage among different photon beam quality parameters. Due to the lack of FFF beams, smaller radiotherapy centres are able to perform ITV-based VMAT lung SBRT treatment in a safe way. Radiotherapy department having FFF beams could perform safe, fast and efficient ITV-based VMAT lung SBRT without a concern about significance of interplay effects.     

A surface energy spectral study on the bone heterogeneity and beam obliquity using the flattened and unflattened photon beams

Aim

Using flattened and unflattened photon beams, this study investigated the spectral variations of surface photon energy and energy fluence in the bone heterogeneity and beam obliquity.

Background

Surface dose enhancement is a dosimetric concern when using unflattened photon beam in radiotherapy. It is because the unflattened photon beam contains more low-energy photons which are removed by the flattening filter of the flattened photon beam.

Materials and methods

We used a water and bone heterogeneity phantom to study the distributions of energy, energy fluence and mean energy of the 6 MV flattened and unflattened photon beams (field size = 10 cm × 10 cm) produced by a Varian TrueBEAM linear accelerator. These elements were calculated at the phantom surfaces using Monte Carlo simulations. The photon energy and energy fluence calculations were repeated with the beam angle turned from 0° to 15°, 30° and 45° in the water and bone phantom.

Results

Spectral results at the phantom surfaces showed that the unflattened photon beams contained more photons concentrated mainly in the low-energy range (0–2 MeV) than the flattened beams associated with a flattening filter. With a bone layer of 1 cm under the phantom surface and within the build-up region of the 6 MV photon beam, it is found that both the flattened and unflattened beams had slightly less photons in the energy range <0.4 MeV compared to the water phantom. This shows that the presence of the bone decreased the low-energy photon backscatters to the phantom surface. When both the flattened and unflattened photon beams were rotated from 0° to 45°, the number of photon and mean photon energy increased. This indicates that both photon beams became more hardened or penetrate when the beam angle increased. In the presence of bone, the mean energies of both photon beams increased. This is due to the absorption of low-energy photons by the bone, resulting in more beam hardening.

Conclusions

This study explores the spectral relationships of surface photon energy and energy fluence with bone heterogeneity and beam obliquity for the flattened and unflattened photon beams. The photon spectral information is important in studies on the patient''s surface dose enhancement using unflattened photon beams in radiotherapy.     

An external dosimetry audit programme to credential static and rotational IMRT delivery for clinical trials quality assurance

PurposeExternal dosimetry audits give confidence in the safe and accurate delivery of radiotherapy. The RTTQA group have performed an on-site audit programme for trial recruiting centres, who have recently implemented static or rotational IMRT, and those with major changes to planning or delivery systems.MethodsMeasurements of reference beam output were performed by the host centre, and by the auditor using independent equipment. Verification of clinical plans was performed using the ArcCheck helical diode array.ResultsA total of 54 measurement sessions were performed between May 2014 and June 2016 at 28 UK institutions, reflecting the different combinations of planning and delivery systems used at each institution. Average ratio of measured output between auditor and host was 1.002 ± 0.006. Average point dose agreement for clinical plans was −0.3 ± 1.8%. Average (and 95% lower confidence intervals) of gamma pass rates at 2%/2 mm, 3%/2 mm and 3%/3 mm respectively were: 92% (80%), 96% (90%) and 98% (94%). Moderately significant differences were seen between fixed gantry angle and rotational IMRT, and between combination of planning systems and linac manufacturer, but not between anatomical treatment site or beam energy.ConclusionAn external audit programme has been implemented for universal and efficient credentialing of IMRT treatments in clinical trials. Good agreement was found between measured and expected doses, with few outliers, leading to a simple table of optimal and mandatory tolerances for approval of dosimetry audit results. Feedback was given to some centres leading to improved clinical practice.     

On the Use of Optically Stimulated Luminescent Dosimeter for Surface Dose Measurement during Radiotherapy

This study was carried out to investigate the suitability of using the optically stimulated luminescence dosimeter (OSLD) in measuring surface dose during radiotherapy. The water equivalent depth (WED) of the OSLD was first determined by comparing the surface dose measured using the OSLD with the percentage depth dose at the buildup region measured using a Markus ionization chamber. Surface doses were measured on a solid water phantom using the OSLD and compared against the Markus ionization chamber and Gafchromic EBT3 film measurements. The effect of incident beam angles on surface dose was also studied. The OSLD was subsequently used to measure surface dose during tangential breast radiotherapy treatments in a phantom study and in the clinical measurement of 10 patients. Surface dose to the treated breast or chest wall, and on the contralateral breast were measured. The WED of the OSLD was found to be at 0.4 mm. For surface dose measurement on a solid water phantom, the Markus ionization chamber measured 15.95% for 6 MV photon beam and 12.64% for 10 MV photon beam followed by EBT3 film (23.79% and 17.14%) and OSLD (37.77% and 25.38%). Surface dose increased with the increase of the incident beam angle. For phantom and patient breast surface dose measurement, the response of the OSLD was higher than EBT3 film. The in-vivo measurements were also compared with the treatment planning system predicted dose. The OSLD measured higher dose values compared to dose at the surface (Hp(0.0)) by a factor of 2.37 for 6 MV and 2.01 for 10 MV photon beams, respectively. The measurement of absorbed dose at the skin depth of 0.4 mm by the OSLD can still be a useful tool to assess radiation effects on the skin dermis layer. This knowledge can be used to prevent and manage potential acute skin reaction and late skin toxicity from radiotherapy treatments.     

End-to-end dosimetric audit: A novel procedure developed for Irish HDR brachytherapy centres

PurposeA dosimetric audit of Ir-192 high dose rate (HDR) brachytherapy remote after-loading units was carried out in 2019. All six brachytherapy departments on the island of Ireland participated in an end-to-end test and in a review of local HDR dosimetry procedures.Materials and methodsA 3D-printed customised phantom was created to position the following detectors at known distances from the HDR source: a Farmer ionization chamber, GafChromic film and thermoluminescent dosimeters (TLDs). Dedicated HDR applicator needles were used to position an Ir-192 source at 2 cm distance from these detectors. The end-to-end dosimetry audit pathway was performed at each host site and included the stages of imaging, applicator reconstruction, treatment planning and delivery. Deviations between planned and measured dose distributions were quantified using gamma analysis methods. Local procedures were also discussed between auditors and hosts.ResultsThe mean difference between Reference Air Kerma Rate (RAKR) measured during the audit and RAKR specified by the vendor source certificate was 1.3%. The results of end-to-end tests showed a mean difference between calculated and measured dose of 2.5% with TLDs and less than 0.5% with Farmer chamber measurements. GafChromic films showed a mean gamma passing rates of >95% for plastic and metal applicators with 2%/1 mm global tolerance criteria.ConclusionsThe results of this audit indicate dosimetric consistency between centres. The ‘end to end’ dosimetry audit methodology for HDR brachytherapy has been successfully implemented in a multicentre environment, which included different models of Ir-192 sources and different treatment planning systems.The ability to create a 3D-printed water-equivalent phantom customised to accurately position all three detector types simultaneously at controlled distances from the Ir-192 source under evaluation gives good reproducibility for end-to-end methodology.     

Technical note: GAMMORA,a free,open-source,and validated GATE-based model for Monte-Carlo simulations of the Varian TrueBeam

PurposeMonte Carlo (MC) is the reference computation method for medical physics. In radiotherapy, MC computations are necessary for some issues (such as assessing figures of merit, double checks, and dose conversions). A tool based on GATE is proposed to easily create full MC simulations of the Varian TrueBeam STx.MethodsGAMMORA is a package that contains photon phase spaces as a pre-trained generative adversarial network (GAN) and the TrueBeam’s full geometry. It allows users to easily create MC simulations for simple or complex radiotherapy plans such as VMAT. To validate the model, the characteristics of generated photons are first compared to those provided by Varian (IAEA format). Simulated data are also compared to measurements in water and heterogeneous media. Simulations of 8 SBRT plans are compared to measurements (in a phantom). Two examples of applications (a second check and interplay effect assessment) are presented.ResultsThe simulated photons generated by the GAN have the same characteristics (energy, position, and direction) as the IAEA data. Computed dose distributions of simple cases (in water) and complex plans delivered in a phantom are compared to measurements, and the Gamma index (3%/3mm) was always superior to 98%. The feasibility of both clinical applications is shown.ConclusionsThis model is now shared as a free and open-source tool that generates radiotherapy MC simulations. It has been validated and used for five years. Several applications can be envisaged for research and clinical purposes.     

Monte Carlo determination of a nanoDot OSLD response using quality index for diagnostic kilovoltage X-ray beams

PurposeThis study aims to investigate the energy response of an optically stimulated luminescent dosimeter known as nanoDot for diagnostic kilovoltage X-ray beams via Monte Carlo calculations.MethodsThe nanoDot response is calculated as a function of X-ray beam quality in free air and on a water phantom surface using Monte Carlo simulations. The X-ray fluence spectra are classified using the quality index (QI), which is defined as the ratio of the effective energy to the maximum energy of the photons. The response is calculated for X-ray fluence spectra with QIs of 0.4, 0.5, and 0.6 with tube voltages of 50–137.6 kVp and monoenergetic photon beams. The surface dose estimated using the calculated response is verified by comparing it with that measured using an ionization chamber.ResultsThe nanoDot response in free air for monoenergetic photon beams (QI = 1.0) varies significantly at photon energies below 100 keV and reaches a factor of 3.6 at 25–30 keV. The response differs by up to approximately 6% between QIs of 0.4 and 0.6 for the same half-value layer (HVL). The response at the phantom surface decreases slightly owing to the backscatter effect, and it is almost independent of the field size. The agreement between the surface dose estimated using the nanoDot and that measured using the ionization chamber for assessing X-ray beam qualities is less than 2%.ConclusionsThe nanoDot response is indicated as a function of HVL for the specified QIs, and it enables the direct surface dose measurement.     

Monte Carlo dose in a prosthesis phantom based on exact geometry vs streak artefact contaminated CT data as benchmarked against Gafchromic film measurements

PurposeIn radiotherapy, accurate calculation of patient radiation dose is very important for good clinical outcome. In the presence of metallic implants, the dose calculation accuracy could be compromised by metal artefacts generated in computed tomography (CT) images of patients. This study investigates the influence of metal-induced CT artefacts on MC dose calculations in a pelvic prosthesis phantom.MethodsA pelvic phantom containing unilateral Ti prosthesis was CT-scanned and accurate Hounsfield unit (HU) values were assigned to known materials of the phantom as opposed to HU values produced through the artefact CT images of the phantom. Using the DOSXYZnrc MC code, dose calculations were computed in the phantom model constructed from the original CT images containing the artefacts and artefact-free images made from the exact geometry of the phantom with known materials. The dose calculations were benchmarked against Gafchromic EBT3 film measurements using 15 MeV electron and 10 MV photon beams.ResultsThe average deviations between film and MC dose data decreased from 3 ± 2% to 1 ± 1% and from about 6 ± 2% to 3 ± 1% for the artefact and artefact-free phantom models against film data for the electron and photon fields, respectively.ConclusionsFor the Ti prosthesis phantom, the presence of metal-induced CT artefacts could cause dose inaccuracies of about 3%. Construction of an artefact-free phantom model made from the exact geometry of the phantom with known materials to overcome the effect of artefacts is advantageous compared to using CT data directly of which the exact tissue composition is not well-known.     

Establishing the impact of temporary tissue expanders on electron and photon beam dose distributions

PurposeThis study investigates the effects of temporary tissue expanders (TTEs) on the dose distributions in breast cancer radiotherapy treatments under a variety of conditions.MethodsUsing EBT2 radiochromic film, both electron and photon beam dose distribution measurements were made for different phantoms, and beam geometries. This was done to establish a more comprehensive understanding of the implant's perturbation effects under a wider variety of conditions.ResultsThe magnetic disk present in a tissue expander causes a dose reduction of approximately 20% in a photon tangent treatment and 56% in electron boost fields immediately downstream of the implant. The effects of the silicon elastomer are also much more apparent in an electron beam than a photon beam.ConclusionsEvidently, each component of the TTE attenuates the radiation beam to different degrees. This study has demonstrated that the accuracy of photon and electron treatments of post-mastectomy patients is influenced by the presence of a tissue expander for various beam orientations. The impact of TTEs on dose distributions establishes the importance of an accurately modelled high-density implant in the treatment planning system for post-mastectomy patients.     

Detailed analysis of dose difference in using water as tissue-equivalent material in 252Cf brachytherapy

AimThe purpose of this study is to analyse how small variations in the elemental composition of soft tissue lead to differences in dose distributions from a ²⁵²Cf brachytherapy source and to determine the error percentage in using water as a tissue-equivalent material.BackgroundWater is normally used as a tissue-equivalent phantom material in radiotherapy dosimetry.Materials and methodsNeutron energy spectra, neutron and gamma-ray dose rate distributions were calculated for a ²⁵²Cf AT source located at the center of a spherical phantom filled with various types of tissue compositions: adipose, brain, muscle, International Commission on Radiation Units and Measurements (ICRU) report No. 44 9-component soft tissue and water, using Monte Carlo simulation.ResultsThe obtained results showed differences between total dose rates in various tissues relative to water varying between zero and 4.94%. The contributions of neutron and total gamma ray doses to these differences are, on average, 81% and 19%, respectively. It was found that the dose differences between various soft tissues and water depend not only on the soft tissue composition, but also on the beam type emitted from the ²⁵²Cf source and the distance from the source.ConclusionAssuming water as a tissue-equivalent material, although leads to overestimation of dose rate (except in the case of adipose tissue), is acceptable and suitable for use in ²⁵²Cf brachytherapy treatment planning systems based on the recommendation by the ICRU that the uncertainties in dose delivery in radiotherapy should be lower than 5%.     

Assessment of ionization chamber correction factors in photon beams using a time saving strategy with PENELOPE code

The purpose of this study was to investigate Monte Carlo-based perturbation and beam quality correction factors for ionization chambers in photon beams using a saving time strategy with PENELOPE code. Simulations for calculating absorbed doses to water using full spectra of photon beams impinging the whole water phantom and those using a phase-space file previously stored around the point of interest were performed and compared. The widely used NE2571 ionization chamber was modeled with PENELOPE using data from the literature in order to calculate absorbed doses to the air cavity of the chamber. Absorbed doses to water at reference depth were also calculated for providing the perturbation and beam quality correction factors for that chamber in high energy photon beams. Results obtained in this study show that simulations with phase-space files appropriately stored can be up to ten times shorter than using a full spectrum of photon beams in the input-file. Values of k_Q and its components for the NE2571 ionization chamber showed good agreement with published values in the literature and are provided with typical statistical uncertainties of 0.2%. Comparisons to k_Q values published in current dosimetry protocols such as the AAPM TG-51 and IAEA TRS-398 showed maximum percentage differences of 0.1% and 0.6% respectively. The proposed strategy presented a significant efficiency gain and can be applied for a variety of ionization chambers and clinical photon beams.     

Delta4 Discover transmission detector: A comprehensive characterization for in-vivo VMAT monitoring

ObjectiveTo investigate the dosimetric behaviour, influence on photon beam fluence and error detection capability of Delta⁴ Discover transmission detector.MethodsThe transmission detector (TRD) was characterized on a TrueBeam linear accelerator with 6 MV beams. Linearity, reproducibility and dose rate dependence were investigated. The effect on photon beam fluence was evaluated in terms of beam profiles, percentage depth dose, transmission factor and surface dose for different open field sizes. The transmission factor of the 10x10 cm² field was entered in the TPS’s configuration and its correct use in the dose calculation was verified recalculating 17 clinical IMRT/VMAT plans. Surface dose was measured for 20 IMRT fields. The capability to detect different delivery errors was investigated evaluating dose gamma index, MLC gamma index and leaf position of 15 manually modified VMAT plans.ResultsTRD showed a linear dependence on MU. No dose rate dependence was observed. Short-term and long-term reproducibility were within 0.1% and 0.5%. The presence of the TRD did not significantly affect PDDs and profiles. The transmission factor of the 10x10 cm² field size was 0.985 and 0.983, for FF and FFF beams respectively. The 17 recalculated plans met our clinical gamma-index passing rate, confirming the correct use of the transmission factor by the TPS. The surface dose differences for the open fields increase for shorter SSDs and greater field size. Differences in surface dose for the IMRT beams were less than 2%. Output variation ≥2%, collimator angle variations within 0.3°, gantry angle errors of 1°, jaw tracking and leaf position errors were detected.ConclusionsDelta⁴ Discover shows good linearity and reproducibility, is not dependent on dose rate and does not affect beam quality and dose profiles. It is also capable to detect dosimetric and geometric errors and therefore it is suitable for monitoring VMAT delivery.     

DEMAT: A multi-institutional dosimetry audit of rotational and static intensity-modulated radiotherapy

PurposeStatic beam intensity-modulated-radiation-therapy (IMRT) and/or Volumetric-Modulated-Arc-Therapy (VMAT) are now available in many regional radiotherapy departments. The aim of this multi-institutional audit was to design a new methodology based on radiochromic films to perform an independent quality control.MethodsA set of data were sent to all participating centres for two clinical localizations: prostate and Head and Neck (H&N) cancers. The agreement between calculations and measurements was verified in the Octavius phantom (PTW) by point measurements using ionization chambers and by 2D measurements using EBT3 radiochromic films. Due to uncertainties in the whole procedure, criteria were set to 5% and 3% in local dose and 3 mm in distance excluding doses lower than 10% of the maximum doses. No normalization point or area was used for the quantitative analysis.Results13 radiotherapy centres participated in this audit involving 28 plans (12 IMRT, 16 VMAT). For point measurements, mean errors were −0.18 ± 1.54% and 0.00 ± 1.58% for prostate and H&N cases respectively. For 2D measurements with 5%/3 mm criteria, gamma map analysis showed a pixel pass rate higher than 95% for prostate and H&N. Mean gamma index was lower than 0.4 for prostate and 0.5 for H&N. Both techniques yielded similar results.ConclusionThis study showed the feasibility of an independent quality control by peers for conventional IMRT and VMAT. Results from all participating centres were found to be in good agreement. This regional study demonstrated the feasibility of our new methodology based on radiochromic films without dose normalization on a specific point.     

Credentialing of vertebral stereotactic ablative body radiotherapy in a multi-centre trial

Purpose/objectiveStereotactic ablative body radiotherapy (SABR) in multi-centre trials requires rigorous quality assurance to ensure safe and consistent treatment for all trial participants. We report results of vertebral SABR dosimetry credentialing for the ALTG/TROG NIVORAD trial.Material/methodsCentres with a previous SABR site visit performed axial film measurement of the benchmarking vertebral plan in a local phantom and submitted radiochromic film images for analysis. Remaining centres had on-site review of SABR processes and axial film measurement of the vertebral benchmarking plan. Films were analysed for dosimetric and positional accuracy: gamma analysis (>90% passing 2%/2mm/10% threshold) and ≤ 1 mm positional accuracy at target-cord interface was required.Results19 centres were credentialed; 11 had on-site measurement. Delivery devices included linear accelerator, TomoTherapy and CyberKnife systems. Five centres did not achieve 90% gamma passing rate. Of these, three were out of tolerance (OOT) in low (<5Gy) dose regions and > 80% passing rate and deemed acceptable. Two were OOT over the full dose range: one elected not to remeasure; the other also had positional discrepancy greater than 1 mm and repeat measurement with a new plan was in tolerance. The original OOT was attributed to inappropriate MLC constraints. All centres delivered planned target-cord dose gradient within 1 mm.ConclusionCredentialing measurements for vertebral SABR in a multi-centre trial showed although the majority of centres delivered accurate vertebral SABR, there is high value in independent audit measurements. One centre with inappropriate MLC settings was detected, which may have resulted in delivery of clinically unacceptable vertebral SABR plans.     

Assessment of skin dose in breast cancer radiotherapy: on-phantom measurement and Monte Carlo simulation

AimThe main purpose of the present study is assessment of skin dose in breast cancer radiotherapy.BackgroundAccurate assessment of skin dose in radiotherapy can provide useful information for clinical considerations.Materials and methodsA RANDO phantom was irradiated using a 6 MV Siemens Primus linac with medial and tangential radiotherapy fields for simulating breast cancer treatment. Dosimetry was also performed on various positions across the fields using an EBT3 radiochromic film. Similar conditions of measurement on the RANDO phantom including field size, irradiation angle, number of fields, etc. were subsequently simulated via the Monte Carlo N-Particle Transport code (MCNP). Ultimately, dose values for corresponding points from both methods were compared.ResultsConsidering dosimetry using radiochromic films on the RANDO phantom, there were points having underdose and overdose based on the prescribed dose and skin tolerance levels. In this respect, 81.25% and 18.75% of the points had underdose and overdose, respectively. In some cases, several differences were observed between the measurement and the MCNP simulation results associated with skin dose.ConclusionBased on the results of the points which had underdose, it was suggested that a bolus should be used for the given points. With regard to overdose points, it was advocated to consider skin tolerance dose in treatment planning. Differences between the measurement and the MCNP simulation results might be due to voxel size of tally cells in simulations, effect of beam’s angle of incidence, validation time of linac’s head, lack of electronic equilibrium in the build-up region, as well as MCNP tally type.     

Calculation and measurement of doses in the surface layers of a phantom when using Tomotherapy

BackgroundThe calculation and measurement on the surface of the skin presents a significant dosimetric problem because of numerous factors which have an influence on the dose distribution in this region.AimThe overall aim of this study was to check the agreement between doses measured with thermoluminescent detectors (TLD) during tomotherapy photon beam irradiation of the skin area of a solid water cylindrical phantom with doses calculated with Hi-Art treatment planning system (TPS).Material and MethodThe measurements of the dose were made with the use of a solid water cylindrical phantom - Cheese Phantom. Two bolus phantoms were used: 5 mm and 10 mm Six different planning treatments were generated. The doses were measured using TL detectors.ResultsIn the case of a tumor located near the surface of the skin, the mean dose for 0.5 cm bolus was - 1.94 Gy, and for 1 cm bolus - 2.03 Gy. For the tumor located inside the phantom and organ at risk on the same side that TL detectors, for a 0.5 cm bolus, mean dose was 0.658 Gy, and for a 1 cm bolus, 0.62 Gy.ConclusionThe analysis of results showed that the relative percentage difference between measured and planned dose in the field of irradiation was less than 10%, while the largest differences were on the board of the field of radiation and outside of the field of irradiation, where the dose was 0.08 Gy to 1 Gy.     

Comparison of the RayStation photon Monte Carlo dose calculation algorithm against measured data under homogeneous and heterogeneous irradiation geometries

PurposeThis work compares Monte Carlo dose calculations performed using the RayStation treatment planning system against data measured on a Varian Truebeam linear accelerator with 6 MV and 10 MV FFF photon beams.MethodsThe dosimetric performance of the RayStation Monte Carlo calculations was evaluated in a variety of irradiation geometries employing homogeneous and heterogeneous phantoms. Profile and depth dose comparisons against measurement were carried out in relative mode using the gamma index as a quantitative measure of similarity within the central high dose regions.ResultsThe results demonstrate that the treatment planning system dose calculation engine agrees with measurement to within 2%/1 mm for more than 95% of the data points in the high dose regions for all test cases. A systematic underestimation was observed at the tail of the profile penumbra and out of field, with mean differences generally <0.5 mm or 1% of curve dose maximum respectively. Out of field agreement varied between evaluated beam models.ConclusionsThe RayStation implementation of photon Monte Carlo dose calculations show good agreement with measured data for the range of scenarios considered in this work and is deemed sufficiently accurate for introduction into clinical use.     

3D dosimetric validation of ultrasound-guided radiotherapy with a dynamically deformable abdominal phantom

ObjectivesThe purpose of this study was to dosimetrically benchmark gel dosimetry measurements in a dynamically deformable abdominal phantom for intrafraction image guidance through a multi-dosimeter comparison. Once benchmarked, the study aimed to perform a proof-of-principle study for validation measurements of an ultrasound image-guided radiotherapy delivery system.MethodsThe phantom was dosimetrically benchmarked by delivering a liver VMAT plan and measuring the 3D dose distribution with DEFGEL dosimeters. Measured doses were compared to the treatment planning system and measurements acquired with radiochromic film and an ion chamber. The ultrasound image guidance validation was performed for a hands-free ultrasound transducer for the tracking of liver motion during treatment.ResultsGel dosimeters were compared to the TPS and film measurements, showing good qualitative dose distribution matches, low γ values through most of the high dose region, and average 3%/5 mm γ-analysis pass rates of 99.2%(0.8%) and 90.1%(0.8%), respectively. Gel dosimeter measurements matched ion chamber measurements within 3%. The image guidance validation study showed the measurement of the treatment delivery improvements due to the inclusion of the ultrasound image guidance system. Good qualitative matching of dose distributions and improvements of the γ-analysis results were observed for the ultrasound-gated dosimeter compared to the ungated dosimeter.ConclusionsDEFGEL dosimeters in phantom showed good agreement with the planned dose and other dosimeters for dosimetric benchmarking. Ultrasound image guidance validation measurements showed good proof-of-principle of the utility of the phantom system as a method of validating ultrasound-based image guidance systems and potentially other image guidance methods.     

An end-to-end postal audit test to examine the coincidence between the imaging isocenter and treatment beam isocenter of the IGRT linac system for Japan Clinical Oncology Group (JCOG) clinical trials

PurposeThe aim of this study was to develop an end-to-end postal audit test to examine the coincidence between the imaging isocenter and treatment beam isocenter of the image guided radiotherapy (IGRT) linac system for Japan Clinical Oncology Group (JCOG) trials, as a part of IGRT credentialing of institutions participating in JCOG trials.MethodsWe developed an end-to-end postal audit test to verify radiation positional errors associated with IGRT techniques. This test is intended for simulating a clinical IGRT flow and uses a static cubic phantom measuring 15 × 15 × 15 cm³ and weighing approximately 3.4 kg. The phantom has four gold fiducial markers and a spherical dummy target for setup, with known shift values from the phantom center. Two pairs of Gafchromic RTQA2 films were inserted 5 mm from the phantom’s anterior-posterior and right-left surfaces. Radiation positional errors at the isocenter were determined by analyzing the center of the radiation field on the films and the known shift values of the dummy target. The test was performed on 47 IGRT devices at 35 institutions.ResultsRadiation positional errors were within acceptance levels (1 mm/1°) for 42 IGRT devices (89.4%) in the first check. Median time to complete IGRT credentialing was 11.5 days. This audit method was applicable for any radiotherapy machine with an IGRT device.ConclusionsA postal audit test to verify radiation positional errors for JCOG trials was successfully developed. In the postal audit, all but one institution passed this credentialing item within two trials.