Small Radiation Fields Research Articles

Assessment of skin doses in small field radiotherapy for 6 MV photons and beam spectral analysis at skin surface: an EGSnrc based Monte Carlo study.

This study aims at the estimation of skin doses during small field radiotherapy with 6 MV photons and analysis of beam spectra at skin surface. The EGSnrc Monte Carlo code was used for spectral analysis and dose scoring in a water phantom. Percent skin dose (PSD) was calculated at a depth of 70µm (relative to 10cm depth), and the effects of field size, collimation, source-to-surface distance, and tissue inhomogeneity (bone/air) below the skin were evaluated. Low-energy photons and contaminant electrons from the machine head or back-scattered from underlying tissue were found to be the major contributors to skin dose. As the field size was reduced, the beam hardened, while the photon and electron fluences at the skin decreased compared to those at the reference depth of 10cm. This resulted in a PSD reduction for fields smaller than the reference field size. Multi leaf collimators increased the PSD (up to 4%) while variation in source-to-skin dose showed a negligible effect. A substantial increase in PSD has been observed (up to 6%) when high Z material like bone was placed below the skin. In contrast, air as underlying material decreased the skin dose. The skin dose varied considerably with various clinical and geometric parameters. It is concluded that, although the skin doses were low for small fields compared to those for the reference field, skin doses may become substantial when escalated target doses are delivered with multi leaf collimators. Moreover, the presence of high Z materials such as bones or metallic implants below the skin can result in significant enhancement of the skin dose.

Application of radiochromic gel dosimetry to commissioning of a megavoltage research linear accelerator for small-field animal irradiation studies.

To develop and implement an efficient and accurate commissioning procedure for small-field static beam animal irradiation studies on an MV research linear accelerator (Linatron-M9) using radiochromic gel dosimetry. The research linear accelerator (Linatron-M9) is a 9MV linac with a static fixed collimator opening of 5.08cm diameter. Lead collimators were manually placed to create smaller fields of 2×2cm2 , 1×1cm2 , and 0.5×0.5cm2 . Relative dosimetry measurements were performed, including profiles, percent depth dose (PDD) curves, beam divergence, and relative output factors using various dosimetry tools, including a small volume ionization chamber (A14), GAFCHROMIC™ EBT3 film, and Clearview gel dosimeters. The gel dosimeter was used to provide a 3D volumetric reference of the irradiated fields. The Linatron profiles and relative output factors were extracted at a reference depth of 2cm with the output factor measured relative to the 2×2cm2 reference field. Absolute dosimetry was performed using A14 ionization chamber measurements, which were verified using a national standards laboratory remote dosimetry service. Absolute dosimetry measurements were confirmed within 1.4% (k=2, 95% confidence=5%). The relative output factor of the small fields measured with films and gels agreed with a maximum relative percent error difference between the two methods of 1.1 % for the 1×1cm2 field and 4.3 % for the 0.5×0.5cm2 field. These relative errors were primarily due to the variability in the collimator positioning. The measured beam profiles demonstrated excellent agreement for beam size (measured as FWHM), within approximately 0.8mm (or less). Film measurements were more accurate in the penumbra region due to the film's finer resolution compared with the gel dosimeter. Following the van Dyk criteria, the PDD values of the film and gel measurements agree within 11% in the buildup region starting from 0.5cm depth and within 2.6 % beyond maximum dose and into the fall-off region for depths up to 5cm. The 2D beam profile isodose lines agree within 0.5mm in all regions for the 0.5×0.5cm2 and the 1×1cm2 fields and within 1mm for the larger field of 2×2cm2 . The 2D PDD curves agree within approximately 2% of the maximum in the typical therapy region (1-4cm) for the 1×1cm2 and 2×2cm2 and within 5% for the 0.5×0.5cm2 field. This work provides a commissioning process to measure the beam characteristics of a fixed beam MV accelerator with detailed dosimetric evaluation for its implementation in megavoltage small animal irradiation studies. Radiochromic gel dosimeters are efficient small-field relative dosimetry tools providing 3D dose measurements allowing for full representation of dose, dosimeter misalignment corrections and high reproducibility with low inter-dosimeter variability. Overall, radiochromic gels are valuable for fast, full relative dosimetry commissioning in comparison to films for application in high-energy small-field animal irradiation studies.

Microfluidic Chip Calorimeter for Small-Field Radiation Dosimetry

Recent advances in medical radiation technologies have enabled small field radiotherapy with a beam size as small as a few millimeters for precise dose delivery to patients. However, conventional radiation dosimeters for small field radiotherapy suffer from inaccurate dose measurements due to their large absorber size. Herein, we present a nanofabricated chip calorimeter as a new platform for a miniaturized dosimeter for measurements of the absorbed dose. The calorimeter consists of a microscale absorber body built with a SU-8 photoresist and a microfluidic water container. The measurement system was constructed such that the calorimeter operates in the heat compensation mode, where the temperature change of the absorber was proportional to the absorbed energy. The operability of the heat compensation calorimeter was examined with electric heating and a visible light source. The feasibility of application of the microfluidic chip calorimeter for medical radiation sensor was successfully demonstrated by measuring the absorbed dose from the small radiation field of the Gamma Knife.

SciFi detector and associated method for real-time determination of profile and output factor for small fields in stereotactic radiotherapy.

PurposeFor determining small‐field profile and output factor during stereotactic radiotherapy quality assurance (QA) procedures, we propose a novel system based on the scintillating fiber (SciFi) detector with output image acquisition and processing to allow real‐time monitoring of profile and output factor.Materials and methodsThe employed detector is a SciFi detector made of tissue‐equivalent scintillating plastic fibers arranged in 6‐layer fiber ribbons with a fiber pitch of 275 μm in each layer. The scintillating signal at the detector output is acquired by a sCMOS (scientific complementary metal–oxide–semiconductor) camera and represents the projected field profile along the fibers axis. An iterative reconstruction method of the field from its projected profile based on a priori knowledge of some features of the radiation field defined by the stereotactic cones is suggested. The detector with implemented data processing has been tested in clinical conditions, for determining beam profiles and output factors, using cone collimators of different sizes from 4 to 15 mm diameter. The detector under test was placed at 1.4 cm depth and 98.6 cm source to surface distance (SSD) in a water‐equivalent phantom and irradiated by a 6 MV photon beam.ResultsThe reconstructed field profiles obtained from the detector are coherent with data from EBT3 radiochromic films, with differences within ±0.32 mm for both the FWHM and the penumbra region. For real‐time determination of the field output factor, the measured data are also in good agreement with data independently determined by the French Institute for Radiological Protection and Nuclear Safety (IRSN) based on radiochromic films and thermoluminescent 1 × 1 mm2 micro‐cubes dosimeters (TLD). The differences are within ±1.6% for all the tested cone sizes.ConclusionsWe propose and have tested a SciFi plastic scintillating detector with an optimized signal processing method to characterize small fields defined by cone collimators. It allows the determination of key field parameters such as full width at half maximum (FWHM) and field output factors. The results are consistent with those independently measured using TLD and radiochromic films. As the SciFi detector does not require a correction factor, it is in line with the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 recommendations, and can be suitable for online QA of small radiation fields used in photon beam radiotherapy, and is compatible with MRI‐LINAC.

High spatial resolution inorganic scintillator detector for high-energy X-ray beam at small field irradiation.

PurposeSmall field dosimetry for radiotherapy is one of the major challenges due to the size of most dosimeters, for example, sufficient spatial resolution, accurate dose distribution and energy dependency of the detector. In this context, the purpose of this research is to develop a small size scintillating detector targeting small field dosimetry and compare its performance with other commercial detectors.MethodAn inorganic scintillator detector (ISD) of about 200 µm outer diameter was developed and tested through different small field dosimetric characterizations under high‐energy photons (6 and 15 MV) delivered by an Elekta Linear Accelerator (LINAC). Percentage depth dose (PDD) and beam profile measurements were compared using dosimeters from PTW namely, microdiamond and PinPoint three‐dimensional (PP3D) detector. A background fiber method has been considered to quantitate and eliminate the minimal Cerenkov effect from the total optical signal magnitude. Measurements were performed inside a water phantom under IAEA Technical Reports Series recommendations (IAEA TRS 381 and TRS 483).ResultsSmall fields ranging from 3 × 3 cm2, down to 0.5 × 0.5 cm2 were sequentially measured using the ISD and commercial dosimeters, and a good agreement was obtained among all measurements. The result also shows that, scintillating detector has good repeatability and reproducibility of the output signal with maximum deviation of 0.26% and 0.5% respectively. The Full Width Half Maximum (FWHM) was measured 0.55 cm for the smallest available square size field of 0.5 × 0.5 cm2, where the discrepancy of 0.05 cm is due to the scattering effects inside the water and convolution effect between field and detector geometries. Percentage depth dose factor dependence variation with water depth exhibits nearly the same behavior for all tested detectors. The ISD allows to perform dose measurements at a very high accuracy from low (50 cGy/min) to high dose rates (800 cGy/min) and was found to be independent of dose rate variation. The detection system also showed an excellent linearity with dose; hence, calibration was easily achieved.ConclusionsThe developed detector can be used to accurately measure the delivered dose at small fields during the treatment of small volume tumors. The author's measurement shows that despite using a nonwater‐equivalent detector, the detector can be a powerful candidate for beam characterization and quality assurance in, for example, radiosurgery, Intensity‐Modulated Radiotherapy (IMRT), and brachytherapy. Our detector can provide real‐time dose measurement and good spatial resolution with immediate readout, simplicity, flexibility, and robustness.

Melanin has a Small Inhibitory Effect on Cutaneous Vitamin D Synthesis: A Comparison of Extreme Phenotypes

Epidemiology suggests that melanin inhibits cutaneous vitamin D3 synthesis by UVR. Laboratory investigations assessing the impact of melanin on vitamin D production have produced contradictory results. We determined the effect of melanin on vitamin D3 photosynthesis in healthy young volunteers (n= 102) of Fitzpatrick skin types II-VI (white to black). Participants, irrespective of skin type, were exposed to the same suberythemal UVR dose, to 85% body surface area, using solar simulated UVR or narrowband UVB (311 nm). This was repeated five times with intervals of 3-4 days between UVR exposures. Blood was taken before, during, and after the irradiation and assessed for serum 25-hydroxyvitamin D3 (25[OH]D3) as a marker of vitamin D3 status. Linear UVR dose-dependent increases in 25(OH)D3 were highly significant (P ≤ 7.7 x 10-11). The ratios of regression slopes of the different skin type groups were compared, and only skin type II was significantly steeper than the other groups. Comparisons between extreme skin types II and VI showed melanin inhibition factors of approximately 1.3-1.4, depending on the UVR source. We conclude that the inhibitory effect of melanin on vitamin D3 synthesis is small, compared with erythema, but that this difference may be sufficient to explain the epidemiological data.

Beam penumbra reduction of Gamma Knife machine model 4C using Monte Carlo simulation

Background and objectiveIn small radiation fields used in stereotactic radiosurgery penumbra is an important portion of the field size especially when critical organs at risk are located near the treatment sites. This study was aimed to reduce penumbra width (90%–50% isodose lines) of Gamma Knife (GK) machine by investigating of source to diaphragm distance (SDD) and designing compensating filter. MethodsCompensating filters at the end of the helmet collimators with the aim of reducing penumbra as well as reducing hot spots appeared near the edge of beam were modeled using Monte Carlo simulation code. Moreover, the SDD parameter was increased as one of the effective factors on penumbra width. ResultsResults showed that single beam penumbra width using optimal design of filters was decreased by 59.49%, 42.50%, 39.02% and 34.44% with attenuation of 30.53%, 13.67%, 11.43% and 9.82% for 4, 8, 14 and 18 mm field sizes, respectively. ConclusionsThe designed filters lead to considerable reductions in single beams penumbra width as well as a noticeable reduction in maximum dose emerged near the beam edge due to the curved lateral surface of filters.

Reconstruction of the electron source intensity distribution of a clinical linear accelerator using in-air measurements and a genetic algorithm.

Background and purposeThe electron source intensity distribution of a clinical linear accelerator has a great influence on the calculation of output factors for small radiation fields where source occlusion by the collimating devices takes place. The purpose of this study was to present a new method for the electron source reconstruction problem.Materials and methodsThe measurements were performed in-air using diode and 6 MV 1 × 1 cm2 photon field in flattening filter-free mode. In Monte Carlo simulation, an electron target area was divided into a number of square subsources. Then, the in-air doses in 2D silicon chip array were calculated individually from each subsource. A genetic algorithm search was applied in order to determine the optimal weight factors for all subsources that provide the best agreement between simulated and measured doses.ResultsIt was found that the reconstructed electron source intensity from a clinical linear accelerator has the two-dimensional elliptical double Gaussian distribution. The source intensity distribution consisted of two intensity components along the in-plane (x) and cross-plane (y) directions characterized by full width half-maximum (FWHM): FWHMx1 = 0.27 cm, FWHMx2 = 0.08 cm, FWHMy1 = 0.24 cm, FWHMy2 = 0.06 cm, where broader components are 81% and 53% of the total intensity along × and y axis respectively.ConclusionsThe obtained results demonstrated an elliptical double Gaussian intensity distribution of the incident electron source. We anticipate that the proposed method has universal applications independent of the type of linear accelerator, modality or energy.

Effects of metal implants and a metal artifact reduction tool on calculation accuracy of AAA and Acuros XB algorithms in small fields.

In this study, the dosimetric accuracy of analytical anisotropic algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms (Varian Medical Systems, Palo Alto, CA) was investigated for small radiation fields incident on phantoms of various metals that include stainless steel grade 316L (SS316L) and titanium alloy grade 5 (Ti5) implants. In addition, the effects of using metal artifact reduction for orthopedic implants (O-MAR, Philips Healthcare, Cleveland, OH) were evaluated. The evaluations of AAA and AXB were performed by comparing the crossline profiles calculated by AAA and AXB with GafChromicTM EBT3 film measurements at the phantom-implant interfaces and in close vicinity of implant materials for small field sizes (1×1cm2 , 2×2cm2 , 3×3cm2 , and 4×4cm2 ) of a 6MV flattening filter free photon beam. O-MAR corrected and uncorrected (UC) computed tomography (CT) images were used for dose calculations. The values of average and standard deviations (SD) of Hounsfield unit (HU) for selected regions of each case were evaluated. The differences in average dose percentages in defined regions were calculated to quantify the relative dosimetric changes between doses calculated on UC and O-MAR corrected CT images. Compared to UC images, the values of SD were reduced, and the average HU became closer to its reference value in the O-MAR images. There was some discrepancy in average dose percentage differences between calculations using UC and O-MAR images at 1cm above the SS316L implant (average dose percentage differences were AXB/UC=5.9% and AXB/O-MAR=-1.2%; AAA/UC=2.2%, and AAA/O-MAR=-0.8%). Neither AAA nor AXB algorithms predict increase in dose at upper phantom-implant interface (4.9%, 9.9%. 13.5%, and 13.8% for the fields from 1×1cm2 to 4×4cm2 , respectively). At the side of the SS316L implant (where dark streak artifacts exist), dose difference averages were estimated as-1.1% and 22.3% when AXB/O-MAR and AXB/UC calculations are compared with EBT3 measurements, respectively. Dose predictions at 1cm below the SS316L implant were underestimated by AXB/O-MAR (average -0.5%) and AXB/UC (average 2.0%). The O-MAR tool was shown to have a favorable dosimetric effect or no effect on the calculations in the upper proximity of the implant materials. The dose differences between EBT3 film measurements and calculations at upper phantom-implant interfaces were smaller when they were calculated using O-MAR images. However, the dose differences increased when O-MAR corrected images were used for AAA calculations at lower phantom-implant interfaces. Use of O-MAR enabled closer agreement for the AXB algorithm, especially in the dark streak artifact regions. The O-MAR algorithm should be used when the dose is calculated with the AXB algorithm in cases of patients with the metal implants. The estimations using AAA and AXB algorithms, in phantom setups, with Ti5 implant material were found to be closer to the EBT3 film measurements, when compared with the same estimations using SS316L implant material.

Feasibility of linear diode array based small field data acquisition for 6 MV & 15 MV photon beams – An intercomparison with micro ion chamber

This study aims at evaluating the response of different radiation detectors for measurement of small field radiotherapy beam profiles for high energy photon beams. Micro ion chamber (IC, CC01, IBA) and linear diode array (LDA-99SC, IBA) were used to measure beam data on Siemens ONCOR Impression linear accelerator for 6 MV and 15 MV photon beams. For both detectors the beam profiles were acquired in water phantom for nominal small field sizes ranging from 1 × 1 cm2 to 5 × 5 cm2 at depths of maximum dose (dmax), 5 cm and 10 cm. Measured profiles were inter compared using gamma analysis at pass criteria of 1.5%/1.5 mm and 1%/1 mm and characterized with full width half maximum (FWHM) and penumbrae width. The gamma pass rate for all profiles of both energies has been found to be 100% (using 1.5%/1.5 mm) and 90% (using 1%/1 mm) except for 15 MV - 1 × 1 cm2 field size. Though, the effect of measurement depth has been found to be insignificant on beam matching with two detectors. For both energies, no considerable difference has been observed in FWHM of profiles as measured with the two detectors, with a slight increase in penumbrae size for LDA. It has been found that 6 MV beam profiles of IC and LDA show a reasonable match whereas a noticeable difference has been observed for 15 MV beam in penumbrae region. In addition, a substantial dose overestimation can be seen for LDA in the out of field region for both energies.

Penumbra Measurements and Comparison of In-House and Standard Circular Cones by the Gafchoromic Film, Pinpoint Ion Chamber, and MCNPX Monte Carlo Simulation

Introduction: Penumbra is an important property of the radiation beam to obtain a suitable margin surrounding the target volume. Therefore, the precise penumbra width determination in stereotactic radiotherapy is necessary for treatment planning. This study aimed to compare the obtained results of penumbra width by in-house and standard circular cones by different dosimeters, as well as evaluating the function of EBT3 for dosimetric properties of the small field radiation. Material and Methods: Different circular cones were mounted on the head of the accelerator to produce 12, 20, and 40 mm field sizes at isocenter. Dosimetric measurements were performed with the EBT3 film, PinPoint ion chamber. Afterwards, MCNPX Monte Carlo simulation was used to evaluate the dosimetric parameters. Results: According to the obtained results, the penumbra width was increased by larger diameters of circular cones. The obtained measured data by PinPoint ion chamber showed a larger penumbra width compared to those calculated by Monte Carlo at all field sizes. The gamma index analysis revealed distance-to-agreement and dose-difference of 2 mm /2%/ at all points. The results of this study showed that source to diaphragm distance had a major role in penumbra size determination of small field dosimetry with PinPoint ion chamber, EBT3 film, and Monte Carlo simulation. Conclusion: As findings of this study reported, EBT3 films are reliable detectors for relative dosimetry due to high spatial resolution for small field sizes. Furthermore, they can be used for measuring beam profile and percentage depth dose curves.

EP-1478 Adjuvant small pelvic field radiotherapy in cervical cancer with intermediate risk factors

EP-1478 Adjuvant small pelvic field radiotherapy in cervical cancer with intermediate risk factors

Absolute dosimetry with polymer gels—a TLD reference system

Background and purpose. Absolute dosimetry in 3D with polymer gels (PG) is generally complicated and usually requires a second independent measurement with conventional detectors. This is why, PG are often used only for relative dosimetry. To overcome this drawback, we combine PG with a 1D thermoluminescence (TL) detector within the same measurement. The TL detector information is then used as additional information for calibration of the gel. Materials and methods. The PAGAT dosimetry gel was used in combination with TLD600 (LiF:Mg,Ti). TL detectors were attached on the surface of the PG container placed inside a cylindrical phantom. To test the usability of this setup, two irradiation geometries were carried out: (a) homogeneous target coverage and (b) small-field irradiation. PG was evaluated with magnetic resonance imaging (MRI) and the TL detectors with a Harshaw 5500 hot gas reader. Results. PG dosimetry alone showed deviations of up to 4% as compared to calculations. Including additionally the dose information of the TL detectors for PG calibration, this deviation was decreased to less than 1% for both irradiation geometries. This is also reflected by the very high -passing rates of > 96% (3%/3 mm) and >93% (2%/2 mm), respectively. Conclusion. This study presents a novel method combining 3D PG and TL dose measurements for the purpose of absolute 3D dose measurements that can also be applied in complex anthropomorphic phantoms using only a single measurement. The method was validated for two different irradiation geometries including a homogeneous large field as well as a small field irradiation with sharp dose gradients.

Compatibility of 3D printing materials and printing techniques with PAGAT gel dosimetry

Polymer gel (PG) dosimetry enables three dimensional (3D) measurement of complex dose distributions. However, PGs are strongly reactive with oxygen and other contaminations, limiting their applicability by the need to use specific container materials. We investigate different 3D printing materials and printing techniques for their compatibility with PG. Suitable 3D printing materials may provide the possibility to perform PG dosimetry in complex-shaped phantoms. 3D printed and PG-filled test vials were irradiated homogenously. The signal response was evaluated with respect to homogeneity and compared to the signal in already validated reference vials. In addition, for the printing material VeroClear™ (StrataSys, Eden Prairie, USA) different methods to remove support material, which was required during the printing process, were investigated. We found that the support material should be used only on the outer side of the container wall with no direct contact to the PG. With the VeroClear™ material a homogenous signal response was achieved with a mean deviation of relative to the reference vials. In addition, the homogeneous irradiation of an irregularly-shaped gel container designed with the same printing material and technique also lead to a homogenous PG response. Furthermore, a small field irradiation of an additional test-vial showed an accurate representation of steep dose gradients with a deviation of the maximum position of relative to the reference vial.

Estimation of Dosimetric Parameters based on KNR and KNCSF Correction Factors for Small Field Radiation Therapy at 6 and 18 MV Linac Energies using Monte Carlo Simulation Methods.

Estimating dosimetric parameters for small fields under non-reference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. Hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in non-reference conditions at different depths and field sizes. Determining correction factors (KNR and KNCSF) recommended recently for small field dosimetry formalism by American Association of Physicists in Medicine (AAPM) for different detectors at 6 and 18 MV photon beams. EGSnrc Monte Carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended KNR and KNCSF correction factors for various circular small field sizes ranging from 5-30 mm diameters. KNR and KNCSF correction factors were determined for different active detectors (a pinpoint chamber, EDP-20 and EDP-10 diodes) in a homogeneous phantom irradiated to 6 and 18 MV photon beams of a Varian linac (2100C/D). KNR correction factor estimated for the highest small circular field size of 30 mm diameter for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.993, 1.020 and 1.054; and 0.992, 1.054 and 1.005 for the 6 and 18 MV beams, respectively. The KNCSF correction factor estimated for the lowest circular field size of 5 mm for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.994, 1.023, and 1.040; and 1.000, 1.014, and 1.022 for the 6 and 18 MV photon beams, respectively. Comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (EDP-20 and EDP-10) can confidently be recommended for small field dosimetry as their correction factors (KNR and KNCSF) was close to 1.0 for all small field sizes investigated and are mainly independent from the electron beam spot size.

Estimation of Dosimetric Parameters based on KNR and KNCSF Correction Factors for Small Field Radiation Therapy at 6 and 18 MV Linac Energies using Monte Carlo Simulation Methods

Background: Estimating dosimetric parameters for small fields under non-reference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. Hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in non-reference conditions at different depths and field sizes.Objective: Determining correction factors (KNR and KNCSF) recommended recently for small field dosimetry formalism by American Association of Physicists in Medicine (AAPM) for different detectors at 6 and 18 MV photon beams.Methods: EGSnrc Monte Carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended KNR and KNCSF correction factors for various circular small field sizes ranging from 5-30 mm diameters. KNR and KNCSF correction factors were determined for different active detectors (a pinpoint chamber, EDP-20 and EDP-10 diodes) in a homogeneous phantom irradiated to 6 and 18 MV photon beams of a Varian linac (2100C/D).Results: KNR correction factor estimated for the highest small circular field size of 30 mm diameter for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.993, 1.020 and 1.054; and 0.992, 1.054 and 1.005 for the 6 and 18 MV beams, respectively. The KNCSF correction factor estimated for the lowest circular field size of 5 mm for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.994, 1.023, and 1.040; and 1.000, 1.014, and 1.022 for the 6 and 18 MV photon beams, respectively.Conclusion: Comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (EDP-20 and EDP-10) can confidently be recommended for small field dosimetry as their correction factors (KNR and KNCSF) was close to 1.0 for all small field sizes investigated and are mainly independent from the electron beam spot size.

Clinical Evaluation of a Two-dimensional Liquid-Filled Ion chamber Detector Array for Verification of High Modulation Small Fields in Radiotherapy.

Introduction:Clinical evaluation of a two-dimensional (2D) liquid-filled ion chamber detector array used in the verification of highly modulated small beams of stereotactic body radiation therapy (SBRT) has been conducted.Materials and Methods:Measurements with the Octavius 1000 SRS (PTW, Freiburg, Germany) detector with 977 liquid-filled ion chambers were compared against EDR2 film and PTW Octavius Seven29. The performance of detector array has been evaluated on ten SBRT patient plans. Dose profiles of individual and composite fields' calculated using Pinnacle3 treatment planning system were compared against measurements with Octavius 1000 SRS detector array, EDR2 film, and Octavius Seven29 detector. Gamma index and profile comparison were used in the evaluation and assessment of the detector's performance.Results:The Gamma index measurements show agreement between Pinnacle3 computations and Octavius 1000 SRS array, PTW Octavius Seven29, and EDR2 film for >90% of the points using 2%, 2 mm tolerance criteria. Profiles obtained with the Octavius 1000 SRS were in agreement with the EDR2 film profiles, demonstrating the detector's superior sampling rate.Conclusions:The Octavius 1000 SRS is a dosimetrically accurate device to perform quality assurance checks in SBRT treatments. The broad range of measurements performed in this study quantified the dosimetric accuracy of Octavius 1000 SRS detector in the clinical setup of the small fields in radiotherapy.

On the Instantaneous Dose Rate and Angular Dependence of Monolithic Silicon Array Detectors

The use of small radiation fields characterized by very steep dose gradients is common in modulated arc X-ray radiotherapy. Dosimeters for quality assurance (QA) applications would ideally have sensitive volumes (SVs) relatively small with respect to the field, along with negligible instantaneous dose rate and angular dependence. Silicon-based dosimeters can be fabricated with SVs $ across, provide a stable and near energy-independent response in megavoltage photon beams, along with a good linearity with accumulated dose and real-time readout. However, their sensitivity is instantaneous dose rate and angular dependent. Monolithic silicon array detectors with sufficiently small SVs and pitch are suitable for high-resolution 2-D dose mapping in radiation fields with steep dose gradients. The Octa is a 2-D monolithic silicon array detector. It has 512 diode- SVs arranged with a sub-millimeter pitch. The physical characteristics of the substrate on which it is based, in terms of resistivity and defects concentration, strongly affect its performance. We report on the experimental characterization of two versions of the Octa, manufactured on a bulk and on an epitaxial substrate, respectively. Their performance is compared and discussed in terms of their instantaneous dose rate and angular dependence in the context of QA applications in small radiation fields delivered with modulated arc radiotherapy.

An Experimental Slope Method for a More Accurate Measurement of Relative Radiation Doses using Radiographic and Radiochromic Films and Its Application to Megavoltage Small-Field Dosimetry.

Purpose:An experimental method using the linear portion of the relative film dose–response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.Materials and Methods:The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.Results:The maximum percentage deviation of PDDFS with PDDIC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDDFS with PDDIC for electron beams was within 3% for energy range studied of 8–15 MeV. The gamma passing rates of PDDFS with PDDIC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDDFS with PDDIC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROFFS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDDFS and PDDMC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROFFS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.Conclusions:The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.

Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields.

PurposeTo explain the deviation observed between measured and Monaco calculated dose profiles for a small field (i.e., alternating open‐closed MLC pattern). A Monte Carlo (MC) model of an Elekta Infinity linac with Agility MLC was created and validated against measurements. In addition, an analytic model which predicts the fluence at the isocenter plane was used to study the impact of multiple beam parameters on the accuracy of dose calculations for small fields.MethodsA detailed MC model of a 6 MV Elekta Infinity linac with Agility MLC was created in EGSnrc/BEAMnrc and validated against measurements. An analytic model using primary and secondary virtual photon sources was created and benchmarked against the MC simulations and the impact of multiple beam parameters on the accuracy of the model for a small field was investigated. Both models were used to explain discrepancies observed between measured/EGSnrc simulated and Monaco calculated dose profiles for alternating open‐closed MLC leaves.Results MC‐simulated dose profiles (PDDs, cross‐ and in‐line profiles, etc.) were found to be in very good agreements with measurements. The best fit for the leaf bank rotation was found to be 9 mrad to model the defocusing of Agility MLC. Moreover, a very good agreement was observed between results from the analytic model and MC simulations for a small field. Modifying the radial size of the incident electron beam in the BEAMnrc model improved the agreement between Monaco and EGSnrc calculated dose profiles by approximately 16% and 30% in the position of maxima and minima, respectively.ConclusionAccurate modeling of the full‐width‐half‐maximum (FWHM) of the primary photon source as well as the MLC leaf design (leaf bank rotation, etc.) is essential for accurate calculations of dose delivered by small radiation fields when using virtual source or MC models of the beam.