Measured Dose Distributions Research Articles

Measurement-based range evaluation for quality assurance of CBCT-based dose calculations in adaptive proton therapy.

The implementation of volumetric in-room imaging for online adaptive radiotherapy makes extensive testing of this image data for treatment planning necessary. Especially for proton beams the higher sensitivity to stopping power properties of the tissue results in more stringent requirements. Current approaches mainly focus on recalculation of the plans on the new image data, lacking experimental verification, and ignoring the impact on the plan re-optimization process. The aim of this study was to use gel and film dosimetry coupled with a three-dimensional (3D) printed head phantom (based on the planning CT of the patient) for 3D range verification of intensity-corrected cone beam computed tomography (CBCT) image data for adaptive proton therapy. Single field uniform dose pencil beam scanning proton plans were optimized for three different patients on the patients' planning CT (planCT) and the patients' intensity-corrected CBCT (scCBCT) for the same target volume using the same optimization constraints. The CBCTs were corrected on projection level using the planCT as a prior. The dose optimized on planCT and recalculated on scCBCT was compared in terms of proton range differences (80% distal fall-off, recalculation). Moreover, the dose distribution resulting from recalculation of the scCBCT-optimized plan on the planCT and the original planCT dose distribution were compared (simulation). Finally, the two plans of each patient were irradiated on the corresponding patient-specific 3D printed head phantom using gel dosimetry inserts for one patient and film dosimetry for all three patients. Range differences were extracted from the measured dose distributions. The measured and the simulated range differences were corrected for range differences originating from the initial plans and evaluated. The simulation approach showed high agreement with the standard recalculation approach. The median values of the range differences of these two methods agreed within 0.1mm and the interquartile ranges (IQRs) within 0.3mm for all three patients. The range differences of the film measurement were accurately matching with the simulation approach in the film plane. The median values of these range differences deviated less than 0.1mm and the IQRs less than 0.4mm. For the full 3D evaluation of the gel range differences, the median value and IQR matched those of the simulation approach within 0.7 and 0.5mm, respectively. scCBCT- and planCT-based dose distributions were found to have a range agreement better than 3mm (median and IQR) for all considered scenarios (recalculation, simulation, and measurement). The results of this initial study indicate that an online adaptive proton workflow based on scatter-corrected CBCT image data for head irradiations is feasible. The novel presented measurement- and simulation-based method was shown to be equivalent to the standard literature recalculation approach. Additionally, it has the capability to catch effects of image differences on the treatment plan optimization. This makes the measurement-based approach particularly interesting for quality assurance of CBCT-based online adaptive proton therapy. The observed uncertainties could be kept within those of the registration and positioning. The proposed validation could also be applied for other alternative in-room images, e.g. for MR-based pseudoCTs.

Dosimetric verification of stereotactic body radiotherapy treatment plan via ArcCHECK-3DVH system.

To study the feasibility of ArcCHECK-3DVH system in dosimetric verification for stereotactic body radiaotherapy (SBRT) with flattening filter free (FFF) model. SBRT treatment plans for 57 patients were introduced into ArcCHECK phantom and recalculated. The calculated dose distribution of treatment planning system and the measured dose distribution of ArcCHECK phantom were compared by γ analysis. Then the 3 dimensional dose distribution of target and organs at risk was reconstructed by 3DVH software. The reconstructed dose and calculated dose with treatment planning system (TPS) were compared, and the dose volume γ pass rate and deviation of dose volume parameters to the target and organs at risk were quantitatively valuated. Based on the threshold criteria (3%, 3 mm, 10%), namely the deviation of measuring points between the planned value and the measured value was less than 3%, and the proportion of points with similar values in the plane or sphere with the center of the point and the radius of 3 mm was 10%, the relative and absolute dose pass rates of SBRT treatment plans in ArcCHECK system via γ analysis were greater than 95%. Based on the stricter threshold criteria (2%, 2 mm, 10%), the relative and absolute dose pass rates of SBRT treatment plan in ArcCHECK system via γ analysis were about 93%. In 3DVH dose verification, the γ pass rate of target and organs at risk was exceed 97%, and the deviations in 3DVH of the target and organs at risk were less than ±5%. The ArcCHECK-3DVH system in dose verification can provide more comprehensive dose distribution information to reasonably evaluate the SBRT plan, with more significance for guiding clinical treatment.

Evaluation of the dosimetric effect of scattered protons in clinical practice in passive scattering proton therapy

The present study verified and evaluated the dosimetric effects of protons scattered from a snout and an aperture in clinical practice, when a range compensator was included. The dose distribution calculated by a treatment planning system (TPS) was compared with the measured dose distribution and the dose distribution calculated by Monte Carlo simulation at several depths. The difference between the measured and calculated results was analyzed using Monte Carlo simulation with filtration of scattering in the snout and aperture. The dependence of the effects of scattered protons on snout size, beam range, and minimum thickness of the range compensator was also investigated using the Monte Carlo simulation. The simulated and measured results showed that the additional dose compared with the results calculated by the TPS at shallow depths was mainly due to protons scattered by the snout and aperture. This additional dose was filtered by the structure of the range compensator so that it was observed under the thin region of the range compensator. The maximum difference was measured at a depth of 16 mm (8.25%), with the difference decreasing with depth. Analysis of protons contributing to the additional dose showed that the contribution of protons scattered from the snout was greater than that of protons scattered from the aperture when a narrow snout was used. In the Monte Carlo simulation, this effect of scattered protons was reduced when wider snouts and longer‐range proton beams were used. This effect was also reduced when thicker range compensator bases were used, even with a narrow snout. This study verified the effect of scattered protons even when a range compensator was included and emphasized the importance of snout‐scattered protons when a narrow snout is used for small fields. It indicated that this additional dose can be reduced by wider snouts, longer range proton beams, and thicker range compensator bases. These results provide a better understanding of the additional dose from scattered protons in clinical practice.

Implementation of free breathing respiratory amplitude‐gated treatments

PurposeThe purpose of this study was to provide guidance in developing and implementing a process for the accurate delivery of free breathing respiratory amplitude‐gated treatments.MethodsA phase‐based 4DCT scan is acquired at time of simulation and motion is evaluated to determine the exhale phases that minimize respiratory motion to an acceptable level. A phase subset average CT is then generated for treatment planning and a tracking structure is contoured to indicate the location of the target or a suitable surrogate over the planning phases. Prior to treatment delivery, a 4DCBCT is acquired and a phase subset average is created to coincide with the planning phases for an initial match to the planning CT. Fluoroscopic imaging is then used to set amplitude gate thresholds corresponding to when the target or surrogate is in the tracking structure. The final imaging prior to treatment is an amplitude‐gated CBCT to verify both the amplitude gate thresholds and patient positioning. An amplitude‐gated treatment is then delivered. This technique was commissioned using an in‐house lung motion phantom and film measurements of a simple two‐field 3D plan.ResultsThe accuracy of 4DCBCT motion and target position measurements were validated relative to 4DCT imaging. End to end testing showed strong agreement between planned and film measured dose distributions. Robustness to interuser variability and changes in respiratory motion were demonstrated through film measurements.ConclusionsThe developed workflow utilizes 4DCBCT, respiratory‐correlated fluoroscopy, and gated CBCT imaging in an efficient and sequential process to ensure the accurate delivery of free breathing respiratory‐gated treatments.

Technical Note: Three-dimensional QA of simultaneous integrated boost radiotherapy treatments by a dose-volume histogram methodology and its comparison with 3D gamma results.

Intensity-modulated radiotherapy with simultaneous integrated boost (SIB) presents several attractive advantages to be employed in clinical practice. Its secure application demands a rigorous quality assurance (QA) procedure, ideal for three-dimensional (3D) dose distribution measurements. Thus, a gel dosimetry methodology to evaluate the dose delivery of SIB treatments is presented and compared to conventional gamma evaluation. MAGIC-f gel dosimeter with magnetic resonance images for dose reading were used following its standard procedures. Four SIB QA plans created in gel dosimeter phantoms were used. The gel measured and treatment planning system (TPS) calculated doses were compared using 3D gamma analyses (3%/3mm/15% threshold). Two structures were artificially on the TPS dose distribution expected on the phantom by converting the 1.7 and 2.0Gy isodose levels into structures to represent the treatment. The gel and TPS dose-volume histogram (DVH) were compared based on five dose points: D95%, D90%, D50%, D10%, and D5%. Approvals of 93%, 96%, 98%, and 92% were achieved in the 3D gamma analyses for the plans QA 1, 2, 3, and 4. In the DVH analyses, QA plan 1 measured and expected curves showed a good agreement. QA plan 2 showed deviations in the highest doses for both structures with a maximum deviation (Δmáx ) of 8.0%. QA plans 3 and 4 showed the highest dose variation between the gel and TPS in the smaller doses of the DVH (Δmáx of 7.2% and -8.9%, respectively). For QA plan 4, the curves of the 1.7Gy structure presented a good agreement, but deviations in the smaller dose region of the DVH occurred for the 2Gy structure (Δmáx of 7.7%). A methodology for 3D dose evaluation of complex SIB treatments was proposed. It provided an important overview of the dose distributions. Their results significantly complemented the usual gamma analysis results.

Investigation of The Compliance of Planned Dose Distribution on The Treatment Planning System with Measured Dose Distribution in Rando Phantom For IMRT and Conformal Radiotherapy Applications

Purpose This study investigates planned and measured dose differences for 3 dimensional conformal radiotherapy (3DCRT), Field in Field (FinF) and Intensity-Modulated Radiation Therapy (IMRT) techniques for breast cancer radiotherapy. Material and Method Breast shaped phantom (BSP) was fabricated with water equevelent bolus material and BSP was placed on Alderson rando phantom, then planning computerized tomography (CT) images were obtained. Target volume andnormal tissues were delineated on these images. Treatment planning was performed for 3DCRT, FinF and IMRT techniques (Varian, Eclipse 8,6, PencilBeam). EBT2 films were placed into sagittal and transverse planes of BSP which intersect with the isocenter. After thatfilms were irradiated for each calculated plans in two times. The measured doses with film from the isocenter and from the axes passing 1 cm anterior-posterior, superior – inferior and right-left of the isocenter were compared with the doses read from the treatment planning system (TPS) on the same axes. Results Among applied techniques, the maximum and minimum dose differences for measured and calculated doses were 4.5 % and 0.05 % for FinF and IMRT techniques, respectively. Measured doses were higher than calculated doses for 66.6% of all points; however 93% of total points had a difference less than 3 %. Dose differences were minimum on axes that intersect with the isocenter and were directly correlated with the distance from the isocenter. There was no relation between dose differences and directions on axes. Conclusion Dose distributions calculated in TPS were found to be compatible for all three techniques when compared to dose distributions measured by film on rando Phantom. The best compliance was found for IMRT technique.

‘End to end’ dosimetry audit for HDR brachytherapy

Purpose: A dosimetric audit of Ir-192 high dose rate (HDR) brachytherapy remote afterloading 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 methods: A 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.

Fiducial detection and registration for 3D IMRT QA with organ-specific dose information.

PurposeTwo‐dimensional (2D) IMRT QA has been widely performed in Radiation Oncology clinic. However, concerns regarding its sensitivity in detecting delivery errors and its clinical meaning have been raised in publications. In this study, a robust methodology of three‐dimensional (3D) IMRT QA using fiducial registration and structure‐mapping was proposed to acquire organ‐specific dose information.MethodsComputed tomography (CT) markers were placed on the PRESAGE dosimeter as fiducials before CT simulation. Subsequently, the images were transferred to the treatment planning system to create a verification plan for the examined treatment plan. Patient’s CT images were registered to the CT images of the dosimeter for structure mapping according to the positions of the fiducials. After irradiation, the 3D dose distribution was read‐out by an optical‐CT (OCT) scanner with fiducials shown on the OCT dose images. An automatic localization algorithm was developed in MATLAB to register the markers in the OCT images to those in the CT images of the dosimeter. SlicerRT was used to show and analyze the results. Fiducial registration error was acquired by measuring the discrepancies in 20 fiducial registrations, and thus the fiducial localization error and target registration error (TRE) was estimated.ResultsDosimetry comparison between the calculated and measured dose distribution in various forms were presented, including 2D isodose lines comparison, 3D isodose surfaces with patient’s anatomical structures, 2D and 3D gamma index, dose volume histogram and 3D view of gamma failing points. From the analysis of 20 fiducial registrations, fiducial registration error was measured to be 0.62 mm and fiducial localization error was calculated to be 0.44 mm. Target registration uncertainty of the proposed methodology was estimated to be within 0.3 mm in the area of dose measurement.ConclusionsThis study proposed a robust methodology of 3D measurement‐based IMRT QA for organ‐specific dose comparison and demonstrated its clinical feasibility.

82786 Quantification of the Accuracy of Stereotactic Radiosurgery using Surface Guided Imaging with 3D Printed Head Phantoms

ABSTRACT IMPACT: This work assesses clinical implementation of a surface guided imaging system to improve the accuracy radiation delivery for treatment of brain lesions using a patient CT derived head phantom. OBJECTIVES/GOALS: Advancements in radiotherapy design have made clinical demand for efficient and accurate methods to deliver stereotactic radiosurgery (SRS) for treatment of intracranial lesions. This study assesses the potential of using surface guided imaging for setup using a 3D patient specific head phantom. METHODS/STUDY POPULATION: A single isocenter, multiple metastases SRS plan was generated on a CT derived RTsafe Prime phantom made of tissue equivalent materials and a polymer gel insert. Five targets of varying diameters were treated with 8Gy of radiation using two different positioning techniques. The first gel insert was irradiated within the phantom according to internal alignment with standard orthogonal x-ray imaging while the second setup used surface guided imaging, based on external anatomy. 42 hours after irradiation, the phantom was scanned in a head coil using a 1.5T MRI. MR images were fused with the patient CT data and structure set to further evaluate calculated and measured dose distributions. RESULTS/ANTICIPATED RESULTS: Discrepancies in phantom setup according to standard orthogonal x-ray imaging compared to surface guided imaging demonstrated to be <1mm in each translational (vertical, longitudinal, and lateral) and angular (rot, roll, pitch) directions. The 3D gel inserts permitted spatial analysis to compare dose distributions of measured values to those calculated in a treatment planning system (TPS). 3D GI (Gamma Index) analysis showed good alignment in high dose regions and resulted in passing rates >94% (5%/2mm) and >87% (3%/2mm). Finally, 3 of 5 targets showed better 3D GI passing rates and less geometric offset for positioning with the surface guided imaging. DISCUSSION/SIGNIFICANCE OF FINDINGS: 3D spatial analysis of human like phantoms demonstrated that patient positioning according to external anatomy performed comparable to standard methods aligning to the internal anatomy, for a multiple met SRS treatment.

Measurements of Electron Beam Dose Distributions in Perspex Block for Different Field Size

The linear accelerator is used in radiotherapy to treat tumors whether it was benign or malignant. The megavoltage electron beam is used to treat superficial tumors that do not exceed 5cm depth. In this work the dose distribution before irradiating the patient to check whether the prescribed dose is matched with the irradiated dose to help the physicist to fix the errors or deviations if it is detected. The quality assurance (QA) measured of electron beam with energy 12 MeV at common different depths and field sizes using StarTrack 2D detector and to ensure that it does not exceed the recommended limits. The equipped QA device is a StarTrack 2D detector under the linear accelerator infinity from Elekta at 100 cm from source-to surface distance SSD. The tested energy 12 MeV at depths 0.5 cm and 1.5 cm for field sizes 6cm×6cm, 10cm×10 cm and 14cm×14cm as limits measured according to the International Electrotechnical Commissioning (IEC) protocols. Results show that the revealed an error in the output dose at 6cm×6cm and 10cm×10 cm field sizes for 1.5 cm depth but for 14cm×14cm field size at 0.5 cm depth, the dose found to be above the tolerance. Also it’s found that the output dose is highly reached to 1.5 cm depth than the 0.5 cm. Furthermore, all the rotation axis of the collimator are within the limits with a few noises in the signal at the inline and the crossline axis. We conclude that there was an error in the output does need to be recalibrated before irradiating the patients to electron beam therapy at 12 MeV to assure that the treatment is qualified and efficient.

Two-dimensional dose distribution measurement based on rotational optical fiber array: A Monte Carlo simulation study

Quality assurance is particularly important for modulated radiotherapies which improve dose conformity and involve complex processes of dose delivery. This work proposes a dose measurement method based on a rotational optical fiber array and the Cherenkov effect as an alternative pretreatment quality assurance method. Monte Carlo simulation toolkit Geant4 is used to study the accuracy and influencing factors of this method. Four different shapes of radiation field are designed to explore the influence of different reconstruction algorithms, fiber spacing, rotation angles, and radiation types on the accuracy of dose field measurement. The results show that the difference between the reconstructed dose field and the reference dose field is within 8% under the irradiation of uniform radiation field. To obtain high-precision reconstructed images, the fiber spacing should be within 2 mm, and at least 60 angles of projection data should be acquired when the fiber array is rotated 180°. In conclusion, the proposed new method using a rotating fiber array can reconstruct the radiation field distribution rapidly, and provides accurate information on the shape and intensity distribution of the radiation field. Further studies are needed to improve the accuracy and feasibility in real clinical practice.

MAGIC-f gel dosimeter reading: a comparison between an in-house optical CT and MRI imaging

Introduction: According to new developments in radiation therapy techniques, accurate ‎dose verification in three-dimensions has become more critical. Polymer gel dosimeters ‎‎(PGDs) are valuable tools to be used for this purpose. Nowadays, various imaging ‎modalities are employed to read out the gels. This study was aimed to investigate the ‎measured dose distribution recorded in MAGIC-f PGD with optical computed tomography ‎‎(OCT) by comparison with MRI. ‎Material and methods: We developed an in-house CCD based cone-beam OCT scanner. A ‎phantom of MAGIC-f PGD was used to measure a four-field box dose distribution. MRI and ‎OCT scanners were performed for gel readouts. Both measurement results were compared ‎by gamma index analysis with various acceptance criteria. The temporal stability of the gel ‎was also evaluated with the OCT readout system.‎Results: The percentage of isodose lines from two measured datasets agreed well together. ‎The pass rates were 99.02%, 96.8%, and 89.8% with 5%/5mm, 4%/4mm, and 3%/3mm ‎criteria, respectively, at the phantom's central axial slice. ‎Conclusion: The results indicate that the performance of this OCT system is almost the ‎same with acceptable discrepancies to the MRI as accepted standard readout modality, and ‎it can be used for three-dimensional dose verifications.‎

An experimental study of focused very high energy electron beams for radiotherapy

Very high energy electron (VHEE) beams have been proposed as an alternative radiotherapy modality to megavoltage photons; they penetrate deeply without significant scattering in inhomogeneous tissue because of their high relativistic inertia. However, the depth dose distribution of a single, collimated VHEE beam is quasi-uniform, which can lead to healthy tissue being overexposed. This can be largely overcome by focusing the VHEE beam to a small spot. Here, we present experiments to demonstrate focusing as a means of concentrating dose into small volumetric elements inside a target. We find good agreement between measured dose distributions and Monte Carlo simulations. Focused radiation beams could be used to precisely target tumours or hypoxic regions of a tumour, which would enhance the efficacy of radiotherapy. The development of new accelerator technologies may provide future compact systems for delivering these focused beams to tumours, a concept that can also be extended to X-rays and hadrons.

Quality assurance for dynamic tumor tracking

The purpose of this work was to develop a treatment plan verification routine for a linear accelerator dedicated to SBRT treatments with gimbal based dynamic tumor tracking using three commercially available phantoms.The accelerator system has two special features: It operates with a rotation of the ring shaped gantry instead of a couch rotation and target motion can be compensated for via a gimbal system (dynamic tumor tracking, DTT). DTT plans were each measured with the three different phantoms. Afterwards the measured dose distribution was compared with the calculated dose distribution via global Gamma Index analysis (3mm / 3%, threshold: 10%).The global gamma pass rates were on average (93.5±7.2) % for ArcCHECK, (98.0±2.6) % for OCTAVIUS® 4D and (98.4±4.2) % for MatriXX Evolution.All three systems could be used for quality assurance with ring rotations and DTT, however, each with limitations.

A protocol for absolute dose verification of SBRT/SRS treatment plans using Gafchromic™ EBT-XD films.

To provide a practical protocol for absolute dose verification of stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) treatment plans, based on our clinical experience. It aims to be a concise summary of the main aspects to be considered when establishing an accurate film dosimetry system. Procedures for film calibration and conversion to dose are described for a dosimetry system composed of Gafchromic™ EBT-XD films and a flatbed document scanner. Factors that affect the film-scanner response are also reviewed and accounted for. The accuracy of the proposed methodology was assessed by taking a set of strips irradiated to known doses and its applicability is illustrated for ten SBRT/SRS treatment plans. The film response was converted to dose using red and triple channel dosimetry. The agreement between the planned and measured dose distributions was evaluated using global gamma analysis with criteria of 3%/2mm 10% threshold (TH), 2%/2mm 10% TH, and 2%/2mm 20% TH. The differences between the expected and determined doses from the strips analysis were 0.9±0.6% for the red channel and 1.1±0.7% for the triple channel method. Regarding the SBRT/SRS plans verification, the mean gamma passing rates were 99.5±1.0% vs 99.6±1.0% (3%/2mm 10% TH), 96.9±3.5% vs 99.1±1.3% (2%/2mm 10% TH) and 98.4±1.8% vs 98.8±1.5% (2%/2mm 20% TH) for red and triple channel dosimetry, respectively. The proposed protocol allows for accurate absolute dose verification of SBRT/SRS treatment plans, applying both single and triple channel methods. It may work as a guide for users that intend to implement a film dosimetry system.

Investigation of Verification Methods of Field Matching Using Polymer Gel Dosimeter in Proton Therapy

In our proton beam therapy center, we use imaging plates (IP) for dose verification of field matching in irradiation (called patch-field technique). In this study, a polymer gel dosimeter, which can perform three- dimensional dose distribution measurement, was used as a new tool for the verification of the patch field irradiation method corresponding to the rectangular irradiation. The results of measurements of the PAGAT gel dosimeter to irradiate to two rectangular fields called as patch irradiation fields, which were created using the treatment planning system, were evaluated and compared to the results of IP and plan in profiles near the boundary of two fields. In the case of no gap between the two fields, the relative dose using the gel dosimeter was 10.1% higher compared to that measured with the IP in the midpoint of two fields. In case of overlap (called hot region), the result of gels was 6.3% higher than that of IP. In the case of space (called cold region), the result of gels was 14.9% higher than that of IP. The results of the difference between gel and plan in the midpoint of two fields were 14.2% (no gap), -5.0% (hot), and 10.5% (cold). We found that the gel dosimeter was a 3-D dosimetric tool and possibility method for dose verification of patch fields. In this study, the results were preliminary and included several error factors. In the future, it is necessary to develop a dosimeter with improved and more precise measurements.

Khảo sát một số đặc tính bức xạ của buồng ion hóa CC13 và diode bán dẫn razor đối với các chùm photon kích thước nhỏ trên máy gia tốc tuyến tính xạ trị Clinac IX

Purpose: Compare percent depth dose (PDD) and off-center ratio (OCR) measured by the CC13 ionization chamber and the RAZOR silicon diode in small photon beams. Method and Materials: Some dosimetric characteristics, such as PDD, OCR, penumbra and radiation field size, were considered in this study for 2x2, 3x3, and 4x4 cm2 field sizes. We used the CC13 ionization chamber and the RAZOR silicon diode to measure dose distribution with depth along the axis and off-center of the beam. From the results obtained, the team investigated the differences in radiation parameters measured by the two types of probes above. Results: There are significant differences in the radiation parameters investigated for the CC13 ionization chamber and the RAZOR silicon diode, especially the width of penumbra. For PDD curves, the difference is less than 5% from dmax to 30 cm, however the difference becomes greater in the build-up region, which reaches to 33% at the water phatom surface. The width of penumbra measured by CC13 is always larger than that of RAZOR, the ratio of the penumbra width between two detectors is 1.8 and 1.3 for energies of 6 MV and 15 MV, respectively. Conclusion: The RAZOR silicon diode has better dose response than the CC13 ionization chamber for measuring the PDD and the OCR in small photon beams.

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

A 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. A 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 2cm 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. The 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. The 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.

3D relative dose measurement with a μm thin dosimetric layer

A μm thin dosimetric coating material based on inorganic phosphor particles NaYF4:Yb3+, Er3+ has been developed. A dose dependency of the luminescence decay time is observed in the μs - ms domain which allows relative dose measurement in the kGy range. Dose values are obtained from decay time evaluation which facilitates a measurement independent of luminescence emission intensity. Here, near-IR luminescence after excitation with 5 ms pulses of near-infrared light is analyzed. The dosimetric material can be directly sprayed onto the surface of objects and therefore renders particularly useful for dose distribution measurements on non-planar geometries. Because of the μm thickness and the direct contact of the dosimetric material to the entire surface of an object, the results can be considered as ‘true’ surface doses. Three dimensional dose maps were derived by scanning the entire surfaces of irradiated objects. Proof-of-concept is provided by good agreement in dose results as well as dose pattern with film dosimetry (Risø B3 strips) and the approach therefore qualifies especially for use in 3D dose mapping of complex geometries. This novel approach provides unprecedented insights into dose distributions of irradiated objects of complex geometries, as exemplified by dose results for outer and inner surfaces of e-beam irradiated bottles for two different irradiation set-ups.

PO-1606: Measurement of dose distribution of cardiosynchronous brain motion in microbeam radiation therapy

PO-1606: Measurement of dose distribution of cardiosynchronous brain motion in microbeam radiation therapy