Solid Water Phantom Research Articles

Out-of-field dose assessment for a 1.5 T MR-Linac with optically stimulated luminescence dosimeters.

PurposeTo assess the out‐of‐field surface and internal dose of the 1.5 T MR‐Linac compared to the conventional external beam linac using optically stimulated luminescence dosimeters (OSLDs), and evaluate the out‐of‐field dose calculation accuracy of the Monaco treatment planning system (TPS) of the 1.5T MR‐Linac.MethodsA cubic solid water phantom, with OSLDs on the surface, was vertically irradiated by MR‐Linac square fields with different sizes. In addition, OSLDs were arranged out of the beam edges in four directions. An anthropomorphic adult phantom, with 125 cm3 simulated volume, was irradiated in four orthogonal directions by both MR‐Linac and conventional linac at the head, thoracic, and pelvic sites. Out‐of‐field doses were measured by OSLDs on both the surface and internal emulational organs at risk (OARs). The results were compared to the simulated dose from Monaco TPS.ResultsAt different field sizes (5 × 5 to 20 × 20 cm2) and distances (1 to 10 cm) to beam edge, the out‐of‐field surface dose measured on MR‐Linac varied from 0.16 % (10 cm to 5 × 5 cm2 edge) to 7.02 % (1 cm to 20 × 20 cm2 edge) of the maximum dose laterally and from 0.14 % (10 cm to 5 × 5 cm2 edge) to 8.56 % (1 cm to 20 × 20 cm2 edge) of the maximum dose longitudinally. Compared to the OSLDs measured data, the Monaco TPS presented an overestimate of the out‐of‐field dose of OARs at 0–2 % isodose area on both surface and internal check points, and the overestimation gets greater as the distance increases. The underestimation was found to be 0–35% at 2–5% isodose area on both surface and internal check points. Compared to the conventional linac, MR‐Linac delivered higher average values of out‐of‐field dose on surface check points (20%, 19%, 21%) and internal simulated OARs (42%, 37%, 9%) of the anthropomorphic phantom at head, thoracic, and pelvic irradiations, respectively.ConclusionsCompared to the conventional linac, MR‐Linac has the same out‐of‐field dose distribution. However, considering the absolute dose values, MR‐Linac delivered relatively higher out‐of‐field doses on both surface and internal OARs. Additional radiation shielding to patients undergoing MR‐Linac may provide protection from out‐of‐field exposure.

Evaluation of SrAl2 O4 :Eu, Dy phosphor for potential applications in thermoluminescent dosimetry.

PurposeTo evaluate the use of commercial‐grade strontium aluminate phosphorescent powder as a thermoluminescent (TL) dosimeter for clinical radiotherapy beams.Materials and MethodCommercially available Eu2+, Dy3+ co‐doped strontium aluminate powder (SrAl2O4:Eu, Dy) was annealed and then irradiated using 20 × 20 cm2 field size, with 6‐MV (PDD10 = 70.7) and 18‐MV (PDD10 = 79.4) photon beams and and 9‐MeV (R50 = 3.6), 15 MeV (R50 = 5.9) and 18‐MeV (R50 = 7.2) electron beams. To calibrate the relationship between the TL readings and the irradiated doses, TL glow curves were acquired for doses up to 600 cGy at all beam energies. For the percentage depth dose (PDD) measurement, the SrAl2O4:Eu, Dy powder was sandwiched by solid water phantoms, with varying thickness of solid water placed above to determine the depth. PDDs were measured at four representative depths and compared against the commissioning depth dose data for each beam energy.ResultsLinear dose response models of doses up to 200 cGy were created for all beam energies. Superlinearity was observed with doses greater than 200 cGy. The PDD measurement acquired experimentally agrees well with the commissioning data of the medical linear accelerator. Trapping parameters such as order of kinetics, activation energy and frequency factor have been obtained via TL glow curve analysis.ConclusionThe linear dose response demonstrates that SrAl2O4:Eu, Dy is a potential TLD dosimeter for both electron beams and photon beams at different beam energies. The PDD measurements further support its potential use in quality assurance and radiation dosimetry.

Anisotropy function of a new 192-Ir brachytherapy source

Brachytherapy is a type of radiotherapy that uses radioactive sources (seeds, wires, among others) close to the tumor. Is important to provide a detailed description of seed dosimetry, so only the tumor will be irradiated avoiding unnecessary dose on adjacent organs and structures. To evaluate the dosimetric parameter of the anisotropy function for a new brachytherapy source, this work proposes the use of microcube TLD-100 dosimeters to find the dose rate using the AAPM Task Group 43 protocol (TG-43). The anisotropy function represents dose distribution around the source and has a major role for characterization of a new iridium source being implemented in Brazil. The value of D(r,θ) was measured using Solid Water phantoms, r value being the distance from the geometric center of the source to the position of the dosimeter on the phantom, and θ being the angle formed between the longitudinal axis of the source and the line connecting the geometric center to the TLD. Monte Carlo calculations were performed to evaluate the anisotropy function to validate the experimental measurements. For each distance value (r), an anisotropy function was plotted (1.0, 2.0, 3.0, 4.0, 5.0, and 10.0 cm). The results obtained with Monte Carlo calculations agreed ±2% with the experimental values for r greater than 3.0 cm, so these results show a good distribution of dose around the seed considering the high energy of 192-Ir (average of 380 KeV) and encapsulation thickness.

Ultrahigh dose-rate (FLASH) x-ray irradiator for pre-clinical laboratory research

FLASH irradiation has been shown to reduce significantly normal tissue toxicity compared to conventional irradiation, while maintaining tumor control probability at similar level. Clinical translation of FLASH irradiation necessitates comprehensive laboratory studies to elucidate biological effects as well as pertinent technological and physical requirements. At present, FLASH research employs complex accelerator technologies of limited accessibilities. Here, we study the feasibility of a novel self-shielded x-ray irradiation cabinet system, as an enabling technology to enhance the preclinical research capabilities. The proposed system employs two commercially available high capacity 150 kVp fluoroscopy x-ray sources with rotating anode technology in a parallel-opposed arrangement. Simulation was performed with the GEANT4 Monte-Carlo platform. Simulated dosimetric properties of the x-ray beam for both FLASH and conventional dose-rate irradiations were characterized. Dose and dose rate from a single kV x-ray fluoroscopy source in solid water phantom were verified with measurements using Gafchromic films. The parallel-opposed x-ray sources can deliver over 50 Gy doses to a 20 mm thick water equivalent medium at ultrahigh dose-rates of 40–240 Gy s−1. A uniform depth-dose rate (±5%) is achieved over 8–12 mm in the central region of the phantom. Mirrored beams minimize heel effect of the source and achieve reasonable cross-beam uniformity (±3%). Conventional dose-rate irradiation (≤0.1 Gy s−1) can also be achieved by reducing the tube current and increasing the distance between the phantom and tubes. The rotating anode x-ray source can be used to deliver both FLASH and conventional dose-rate irradiations with the field dimensions well suitable for small animal and cell-culture irradiations. For FLASH irradiation using parallel-opposed sources, entrance and exit doses can be higher by 30% than the dose at the phantom center. Beam angling can be employed to minimize the high surface doses. Our proposed system is amendable to self-shielding and enhance research in regular laboratory setting.

Characteristics of a bolus created using thermoplastic sheets for postmastectomy radiation therapy

This study applied a "shell bolus," an immobilizing thermoplastic shell locally thickened with extra layers over the radiation target, during postmastectomy radiation therapy (PMRT). We performed ion chamber and film measurements for a solid water phantom for thermoplastic sheets and a gel bolus for dosimetric characterization using a 6-MV X-ray flattening-filter-free (FFF) beam. The air gaps between the body surface for the gel and shell bolus were measured using computed tomography (CT) images in patients who underwent PMRT. This included seven and 13 patients treated with the gel and shell boluses, respectively. A comparison of the dose differences between a 10-mm gel bolus and a 9.6-mm-thick thermoplastic sheet at the surface and 5cm below the surface showed a 4.2% higher surface dose and 0.5% lower dose at 5-cm depth for the thermoplastic sheet compared to those for the gel bolus. The mean (p = 0.029) and maximum (p < 0.001) air gaps of the shell bolus were significantly thinner than those of the gel bolus. Thus, the shell bolus provided a close fit and robust bolus effect. In addition, the shell bolus reduced respiratory motion and eliminated the need for skin marking. Therefore, this system can be effectively used as a bolus for PMRT.

Reduction of superficial radiation dose with bolus in passive scattering proton beam therapy.

PurposeIn passive scattering proton beam therapy, scattered protons from the snout and aperture increase the superficial dose, however, treatment planning systems (TPSs) based on analytic algorithms (such as proton convolution superposition) are often inaccurate in this aspect. This additional dose can cause permanent alopecia or severe radiation dermatitis. This study aimed to evaluate the effect of bolus on the superficial radiation dose in passive scattering proton beam therapy.MethodsWe drew a clinical target volume (CTV) and a scalp‐p (phantom), and created plans using a TPS for a solid water phantom with and without bolus. We calculated the dose distribution in the established plans independently with Monte Carlo (MC) simulation and measured the actual dose distribution with an array of ion chambers and radiochromic films. To assess the clinical impact of bolus on scalp dose, we conducted independent dose verification using MC simulation in a clinical case.ResultsIn the solid water phantom without bolus, the calculated scalp‐p volume receiving 190 cGy was 20% with TPS but 80% with MC simulation when the CTV received 200 cGy. With 2 cm bolus, this decreased from 80% to 10% in MC simulation. With the measurements, average superficial dose to the scalp‐p was reduced by 5.2% when 2 cm bolus was applied. In the clinical case, the scalp‐c (clinical) volume receiving 3000 cGy decreased from 74% to 63% when 2 cm bolus was applied.ConclusionThis study revealed that bolus can reduce radiation dose at the superficial body area and alleviate toxicity in passive scattering proton beam therapy.

Electron FLASH Delivery at Treatment Room Isocenter for Efficient Reversible Conversion of a Clinical LINAC

In this study, procedures were developed to achieve efficient reversible conversion of a clinical linear accelerator (LINAC) and deliver ultrahigh-dose-rate (UHDR) electron or conventional beams to the treatment room isocenter for FLASH radiation therapy. The LINAC was converted to deliver UHDR beam within 20 minutes by retracting the x-ray target from the beam's path, positioning the carousel on an empty port, and selecting 10 MV photon beam energy in the treatment console. Dose rate surface and depth dose profiles were measured in solid water phantom at different field sizes with Gafchromic film and an optically stimulated luminescent dosimeter (OSLD). A pulse controller counted the pulses via scattered radiation signal and gated the delivery for a preset pulse count. A fast photomultiplier tube-based Cherenkov detector measured the per pulse beam output at a 2-ns sampling rate. After conversion back to clinical mode, conventional beam output, flatness, symmetry, field size, and energy were measured for all clinically commissioned energies. The surface average dose rates at the isocenter for 1-cm diameter and 1.5-in diameter circular fields and for a jaws-wide-open field were 238 ± 5 Gy/s, 262 ± 5 Gy/s, and 290 ± 5 Gy/s, respectively. The radial symmetry of the beams was within 2.4%, 0.5%, and 0.2%, respectively. The doses from simultaneous irradiation of film and OSLD were within 1%. The photomultiplier tube showed the LINAC required ramp up time in the first 4 to 6 pulses before the output stabilized, after which its stability was within 3%. At the isocenter of the treatment room, 10 MeV UHDR beams were achieved. The beam output was reproducible but requires further investigation of the ramp up time, equivalent to ∼1 Gy, requiring dose monitoring. The UHDR beam can irradiate both small and large subjects to investigate potential FLASH radiobiological effects in minimally modified clinical settings, and the dose rate can be further increased by reducing the source-to-surface distance.

Comparative Studies of Absolute Dose in Water Phantom, Solid Water Phantom and MatriXX with MULTICube Phantom

Introduction: Radiotherapy is one of the important treatment modalities in cancer treatment. To maintain the treatment procedure accuracy, the phantom is an essential tool for absolute dosimetry conformation and Quality Assurance routine cheek up. This work aims to study the absorbed dose of various phantoms and hence to make a comparison of the result with the IAEA recommendation (TRS 398) for daily QA of Linac. Materials and Methods: The experiment has been done at the Institute of Nuclear Medical Physics (INMP), AERE, Savar, Dhaka, under the Bangladesh Atomic Energy Commission. For external beam radiation, 6 MV and 15 MV photon beams of Varian Clinac iX Linear Accelerator (Linac) were used. One dimensional (1D) water phantom, solid water phantom, and MatriXX with MULTICube phantom and associate accessories were used to experiment. Results: We have measured and compared the absorbed dose data of the phantoms. The variation of sold water phantom from the 1D water phantom is +2.8% at 6 MV and +3.5% at 15 MV. The variation MatriXX with MULTIcube phantom from the 1D water phantom is +8.0% at 6 MV and +3.2% at 15 MV. This study revealed that the 1D water phantom was the best absolute dose conformation among the other phantoms and the deviation was within the acceptable limit (±5%), except MatriXX with MULTICube Phantom for low energy beam (8%). Conclusion: It was observed that the accuracy of dose estimation was better in the 1D water phantom rather than the other two. It is also known that the 1D water phantom is low cost but needs a long time to set up for the experimental arrangement. Solid water or MatriXX with MULTICube phantom can be used to overcome this problem, which takes only a few minutes for setup and is comparatively faster than 1D water phantom.

Investigation of Collapsed-cone Algorithm Accuracy in Small Fields and Heterogeneous Environments.

Background: The use of small fields has increased by the emergence of advanced radiotherapy. Dose calculations of these fields are complex and challenging for many reasons such as lack of electrical equilibrium even in homogeneous environments, and this complexity will increase in presence of heterogeneity. According to the importance of delivery the accurate prescription dose to the target volume in the patient’s body, the dose calculation accuracy of used commercial algorithms in clinical treatment planning systems (TPS) should be evaluated. Objective: The present study aims to evaluate the accuracy of Collapsed-cone dose measurement algorithm in Isogray treatment planning system. Material and Methods: In this analytical study, the measurements were made in tissue equivalent solid water phantom with lung and bone heterogeneities by Pinpoint dosimeter (0.015 cm3 sensitive volume) in several radiation fields (1×1 to 5×5 cm2). The phantoms were irradiated with 6, 10 and 18 MV photon beams and finally, the results of experimental calculations were compared with treatment planning outputs. Results: In all setups, the maximum deviation occurred in the field of 1×1 cm2. Then, the maximum deviation was observed for 2×2 cm2 field size; however, it was up to 5% for homogeneous water phantom and lung heterogeneity. In 3×3 cm2 and larger fields, there was a good agreement between the results of the TPS and experimental dosimetry. The maximum deviation was observed in water-bone heterogeneity. Conclusion: This algorithm was able to pass the standard audit criteria, but it is better to be used more cautiously in bone heterogeneity, especially in low energies.

Validation of Gamma Knife Perfexion Dose Profile Distribution by a Modified Variable Ellipsoid Modeling Technique.

ObjectiveHigh precision and accuracy are expected in gamma knife radiosurgery treatment. Because of the requirement of clinically applying complex radiation and dose gradients together with a rapid radiation decline, a dedicated quality assurance program is required to maintain the radiation dosimetry and geometric accuracy and to reduce all associated risk factors. This study investigates the validity of Leksell Gamma plan (LGP)10.1.1 system of 5th generation Gamma Knife Perfexion as modified variable ellipsoid modeling technique (VEMT) method. MethodsTo verify LGP10.1.1 system, we compare the treatment plan program system of the Gamma Knife Perfexion, that is, the LGP, with the calculated value of the proposed modified VEMT program. To verify a modified VEMT method, we compare the distributions of the dose of Gamma Knife Perfexion measured by Gafchromic EBT3 and EBT-XD films. For verification, the center of an 80 mm radius solid water phantom is placed in the center of all sectors positioned at 16 mm, 4 mm and 8 mm; that is, the dose distribution is similar to the method used in the x, y, and z directions by the VEMT. The dose distribution in the axial direction is compared and analyzed based on Full-Width-of-Half-Maximum (FWHM) evaluation. ResultsThe dose profile distribution was evaluated by FWHM, and it showed an average difference of 0.104 mm for the LGP value and 0.130 mm for the EBT-XD film. ConclusionThe modified VEMT yielded consistent results in the two processes. The use of the modified VEMT as a verification tool can enable the system to stably test and operate the Gamma Knife Perfexion treatment planning system.

The impact of treatment parameter variation on secondary neutron spectra in high-energy electron beam radiotherapy.

High-energy electron treatment procedures in radiotherapy pose a potential iatrogenic cancer risk as well as a critical health risk to patients with cardiac implantable electronic devices due to the generation of secondary neutrons in the linac head, the treatment room, and the patient. It may be argued that the neutron production from photons is well characterized, but the same is not true for electrons. Therefore, to assess the risk involved in an electron treatment, one must determine the neutron flux spectrum generated by the treatment procedure. The neutron spectrum depends on the treatment parameters used and therefore it is crucial to study its dependence on these parameters. In this work, eight experiments were devised to analyze how eight electron treatment parameters impacted the neutron spectrum. The parameters we considered were the electron beam energy, location of measurement, cutout size, collimator size, applicator size, collimator angle, choice of treatment room, and the presence or absence of a solid water phantom. For each experiment, we used a Nested Neutron Spectrometer™ (NNS) to measure the neutron flux spectra for multiple settings of the treatment parameter of interest. The resulting spectra were plotted and compared. We found that the electron beam energy and the location of measurement had the most impact on the neutron flux spectra, while the other parameters had a smaller or insignificant impact. This report may serve as a reference tool for medical physicists to help estimate the risk associated with a particular high-energy electron treatment procedure.

Evaluation of an Automated Algorithm for Design of 2-piece Molds for Patient-specific, Flexible Bolus

In external beam radiation therapy, use of sheet bolus can lead to compromises in set-up reproducibility and surface conformity, especially in areas of anatomical complexity. Patient-specific, flexible bolus has been shown to improve those aspects; however, manual fabrication involves using molds, which requires 3D modeling expertise and relies heavily on the skills of the technician. The aim of this work is to demonstrate that using 3D printed 2-piece molds, designed with an automated software algorithm, we were able to easily produce flexible silicone bolus for a variety of geometries, with accuracy in bolus thickness comparable to generic flexible bolus sheets, but with superior surface conformity. An anthropomorphic and a solid water phantom were CT scanned. TPS tools were used to create bolus structures for four anatomical sites (scalp, face, ear and breast) and one slab bolus. DICOM CT and contour data was then imported into the mold generating software. The automated algorithm for design of 2-piece mold STL files was then executed: mold design type was selected; a pouring hole was cut in the resulting mold structure and alignment guides were placed. Both block and shell mold designs were produced for each anatomical site. Mold STL files for 3D printing were exported, along with DICOM RT structure set files, for TPS-based verification of the resulting bolus structure accuracy. Once molds were 3D-printed, silicone rubber was poured. The thicknesses of the patient-specific bolus and a set of generic bolus sheets were measured with an ultrasonic thickness gauge. Patient-specific silicone boluses were positioned on phantoms and CT scanned; this process was repeated with standard bolus sheets taped to cover the same regions. Surface conformity was evaluated and compared. Silicone bolus HU homogeneity was evaluated from the CT data. Mold design took approximately five minutes, with an additional five minutes for TPS-based verification of the resulting bolus structure. 63% of all sheet bolus thickness measurements were found to fall within +/- 0.0 - 0.5 mm of the expected 5 mm and 10 mm sheet thickness and this was taken as the criterion for 3D printed mold-based bolus comparison. All silicone mold-based bolus thickness measurements fell within this criterion, for all anatomic and slab bolus sites and CT images demonstrate superior surface conformity for all anatomical site bolus. Mean silicone bolus density was 1.05 ± 0.04 g/cc and HU uniformity was 150 ± 19 HU. The automated algorithm for designing 2-piece molds is shown to be a simple and versatile method for producing spatially accurate, flexible, patient-specific bolus. 3D-printed mold-based silicone bolus provide superior patient fit and comparable thickness variation when compared to sheet bolus, and also demonstrate clinically acceptable radiological properties.

Dosimetry of Gamma-KnifeHybrid Shots With Film, Scintillator and the Microdiamond Detector

The Gamma-Knife Icon houses 192 Co60 sources grouped into 8 sectors and each sector is set on either 4mm, 8mm or 16mm wide collimators. Hybrid shots combine sectors of different collimators to form non-spherical dose distributions. This way, the conformality, especially for targets close to critical structures, can be improved while minimizing beam-on time. Hybrid shots are, however, not part of the quality assurance routine. The authors investigate dosimetric aspects of hybrid shots using film and two small-field detectors. GAFchromic EBT3 film, the PTW microdiamond detector, and the Standard Imaging W2 scintillator were used to measure the absolute dose for one 16mm, one 8mm and all eight 4mm single sector shots as well as twelve hybrid shots with varying collimator settings. The planned dose was 4Gy. The measurements were performed in a Leksell Gamma Knife Icon with the film inserted in the (gray) ABS plastic solid phantom and the two-point detectors inserted into the (red) Solid Water phantom. Both phantoms are 16cm in diameter and have been placed at isocenter (x = y = z = 100mm). The cross-calibration factor for the small field detectors were determined as the ratio of 10Gy delivered with a 16mm-collimator circular field and the corresponding electrometer reading. The film was calibrated in a range of 0.5Gy to 8Gy using the red channel. The mean optical density within a circle of 3mm diameter centered on the maximum density was used to determine the dose. The table below shows the average absolute dose in Gray along with the standard deviation measured by the three kinds of detectors. The film measurement for 4mm single sectors display an alternating dose fluctuation where the average dose for sectors 1, 3, 5 and 7 is 4.39±0.01Gy but 4.61±0.05Gy for sectors 2, 4, 6 and 8. Beam profiles show the peak region is elevated for evenly-numbered sectors but falls off to the same level as the other sectors in the penumbra. This indicates that the effect is not only an offset introduced by angular dependence of detection or readout sensitivity. The angular dependence is not observed with the two cylindrical point detectors. The investigation showed a large spread in absolute dose dependent on the type of detection device. Only the W2 scintillator was within 3% of the planned dose for both single sector and hybrid shots and offset and fluctuation were generally lower for the point detectors. While the film measurements are more complex and time consuming, the superior resolution helps indicating an angular dependence in single sector shots that is hidden by volume averaging of point detectors.Tabled 1Abstract 2646; TableIntegrating volumeEBT3 FilmMicrodiamondW2 scintillator0.13mm/pixel (scan)4mm x 0.001mm cyl.1mm x 1mm cyl.Single Sector4.49±0.134.17±0.073.91±0.08Hybrid Shot4.60±0.114.05±0.063.91±0.04 Open table in a new tab

A Comprehensive Quality Assurance Program of Small Animal Irradiators for Preclinical Radiation Research

Draeger et al. (Red Journal, 2020) raised awareness on the importance of the standardization of dosimetry protocols in pre-clinical radiobiology research and the inclusion of physicists on the research team. At our institute, a QA program of the small animal radiation research platform (SARRP, Xstrahl) that includes daily, monthly and annual tests have been implemented to ensure minimal delivery uncertainties. Details of the QA procedures, feasibility and results will be reported. End-to-end imaging and radiation isocenter coincidence testing using a Winston-Lutz phantom and EBT3 films, as well as CBCT imaging quality visual check, were performed daily by the research team. Monthly QA (mechanical and dosimetric tests such as the star-shot test and output constancy of the reference field), and annual QA (TG61 absolute dosimetry, imaging quality phantom check, output factor, beam profiles and percent depth dose measurements) were performed by the physics staff. The estimated time to complete the daily, monthly and annual tests were twenty minutes, one and two hours, respectively. The frequency of each QA item had been established based on the failure occurrence rate. The tolerance results of the daily, monthly and annual testing are displayed in the table. The results of the daily isocenter coincidence check show a discrepancy between the radiation isocenter and set-up center combined with the uncertainty from the couch movement of the image-guided system smaller than 1.5 mm. This test has successfully caught failures in the entire delivery process. For the monthly QA, diameter of the radiation isocenter was consistently smaller than 0.5 mm and the output has been measured to be within ±1% of baseline. The results of the annual QA, using an ion chamber and EBT3 films, match the beam data of the treatment planning system as well as the data from our in-house Monte Carlo calculation within the tolerance. Additionally, we used EBT3 films and a mouse solid water phantom as tools for verification of dosimetrically complex plans. The maximum dosimetry error observed has been consistently within 2%. With the implemented QA program, the fidelity of overall system delivery was ensured for robust and accurate small animal radiation studies without excess burden on the medical physicists and research team.Tabled 1Abstract 2596; TableProcedureToleranceDailyEnd to end imaging and radiation isocenter coincidence2 mmCBCT imaging quality visual checkBaselineLocalizing lasers1 mmMonthlyOutput constancy2%Isocentricity1 mmGantry position check1 mmAnnualOutput, flatness and symmetry, PDD, output factor1%CBCT imaging quality phantom checkBaseline Open table in a new tab

Estimated radiation doses to ovarian and uterine organs in breast cancer irradiation using radio-photoluminescent glass dosimeters (RPLDs).

IntroductionThe well‐being of breast cancer patients is essential, especially fertility in patients of reproductive age. The objective of this study was to estimate the radiation doses to the ovaries and uterus for different treatment techniques of breast cancer irradiation using radio‐photoluminescent glass dosimeters (RPLDs).MethodsA Farmer‐type ionisation chamber (IBA FC‐65G) and RPLDs were used to measure in‐ and out‐of‐field radiation doses in a solid water phantom. The field sizes were set to 10 × 10 cm2 and 8 × 17 cm2 with the central axis at out‐of‐field measurement distances of 30 or 50 cm. The Rando phantom’s left breast was planned using four different techniques: two tangential standard fields with and without electronic tissue compensator (E‐comp) techniques, intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). The radiation doses in the ipsilateral ovary, contralateral ovary and uterus were measured using RPLDs.ResultsThe percentage ratio of out of field to in field was affected by distance from the central axis to the point of measurement, in addition to the field sizes associated with collimator scatter. Advanced techniques such as IMRT and VMAT produced higher doses to the ovaries and uterus. The estimated results of the worst‐case scenario for the ipsilateral ovary, contralateral ovary and uterus were 0.84% (42 cGy), 0.62% (31 cGy) and 0.76% (38 cGy), respectively, for a 5000 cGy prescription dose.ConclusionThe lowest to highest out‐of‐field radiation doses to the ovarian and uterine organs from breast irradiation were the two tangential field techniques, VMAT and IMRT. These advanced techniques yielded higher radiation leakage, which potentially contributed to the out‐of‐field radiation dose.

Electron Paramagnetic Resonance Spectroscopy for Gamma Knife Dosimetry

Alanine chips exposed to high doses of radiation produces long lived free radicals that could be easily measured with electron paramagnetic resonance (EPR) spectrometers. In this study, the feasibility of using alanine dosimeters for performing rapid quality assurance of Leksell Gamma Knife (LGK) treatment plans was demonstrated. A 3D printed grid was placed inside the LGK spherical solid water phantom (SWP) for measurement of doses at isocentre and off-axis points. The EPR spectroscopy was performed on a Magnettech MS-5000 EPR/ESR spectrometer. A set of dose calibration curves were established prior to the use of alanine chips for LGK dosimetry. Absolute dose, transit dose and dose/timer linearity were performed with the alanine chips positioned at the centre of the LGK solid water phantom (SWP). Five patients of different sites were selected, and patient specific quality assurance (PSQA) was performed in the LGK SWP. The absolute dose measured with the EPR alanine dosimeter agreed well within 2% of the ion chamber results and PSQA results were within 2.1%. Alanine-based EPR dosimetry offers rapid dose measurement with high accuracy and can also be used as a dosimeter for Gamma Knife PSQA.

Feasibility of energy-resolved dose imaging technique in pencil beam scanning mode

Purpose: Proton energy-resolved dose imaging (pERDI) is a recently proposed technique to generate water equivalent path length (WEPL) images using a single detector. Owing to its simplicity in instrumentation, analysis and the possibility of using the in-room x-ray flat panels as detectors, this technique offers a promising avenue towards a clinically usable imaging system for proton therapy using scanned beams. The purpose of this study is to estimate the achievable accuracy in WEPL and Relative Stopping Power (RSP) using the pERDI technique and to assess the minimum dose required to achieve such accuracy. The novelty of this study is the first demonstration of the feasibility of pERDI technique in the pencil beam scanning (PBS) mode. Methods: A solid water wedge was placed in front of a 2D detector (Lynx). A library of energy-resolved dose functions (ERDF) was generated from the dose deposited in the detector by 50 PBS layers of energy varying from 100 MeV to 225 MeV. This set-up is further used to image the following configurations using the pERDI technique: stair-case shaped solid water phantom (configuration 1), electron density phantom (configuration 2) and head phantom (configuration 3). The result from configuration 1 was used to determine the achievable WEPL accuracy. The result from configuration 2 was used to estimate the relative uncertainty in RSP. Configuration 3 was used to evaluate the effect of range mixing on the WEPL. In all three cases, the variation of the accuracy with respect to dose, by varying the number of scanning layers, was also studied. Results: An accuracy of 1 mm in WEPL was achieved using the Lynx detector with an imaging field of 10 PBS layers or more, which is equivalent to a total dose of 5 cGy. The RSP is measured with a precision better than 2% for all homogeneous inserts of tissue surrogates. The pERDI technique failed for tissues surrogates with total WEPL outside the calibration window (WEPL < 70 mm) like in the case of lung exhale and lung inhale. The imaging of an anthropomorphic head phantom, in the same condition, produced a WEPL radiograph and compared to the WEPL derived from CT using gamma index analysis. The gamma index failed in the heterogeneous areas due to range mixing. Conclusions: The pERDI technique is a promising clinically usable imaging modality for reducing range uncertainties and set-up errors in proton therapy. The first results have demonstrated that WEPL and RSP can be estimated with clinically acceptable accuracy using the Lynx detector. Similar accuracy is also expected with in-room flat-panel detectors but at significantly reduced imaging dose. Though the issue of range mixing is still to be addressed, we expect that a statistical moment analysis of the ERDFs can be implemented to filter out the regions with high gradient of range mixing.

Dose profile evaluation of a 137Cs source using a solid water phantom

The precision in the dose values delivered in irradiation processes is essential for the efficiency and quality control of these processes. Radiochromic films can be used to record doses and the calibration of these films must be performed so that they can be used as dosimeters. The planning and control of the radiation released in a process allows to adjust the desired dose in the irradiated object. The photons in the primary beam interact with the matter of the object and the beam energy is attenuated due to these interactions. The attenuation depends on the characteristics of the beam and the composition of the irradiated matter. When a beam of photons propagates on an object, it tends to deposit more energy close to the surface and after reaching the maximum dose value, it decreases the dose values with depth. The films used in this work are of the Gafchromic External Beam Therapy (EBT) type, insensitive to visible light and can be prepared in places where sunlight and artificial light exists. Like many other dosimeters, which follow certain protocols, radiochromic films can provide an absolute dose measurement. Radiochromic films are characterized by their linearity, reproducibility, uniformity, sensitivity, and stability after irradiation. For the realization of the experiments, a part of the film to be irradiated was removed designated as background (BG). BG represents a piece of radiochromic film that will not change and reflects changes in film absorption in relation to environmental conditions such as temperature, visible light and scanning light, for example and that must be handled from it way that the film radiated. In this work, irradiations of a solid water phantom were performed using a source of cesium-137 with the deposition of a maximum absorbed dose value of 2.0 Gy. The phantom was placed 1,0 m far from the source collimator. Radiochromic films were placed inside the phantom to obtain the depth variation dose profile and axial dose profiles measured at 1.0 cm depth in the phantom. The dose variation profile in depth allowed to verify that the maximum dose value happened at a depth between 10 and 13 mm, very close to the surface due to the beam energy range (keV). The axial profiles presented a flatness of about 9.4 cm with a total field of 12 cm in diameter.

Head and Neck Immobilization Masks: Increase in Dose Surface Evaluated by EBT3, TLD‐100 and PBC Method

AbstractPositioning and immobilization tools are considered essential components of radiotherapy treatments to guarantee that the planned dose distribution can be efficiently reached. However, the benefits brought by their use are met by an apparent increase in the patient skin entrance dose. In the current study, we evaluated the dose surface effects provoked by the use of immobilization thermoplastic masks in head and neck radiotherapy treatments, carried out using a 6‐MV linear accelerator beam. The study was carried out using an anthropomorphic head–neck phantom and three dosimetric techniques: (i) thermoluminescent dosimetry (TL); (ii) radiochromic film dosimetry; and (iii) computational simulation using the pencil beam convolution (PBC) method. For calibration purposes, TLD chips and radiochromic (EBT3) small 2.0‐cm2 strip dosimeters were positioned between two virtual solid water plates, and exposed to absorbed doses ranging from 25 to 200 cGy. The use of an anthropomorphic head–neck phantom allows the dose variation in non‐flat surfaces to be taken into account. TLD chips, positioned on the surface of the supraclavicular fossa anatomical region, covered with a thermoplastic mask, detected an entrance skin dose that was approximately 38.4% higher than that measured without a mask. The EBT3 dosimeters, averaged among all strips used, also detected a medium increase of 58.6%. Both TLD and EBT3 detected increased doses for all measured points, and measured similar averaged surface doses without the use of immobilization masks; that is, 50.5% for EBT3 and 53.7% for TLD‐100. The pencil beam convolution simulation results suggested an increase for most of the measured points; however, no increased, and in some cases even decreased, doses were observed. The surface dose data of three other commercial thermoplastic masks irradiated in a solid water phantom are also provided.

Radiobiological effectiveness difference of proton arc beams versus conventional proton and photon beams

This paper aims to demonstrate the difference in biological effectiveness of proton monoenergetic arc therapy (PMAT) compared to intensity modulated proton therapy (IMPT) and conventional 6 MV photon therapy, and to quantify this difference when exposing cells of different radiosensitivity to the same experimental conditions for each modality.V79, H1299 and H460 cells were cultured in petri dishes placed in the central axis of a cylindrical and homogeneous solid water phantom of 20 cm in diameter. For the PMAT plan, cells were exposed to 13 mono-energetic proton beams separated every 15° over a 180° arc, designed to deliver a uniform dose of higher LET to the petri dishes. For the IMPT plans, 3 fields were used, where each field was modulated to cover the full target. Cells were also exposed to 6 MV photon beams in petri dishes to characterize their radiosensitivity. The relative biological effectiveness of the PMAT plans compared with those of IMPT was measured using clonogenic assays. Similarly, in order to study the quantity and quality of the DNA damage induced by the PMAT plans compared to that of IMPT and photons, γ-H2AX assays were conducted to study the relative amount of DNA damage induced by each modality, and their repair rate over time.The clonogenic assay revealed similar survival levels to the same dose delivered with IMPT or x-rays. However, a systematic average of up to a 43% increase in effectiveness in PMAT plans was observed when compared with IMPT. In addition, the repair kinetic assays proved that PMAT induces larger and more complex DNA damage (evidenced by a slower repair rate and a larger proportion of unrepaired DNA damage) than IMPT. The repair kinetics of IMPT and 6 MV photon therapy were similar.Mono-energetic arc beams offer the possibility of taking advantage of the enhanced LET of proton beams to increase TCP. This study presents initial results based on exposing cells with different radiosensitivity to other modalities under the same experimental conditions, but more extensive clonogenic and in-vivo studies will be required to confirm the validity of these results.