EBT3 Film Research Articles

Using convergent half width half Maximum(c-HWHM) tilting method to determine the virtual source position of scanning-pass scattering beam in carbon ion therapy

IntroductionWe developed a half-width half-maximum tilting angle convergent method to replace the conventional measurement of the virtual source position for scanning-pass scattering beam in carbon ion therapy. Materials and methodsA homemade large-format CMOS sensor and Gaf Chromic EBT3 films were used for the virtual source position measurement. The Gaf films were embedded in a self-designed rectangular plastic frame to tighten the films and set up on a treatment couch for irradiation in the air with the film perpendicular to the carbon ion beam at the nominal source-axis-distance (SAD) as well as upstream and downstream from the SAD. The radiation rectangular field size in horizontal and vertical directions on film or detector delivered by 5 carbon ion energies were analyzed separately in this study. The virtual source position was determined with film or detector at a distance upstream or downstream from the various source-film-distance by the half width half maximum tilting angle convergent method to compare the results conducted with the same measurement condition by the back projecting the FWHM to zero methods. ResultsThe determination of virtual source position by using half width half maximum tilting angle convergent method agrees with the method by back projecting the FWHM to zero, and for higher carbon ion energy has an obvious longer virtual source position since the more carbon ion beam energy, hence, the less side scatter results in the less divergent at the downstream, then the virtual source positions is distal for high energies than low energies when back projects the field sizes to zero. ConclusionWe have developed a technique capable of dealing with the virtual source position with a geometric convergent method to avoid any mistakes in the passive scatter carbon ion beam. The method for investigating the virtual source position in the carbon ion beam in this study can also be used for electrons and proton beams.

Experimental validation of a novel method of dose accumulation for the rectum

Background Dose-surface maps (DSMs) are an increasingly popular tool to evaluate spatial dose-outcome relationships for the rectum. Recently, DSM addition has been proposed as an alternative method of dose accumulation from deformable registration-based techniques. In this study, we performed the first experimental investigation of the accuracy at which DSM accumulation can capture the total dose delivered to a rectum’s surface in the presence of inter-fraction motion. Material and methods A custom PVC rectum phantom capable of representing typical rectum inter-fraction motion and filling variations was constructed for this project. The phantom allowed for the placement of EBT3 film sheets on the representative rectum surface to measure rectum surface dose. A multi-fraction prostate VMAT treatment was designed and delivered to the phantom in a water tank for a variety of inter-fraction motion scenarios. DSMs for each fraction were calculated in two ways using CBCT images acquired during delivery and summed to produce accumulated DSMs. Accumulated DSMs were then compared to film measurements using gamma analysis (3%/2 mm criteria). Similarity of isodose clusters between films and DSMs was also investigated. Results Baseline agreement between film measurements and accumulated DSMs for a stationary rectum was 95.6%. Agreement between film and accumulated DSMs in the presence of different types of inter.-fraction motion was ≥92%, and isodose cluster mean distance to agreement was within 1.5 mm for most scenarios. Overall, DSM accumulation performed the best when using DSMs that accounted for changes in rectum path orientation. Conclusion Dose accumulation performed with DSMs was found to accurately replicate total delivered dose to a rectum phantom in the presence of inter-fraction motion.

Small field output correction factors at 18MV.

The response of various detectors in the radiotherapy energy range has been investigated, especially for 6 and 10MV energies for small fields, and is summarized in TRS-483. However, data for accelerator energies above 10MV are sparse or unavailable for many detectors, especially for the energy of 18MV. Small variations in field output factors for the commissioning of a treatment planning system can have a high impact on calculation of dosedistributions. Many studies describe an energy dependence of the response for a large number of detectors. We wanted to close the gap for the 18MV energy regime and determined field output correction factors for different detectors at18MV. An ELEKTA Versa HD accelerator at 18MV was used together with a PTW MP3 water phantom at an SSD of 90cm. The following detectors were examined: PTW Semiflex 31021, PinPoint 3D 31022, diode 60012, diode 60008 and microDiamond 60019, Sun Nuclear EDGE detector, IBA PFD, SFD, Razor Chamber, Razor Nano Chamber and Razor Diode, Standard Imaging Scintillator Exradin W2 1x3, W2 1x1 and Gafchromic EBT3 film. The dose response was determined at a depth of 10cm for square fields between 0.5 and 10cm side length. As reference data a composure of radiochromic film data for small fields ( cm) and data of all compatible chambers for larger fields ( cm) was used. The effective field sizes of small fields were determined from profiles obtained on radiochromic film. The obtained field output correction factors obey the rules of the TRS-483protocol. The W2 1x1 scintillator and the Razor Chamber showed the smallest deviations from the reference curve. The shielded diodes (diode 60008, EDGE detector) showed the highest over-response at small fields, followed by PFD, microDiamond and the unshielded diodes (diode 60012, SFD). The ionization chambers exhibited the well-known volume effect, that is, strong under-response at small fields of up to 9% for the PinPoint 3D, 7% for the Razor Chamber and up to 30% for the Semiflex detector for the smallest studied field size. The small chambers showed a polarity effect in axial orientation, especially the Razor Nano Chamber. Corrections at 18 MV are generally larger than those provided by TRS-483, continuing the trend of increasing corrections between 6 and 10 MV also at a higher accelerator energy. Only the PinPoint 3D Chamber showed a slightly smallercorrection. Field output correction factors were determined for square field sizes between 0.5 and 10cm at 18MV. Most detectors needed a larger correction than at 6 and 10MV. Thus, the use of correction factors will improve beam data for18MV.

Dosimetric Investigation of Six Ru-106 Eye Plaques by EBT3 Radiochromic Films and Monte Carlo Simulation.

Ophthalmic brachytherapy using radioactive plaques is an effective technique for the treatment of uveal melanoma. Ru-106 eye plaques are considered as interesting issue due to their steep gradient dose. The pre-planning evaluation of dosimetric parameters is essential for the treatment planning system. The current study aims at providing dose distributions of six Ru-106 eye plaques (CCA, CCB, CGD, CIB, COB and COD) using radiochromic EBT3 film, Geant4 Monte Carlo toolkit and the treatment planning software (Plaque Simulator). In this experimental study, an in-house phantom was employed for depth dose measurements with EBT3 films. Also, Geant4.10.5 scoring mesh was implemented to obtain the 2D dose distribution of the plaques. The results were compared with Plaque Simulator software and the manufacturer's (BEBIG) data. The gamma index criterion (3%/3 mm) was used to evaluate dose distributions obtained by the film measurements and Geant4 simulation. A good agreement was achieved between simulation and experimental results. Gamma index passing rate was 94.2%, 89.3%, 88.2%, 82.2%, 92.2% and 90.1% for CCA, CCB, CGD, CIB, COB and COD plaques, respectively. Absolute dose rate (mGy/min) obtained by EBT3 film at the depth of 2 mm was 79.4 mGy/min, 81.0 mGy/min, 78.6 mGy/min, 62.2 mGy/min, 75.2 mGy/min and 81.2 mGy/min for CCA, CCB, CGD, CIB, COB and COD plaques, respectively. The measured dose distributions and lateral dose profiles may be utilized in the treatment planning system to cover clinical volumes such as the clinical target volume and the gross tumor volume.

Evaluation of a planar diode matrix for SRS patient-specific QA in comparison with GAFchromic films.

We validate the routine use of a two-dimensional (2D) diode matrix for patient specific pre-treatment verification for Cyberknife (CK) stereotactic radiosurgery and to compare it with film dosimetry. A total of 46 patients were selected according to the most frequent diseases treated at our institution with the CK system, that is, brain metastases, meningiomas, spine metastases, and prostate tumors. All cases were evaluated with GAFChromic EBT-3 films and SRS MapCHECK for Fixed cone, IRIS, and MLC collimators of the CK. The highest mean passing rate was observed for the SRS MapCHECK system compared to films. In order to assess if the two techniques provide statistically different results, a Wilcoxon Signed-Rank non-parametric test was performed (p<0.05) and we found gamma values significantly lower for EBT-3 films with respect to the SRS MapCHECK. We noticed a moderately significant association between the two techniques using Spearman's rank correlation coefficient (rs>0.4). We also performed the Bland-Altman statistical method: less than 5% of the differences resulted outside the range (mean±1.96×SD), so the two methods can be considered interchangeable within the combined inaccuracy. The use of SRS MapCHECK for CK patient specific quality assurance (QA) is feasible for a variety of clinical districts and could be reliably used as a replacement for radiochromic films.

Comprehensive evaluation and new recommendations in the use of Gafchromic EBT3 film.

Gafchromic film's unique properties of tissue-equivalence, dose-rate independence, and high spatial resolution make it an attractive choice for many dosimetric applications. However, complicated calibration processes and film handling limits its routine use. We evaluated the performance of Gafchromic EBT3 film after irradiation under a variety of measurement conditions to identify aspects of film handling and analysis for simplified but robust film dosimetry. The short- (from 5min to 100h) and long-term (months) film response was evaluated for clinically relevant doses of up to 50Gy for accuracy in dose determination and relative dose distributions. The dependence of film response on film-read delay, film batch, scanner type, and beam energy was determined. Scanning the film within a 4-h window and using a standard 24-h calibration curve introduced a maximum error of 2% over a dose range of 1-40Gy, with lower doses showing higher uncertainty in dose determination. Relative dose measurements demonstrated <1mm difference in electron beam parameters such as depth of 50% of the maximum dose value (R50 ), independent of when the film was scanned after irradiation or the type of calibration curve used (batch-specific or time-specific calibration curve) if the same default scanner was used. Analysis of films exposed over a 5-year period showed that using the red channel led to the lowest variation in the measured net optical density values for different film batches, with doses >10Gy having the lowest coefficient of variation (<1.7%). Using scanners of similar design produced netOD values within 3% after exposure to doses of 1-40Gy. This is the first comprehensive evaluation of the temporal and batch dependence of Gafchromic EBT3 film evaluated on consolidated data over 8years. The relative dosimetric measurements were insensitive to the type of calibration applied (batch- or time-specific) and in-depth time-dependent dosimetric signal behaviors can be established for film scanned outside of the recommended 16-24h post-irradiation window. We generated guidelines based on our findings to simplify film handling and analysis and provide tabulated dose- and time-dependent correction factors to achieve this without reducing the accuracy of dose determination.

Measuring dose in lung identifies peripheral tumour dose inaccuracy in SBRT audit

PurposeStereotactic Body Radiotherapy (SBRT) for lung tumours has become a mainstay of clinical practice worldwide. Measurements in anthropomorphic phantoms enable verification of patient dose in clinically realistic scenarios. Correction factors for reporting dose to the tissue equivalent materials in a lung phantom are presented in the context of a national dosimetry audit for SBRT. Analysis of dosimetry audit results is performed showing inaccuracies of common dose calculation algorithms in soft tissue lung target, inhale lung material and at tissue interfaces. MethodsMonte Carlo based simulation of correction factors for detectors in non-water tissue was performed for the soft tissue lung target and inhale lung materials of a modified CIRS SBRT thorax phantom. The corrections were determined for Gafchromic EBT3 Film and PTW 60019 microDiamond detectors used for measurements of 168 SBRT lung plans in an end-to-end dosimetry audit. Corrections were derived for dose to medium (Dm,m) and dose to water (Dw,w) scenarios. ResultsCorrection factors were up to −3.4% and 9.2% for in field and out of field lung respectively. Overall, application of the correction factors improved the measurement-to-plan dose discrepancy. For the soft tissue lung target, agreement between planned and measured dose was within average of 3% for both film and microDiamond measurements. ConclusionsThe correction factors developed for this work are provided for clinical users to apply to commissioning measurements using a commercially available thorax phantom where inhomogeneity is present. The end-to-end dosimetry audit demonstrates dose calculation algorithms can underestimate dose at lung tumour/lung tissue interfaces by an average of 2–5%.

Patient Dose Analysis Using GafchromicTM EBT3 Film: A Retrospective Study with a Four Dual-Field Technique in Total Skin Electron Therapy.

The Six dual-field is the most commonly used treatment technique in total skin electron therapy (TSET). Because of the prolonged treatment period, the patient may experience discomfort, and routine radiotherapy treatments may be affected. This reflects the idea of using a modified technique in TSET. The study aims to report our experience with the four dual-field technique and review the in-vivo dosimetry using gafchromic film. The in-vivo dosimetry reports using gafchromic EBT-3 films of 12 patients who received TSET with the four dual-field techniques in our hospital were analysed in this study. The dosimetric parameter including percentage depth dose, dose homogeneity, flatness and symmetry were analysed in this study. For all the patients, the mean dose to the skin was close to the prescription dose, and it was within 10% (99.3%-103%) of the prescription dose. The standard deviation was observed between 5.8 and 12.4 cGy. According to international standards, all of the measured dosimetric parameters were within the acceptable limit and thereby validating our technique. The in-vivo dosimetry study using radiochromic film in TSET is relatively uncommon. So, based on our results, gafchromic films are a viable choice. The objective of our four dual-field techniques is to reduce the overall treatment time on the machine, whereas our study shows a time reduction when compared to regular techniques, which aids in the smooth operation of daily routines. The preliminary results of this novel modified technique in TSET demonstrated favourable effectiveness with minimal skin toxicity. This four dual-field technique is simple and easy to implement. Comparatively, this study shows the dose homogeneity of ±10% and better dose in the underdose areas proving the reliability and homogeneity of four-dual field technique.

The study of virtual source position with the geometric convergent ratio errors prevention management method of spot scanning beam in carbon ion therapy.

We developed a technique including preventing errors management method capable of dealing with the virtual source position delivered by different carbon ion energies from the pattern of spot scanning beam in this study. A homemade large-format complementary metal-oxide-semiconductor (CMOS) sensor and Gaf Chromic EBT3 films were used for the virtual source position measurement. The Gaf films were embedded in a self-designed rectangular plastic frame to tighten the films and set up on a treatment couch for irradiation in the air with the film perpendicular to the carbon ion beam at the nominal source-axis-distance (SAD) as well as upstream and downstream from the SAD. The horizontal carbon ion beam with five energies at a machine opening field size was carried out in this study. The virtual source position was determined mainly with a linear regression by back projecting the full width half maximum (FWHM) to zero at a distance upstream from the various source-film-distance and double checks additionally with a geometric convergent method to avoid any mistakes caused by manual measurement on FWHM. The virtual source position for higher carbon ion energy has an obvious longer distance from the SAD since the more carbon ion beam energy, the less spreading affected by the horizontal and vertical magnetism, therefore, the distance of virtual source positions is decreased from SAD with high to low energy. The method for investigating the virtual source position in the carbon ion beam in this study can also be used for electrons and the proton. We have developed a technique capable of dealing with the virtual source position with a geometric convergent method to avoid any mistakes in spot scanning carbon ion beam.

Characterization and commissioning of a new collaborative multi-modality radiotherapy platform.

TaiChi, a new multi-modality radiotherapy platform that integrates a linear accelerator, a focusing gamma system, and a kV imaging system within an enclosed O-ring gantry, was introduced into clinical application. This work aims to assess the technological characteristics and commissioning results of the TaiChi platform. The acceptance testing and commissioning were performed following the manufacturer's customer acceptance tests (CAT) and several AAPM Task Group (TG) reports/guidelines. Regarding the linear accelerator (linac), all applicable validation measurements recommended by the MPPG 5.a (basic photon beam model validation, intensity-modulated radiotherapy (IMRT)/volumetric-modulated arc therapy (VMAT) validation, end-to-end(E2E) tests, and patient-specific quality assurance (QA)) were performed. For the focusing gamma system, the absorbed doses were measured using a PTW31014 ion chamber (IC) and PTW60016 diode detector. EBT3 films and a PTW60016 diode detector were employed to measure the relative output factors (ROFs). The E2E tests were performed using PTW31014 IC and EBT3 films. The coincidences between the imaging isocenter and the linac/gamma mechanical isocenter were investigated using EBT3 films. The image quality was evaluated regarding the contrast-to-noise ratio (CNR), spatial resolution, and uniformity. All tests included in the CAT met the manufacturer's specifications. All MPPG 5.a measurements complied with the tolerances. The confidence limits for IMRT/VMAT point dose and dose distribution measurements were achieved according to TG-119. The point dose differences were below 1.68% and gamma passing rates (3%/2 mm) were above 95.1% for the linac E2E tests. All plans of patient-specific QA had point dose differences below 1.79% and gamma passing rates above 96.1% using the 3%/2 mm criterion suggested by TG-218. For the focusing gamma system, the differences between the calculated and measured absorbed doses were below 1.86%. The ROFs calculated by the TPS were independently confirmed within 2% using EBT3 films and a PTW60016 detector. The point dose differences were below 2.57% and gamma passing rates were above 95.3% using the 2%/1 mm criterion for the E2E tests. The coincidences between the imaging isocenter and the linac/gamma mechanical isocenter were within 0.5mm. The image quality parameters fully complied with the manufacturer's specifications regarding the CNR, spatial resolution, and uniformity. The multi-modality radiotherapy platform complies with the CAT and AAPM commissioning criteria. The commissioning results demonstrate that this platform performs well in mechanical and dosimetry accuracy.

Dosimetric comparison and evaluation of two computational algorithms in VMAT treatment plans.

This study aimed to assess the accuracy and dosimetric impact of the Acuros XB (AXB) algorithm compared to the Anisotropic Analytical Algorithm (AAA) in two situations. First, simple phantom geometries were set and analyzed; moreover, volumetric modulated arc therapy (VMAT) clinical plans for Head & Neck and lung cases were calculated and compared. First, a phantom study was performed to compare the algorithms with radiochromic EBT3 film doses using one PMMA slab phantom and two others containing foam or air gap. Subsequently, a clinical study was conducted, including 20 Head & Neck and 15 lung cases irradiated with the VMAT technique. The treatment plans calculated by AXB and AAA were evaluated in terms of planning target volume (PTV) coverage (V95% ), dose received by relevant organs at risk (OARs), and the impact of using AXB with a grid size of 1mm. Finally, patient-specific quality assurance (PSQA) was performed and compared for 17 treatment plans. Phantom dose calculations showed a better agreement of AXB with the film measurements. In the clinical study, AXB plans exhibited lower Conformity Index and PTV V95% , higher maximum PTV dose, and lower mean and minimum PTV doses for all anatomical sites. The most notable differences were detected in regions of intense heterogeneity. AXB predicted lower doses for the OARs, while the calculation time with a grid size of 1mm was remarkably higher. Regarding PSQA, although AAA was found to exhibit slightly higher gamma passing rates, the difference did not affect the AXB treatment plan quality. AXB demonstrated higher accuracy than AAA in dose calculations of both phantom and clinical conditions, specifically in interface regions, making it suitable for sites with large heterogeneities. Hence, such dosimetric differences between the two algorithms should always be considered in clinical practice.

Development of a hydrated electron dosimeter for radiotherapy applications: A proof of concept.

Hydrated electrons, which are short-lived products of radiolysis in water, increase the optical absorption of water, providing a pathway toward near-tissue-equivalent clinical radiation dosimeters. This has been demonstrated in high-dose-per-pulse radiochemistry research, but, owing to the weak absorption signal, its application in existing low-dose-per-pulse radiotherapy provided by clinical linear accelerators (linacs) has yet to beinvestigated. The aims of this study were to measure the optical absorption associated with hydrated electrons produced by clinical linacs and to assess the suitability of the technique for radiotherapy (⩽ 1 cGy per pulse)applications. 40 mW of 660-nm laser light was sent five passes through deionized water contained in a 10 2 cm3 glass-walled cavity by using four broadband dielectric mirrors, two on each side of the cavity. The light was collected with a biased silicon photodetector. The water cavity was then irradiated by a Varian TrueBeam linac with both photon (10 MV FFF, 6 MV FFF, 6 MV) and electron beams (6 MeV) while monitoring the transmitted laser power for absorption transients. Radiochromic EBT3 film measurements were also performed forcomparison. Examination of the absorbance profiles showed clear absorption changes in the water when radiation pulses were delivered. Both the amplitude and the decay time of the signal appeared consistent with the absorbed dose and the characteristics of the hydrated electrons. By using literature value for the hydrated electron radiation chemical yield (3.0±0.3), we inferred doses of 2.1±0.2 mGy (10 MV FFF), 1.3±0.1 mGy (6 MV FFF), 0.45±0.06 mGy (6 MV) for photons, and 0.47±0.05 mGy (6 MeV) for electrons, which differed from EBT3 film measurements by 0.6%, 0.8%, 10%, and 15.7%, respectively. The half-life of the hydrated electrons in the solution was ∼ 24 s. By measuring 660-nm laser light transmitted through a cm-scale, multi-pass water cavity, we observed absorption transients consistent with hydrated electrons generated by clinical linac radiation. The agreement between our inferred dose and EBT3 film measurements suggests this proof-of-concept system represents a viable pathway toward tissue-equivalent dosimeters for clinical radiotherapyapplications.

Current Approach of Small Field Electron Beam for Head and Neck Cases

Regarding the increasing use of small-field photons in clinical treatment, in this study, we investigate the use of small-field electron beams in clinical treatment. This study aimed to evaluate small-field electron beam dosimetry of the nasopharyngeal, thyroid, and ethmoid sinus carcinoma cases. Dose measurement was done using EBT3 film. In nasopharyngeal cases with a homogenous area and irregular surface, the dose discrepancies for 6 MeV energy were unpredictable except for the 5×5 cm2 field size. For all energies in 5×5 cm2 field size, the dose discrepancies were less than 3%. In these cases, we found that a smaller electron beam field will increase the percentage of the dose discrepancy. This is caused by the effect of the lateral scatter disequilibrium in a small field electron beam. For ethmoid sinus cases, dose discrepancy depends on the field size and inhomogeneity of bone and tissue organ. Based on the evaluation of doses on the spinal cord, chiasm, and larynx (OAR), it can be seen that these organs received a very small dose. From this result, a small field electron beam is recommended for cases with a homogeneous target. However, in cases with a heterogenous target, further investigation is needed.

Gadolinium dose enhancement determination by unlaminated EBT3 films irradiated with 192Ir brachytherapy source

In this work, Gafchromic® EBT3 films were carefully delaminated and resealed with a thin mylar film of 12 μm for implementing accuracy measurements of Gd dose enhancement by a192Ir brachytherapy source. The intervened dosimeters were previously characterized in terms of their quality intervention (mechanical damage and dosimetry modification). The dose enhancement measurements were implemented in a phantom formed by a set of acrylic slabs of 30 × 30 × 0.5 cm3 machined to contain in the central axis 2 vials of 1.25 × 1.25 × 4.5 cm3 which were filled with a 20 mg/ml solution of Gd agents and with water simulating a tumoral phantom doped and undoped with Gd. In the center of the phantom, a machined acrylic slab was placed to introduce the 2.5 mm flexible guide of GammaMedplus iX equipment to position the source. Fragments of delaminated and resealed EBT3 of 0.9 × 4 cm2 were placed inside the second vial and 0.5 cm away from it, at 1.95 cm and 3.55 cm from the radioactive source respectively, to measure simultaneously the tumor dose enhancement and the radiation shielding produced at a proximal zone. CT images of the phantom were taken to prescribe 5 Gy to 1.95 cm (EBT3 film position) using a Varian BrachyVision™ planning system. Films were read out by a flatbed scanner Epson V750 PRO, 48 h after irradiation. The measurements in both tumor and healthy tissue were repeated ten times in each condition to analyze the uncertainties of the complete experimental realization. The results showed variations in the response of the intervened dosimeters, less than 1.7% for the range of interest and average fluctuations in the readout of 4.6%. The implemented dosimeter showed the feasibility of measuring dose enhancement by 192Ir source of 13 ± 7% for the concentration studied. Additionally, radiation shielding up to 6% was obtained, consistent with what has been reported in the literature.

High resolution 2D plastic scintillator detectors for radiotherapy departments

Abstract Introduction Plastic scintillation detectors (PSD) have been developed for over four decades and are widely used in a variety of fields, but one can find relatively few reports of their clinical use compared to other dosimetric solutions. Material and methods The inexpensive detector setup made of a Saint-Gobain BC-400 plastic scintillator and commercially available on the market CMOS-based DSLR Pentax camera was investigated. Build PSD detectors were irradiated with 6, 10 and 15 MV flattening filtered (FF) and 6 and 10 MV flattening filter free (FFF) photon beams using a clinical linear accelerator. Data were processed using Matlab software to remove background and artefacts. A comparison of the spatial resolution parameters to the Gafchromic EBT3 films was performed. Results Average dose difference between TPS and PSD was 1.1%. The measured spatial resolution was 0.29 mm, and it differed from the film by 1.1%. MTF50 for PSD was 0.57 mm higher than the Gafchromic film. Signal to dose fit function with an R-square equal to 0.999 was established. The standard deviation of mean pixels value for a series of measurements was below 0.1%, for variable dose rate dependence was below 0.6% and for different energies 1.1%. It was demonstrated that such a setup allows a satisfactory signal-to-dose dependence and provides high spatial resolution at an affordable price compared to a 2D ion chamber or a diode detector array. Moreover, PSDs are reusable and provide a simple readout compared to Gafchromic films commonly used in radiotherapy departments. Conclusions Variable parameters of the camera allow to select signal values at the optimal level. The system presented excellent signal stability, high image resolution and a simple signal-to-dose relationship which encourages further work to investigate PSDs for use in radiation therapy departments.

Experimental dosimetric characterization of proton pencil beam distortion in a perpendicular magnetic field of an in-beam MR scanner.

As it promises more precise and conformal radiation treatments, magnetic resonance imaging-integrated proton therapy (MRiPT) is seen as a next step in image guidance for proton therapy. The Lorentz force, which affects the course of the proton pencil beams, presents a problem for beam delivery in the presence of a magneticfield. To investigate the influence of the 0.32-T perpendicular magnetic field of an MR scanner on the delivery of proton pencil beams inside an MRiPT prototypesystem. An MRiPT prototype comprising of a horizontal pencil beam scanning beam line and an open 0.32-T MR scanner was used to evaluate the impact of the vertical magnetic field on proton beam deflection and dose spot pattern deformation. Three different proton energies (100, 150, and 220 MeV) and two spot map sizes (15 × 15 and 30 × 20 cm2 ) at four locations along the beam path without and with magnetic field were measured. Pencil-beam dose spots were measured using EBT3 films and a 2D scintillation detector. To study the magnetic field effects, a 2D Gaussian fit was applied to each individual dose spot to determine the central position , minimum and maximum lateral standard deviation ( and ), orientation (θ), and the eccentricity (ε). The dose spots were subjected to three simultaneous effects: (a) lateral horizontal beam deflection, (b) asymmetric trapezoidal deformation of the dose spot pattern, and (c) deformation and rotation of individual dose spots. The strongest effects were observed at a proton energy of 100 MeV with a horizontal beam deflection of 14-186 mm along the beam path. Within the central imaging field of the MR scanner, the maximum relative dose spot size decreased by up to 3.66%, while increased by a maximum of 2.15%. The largest decrease and increase in the eccentricity of the dose spots were 0.08 and 0.02, respectively. The spot orientation θ was rotated by a maximum of 5.39°. At the higher proton energies, the same effects were still seen, although to a lesserdegree. The effect of an MRiPT prototype's magnetic field on the proton beam path, dose spot pattern, and dose spot form has been measured for the first time. The findings show that the impact of the MF must be appropriately recognized in a future MRiPT treatment planning system. The results emphasize the need for additional research (e.g., effect of magnetic field on proton beams with range shifters and impact of MR imaging sequences) before MRiPT applications can be employed to treat patients.

Dosimetric Study to Estimate Deviations in Delivered Radiation Dose due to Occluded Air Spaces in Vaginal Vault Brachytherapy Applications.

To develop a dosimetric tool to estimate the dose delivered in the presence of air pockets with EBT3 film while simulating the conditions of vaginal vault brachytherapy (VVBT) with 3.0 diameter cylindrical applicator at a prescription dose distance of 5mm from the surface of it. Six acrylic plates (10 cm x 10 cm, 0.5 cm thick) with four different types of slots were designed and produced locally. They can hold a cylindrical vaginal brachytherapy applicator in the centre, air equivalent material from the applicator's surface [(sizes 4.5 mm (A), 3.0 mm (B), and 2.0 mm (C)], EBT3 film at the prescribed dose distance, and holder rods. Plates were layered together with acrylic rods and assembled in a holding box in a water phantom. Three treatment plans done in TPS with prescription doses of 2 Gy, 3 Gy, and 4 Gy at 5.0 mm with a treatment length of 6 cm, and were executed in Co-60-based HDR brachytherapy unit (M/s SagiNova, Germany) with & without the placement of air equivalent material, and the dose received at slot locations A, B, & C were noted. The mean percentage deviation of measured dose without and with presence of air pocket at A, B and C was 13.9%, 11.0% and 6.4% respectively for all dose prescriptions. As the air pocket size expanded radially from 2.0 mm to 4.5 mm, the increase in dosage ranged from 6.4% to 13.9% which was due to the fact that the film was held at dosage prescription distance and the lack of attenuation of photons radially through air pocket. The present study can be carried out with a 3D printed phantom that simulates VVBT application having air pockets of different dimensions at different locations and also can be analyzed with Monte Carlo simulations.

Characterization of scanning orientation and lateral response artifact for EBT4 Gafchromic film.

The purpose of this study was to investigate the impact of scanning orientation and lateral response artifact (LRA) effects on the dose-response of EBT4 films and compare it with that of EBT3 films. Dose-response curves for EBT3 and EBT4 films in red-green-blue (RGB) color channels in portrait orientation were created for unexposed films and for films exposed to doses ranging from 0 to 1000cGy. Portrait and landscape orientations of the EBT3 and EBT4 films were scanned to investigate the scanning orientation effect in the red channel. EBT3 and EBT4 films were irradiated to assess the LRA in the red channel using a field size of 15×15cm2 and delivered doses of 200, 400, and 600cGy. Films were scanned at the edge of the scanner bed, and the measured doses were compared with the treatment planning system (TPS) calculated doses at a position 100mm lateral to the scanner center. At a dose of 200cGy, the differences in optical density (OD) in the red, green, and blue color channels between EBT3 and EBT4 films were 0.035 (24.8%), 0.042 (49.7%), and 0.022 (64.4%), respectively. The EBT4 film slightly improved the scanning orientation compared to the EBT3 film. The OD difference in the different scanning orientations for the EBT3 and EBT4 films was 0.015 (6.8%) and 0.007 (3.9%), respectively, at a dose of 200cGy. This is equivalent to a 20 or 10cGy variation at a dose of 200cGy. Compared with the TPS calculation, the measurement doses for EBT3 and EBT4 films irradiated at 200cGy were approximately 16% and 13% higher, respectively, at the 100mm off-centered position. The EBT4 film showed an improvement concerning the impact of LRA compared with the EBT3 film. This study demonstrated that the response of EBT4 film to a dose in the blue channel was less sensitive and showed an improvement in the scanning orientation and LRA effects.

Study of a plastic scintillating plate-based quality assurance system for pencil beam scanning proton beams.

The purpose of this study was to evaluate a plastic scintillating plate-based beam monitoring system to perform quality assurance (QA) measurements in pencil beam scanning proton beam. Single spots and scanned fields were measured with the high-resolution dosimetry system, consisting of a plastic scintillation plate coupled to a camera in a dark box at the isocenter. The measurements were taken at 110-190 MeV beam energies with 30° gantry angle intervals at each energy. Spot positions were determined using the plastic scintillating plate-based dosimetry system at the isocenter for 70-230 MeV beam energies with 30° gantry angle intervals. The effect of gantry angle on dose distribution was also assessed by determining the scanning pattern for daily QA and 25 fields treated with intensity-modulated proton therapy. Spot size, field flatness, and field symmetry of plastic scintillating plate-based dosimetry system were consistent with EBT3 at all investigated energies and angles. In all investigated energies and angles, the spot size measured was ±10% of the average size of each energy, the spot position measured was within ±2 mm, field flatness was within ±2%, and field symmetry was within ±1%. The mean gamma passing rates with the 3%/3 mm gamma criterion of the scanning pattern and 25 fields were 99.2% and 99.8%, respectively. This system can be effective for QA determinations of spot size, spot position, field flatness, and field symmetry over 360° of gantry rotation in a time- and cost-effective manner, with spatial resolution comparable to that of EBT3 film.

Preclinical Dosimetry for Small Animal Radiation Research in Proton Therapy: A Feasibility Study.

To evaluate the feasibility of the three-dimensional (3D) printed small animal phantoms in dosimetric verification of proton therapy for small animal radiation research. Two different phantoms were modeled using the computed-tomography dataset of real rat and tumor-bearing mouse, retrospectively. Rat phantoms were designed to accommodate both EBT3 film and ionization chamber. A subcutaneous tumor-bearing mouse phantom was only modified to accommodate film dosimetry. All phantoms were printed using polylactic-acid (PLA) filament. Optimal printing parameters were set to create tissue-equivalent material. Then, proton therapy plans for different anatomical targets, including whole brain and total lung irradiation in the rat phantom and the subcutaneous tumor model in the mouse phantom, were created using the pencil-beam scanning technique. Point dose and film dosimetry measurements were performed using 3D-printed phantoms. In addition, all phantoms were analyzed in terms of printing accuracy and uniformity. Three-dimensionally printed phantoms had excellent uniformity over the external body, and printing accuracy was within 0.5 mm. According to our findings, two-dimensional dosimetry with EBT3 showed acceptable levels of γ passing rate for all measurements except for whole brain irradiation (γ passing rate, 89.8%). In terms of point dose analysis, a good agreement (<0.1%) was found between the measured and calculated point doses for all anatomical targets. Three-dimensionally printed small animal phantoms show great potential for dosimetric verifications of clinical proton therapy for small animal radiation research.