Energy Photon Beams Research Articles

Fluorescence polarization as a precise tool for understanding nonsequential many-photon ionization

Nonsequential two-photon ionization of inner-shell $np$ subshell of neutral atoms by circularly polarized light is investigated. Detection of subsequent fluorescence as a signature of the process is proposed and the dependence of fluorescence degree of polarization on incident photon beam energy is studied. It is generally expected that the degree of polarization remains approximately constant, except when the beam energy is tuned to an intermediate $n's$ resonance. However, strong unexpected change in the polarization degree is discovered for nonsequential two-photon ionization at specific incident beam energy due to a zero contribution of the otherwise dominant ionization channel. Polarization degree of the fluorescence depends less on the beam parameters and its measurements at this specific beam energy, whose position is very sensitive to the details of the employed theory, are highly desirable for evaluation of theoretical calculations of nonlinear ionization at hitherto unreachable accuracy.

Contrast Enhancement for Portal Imaging in Nanoparticle-Enhanced Radiotherapy: A Monte Carlo Phantom Evaluation Using Flattening-Filter-Free Photon Beams.

Our team evaluated contrast enhancement for portal imaging using Monte Carlo simulation in nanoparticle-enhanced radiotherapy. Dependencies of percentage contrast enhancement on flattening-filter (FF) and flattening-filter-free (FFF) photon beams were determined by varying the nanoparticle material (gold, platinum, iodine, silver, iron oxide), nanoparticle concentration (3–40 mg/mL) and photon beam energy (6 and 10 MV). Phase-space files and energy spectra of the 6 MV FF, 6 MV FFF, 10 MV FF and 10 MV FFF photon beams were generated based on a Varian TrueBeam linear accelerator. We found that gold and platinum nanoparticles (NP) produced the highest contrast enhancement for portal imaging, compared to other NP with lower atomic numbers. The maximum percentage contrast enhancements for the gold and platinum NP were 18.9% and 18.5% with a concentration equal to 40 mg/mL. The contrast enhancement was also found to increase with the nanoparticle concentration. The maximum rate of increase of contrast enhancement for the gold NP was equal to 0.29%/mg/mL. Using the 6 MV photon beams, the maximum contrast enhancements for the gold NP were 79% (FF) and 78% (FFF) higher than those using the 10 MV beams. For the FFF beams, the maximum contrast enhancements for the gold NP were 53.6% (6 MV) and 53.8% (10 MV) higher than those using the FF beams. It is concluded that contrast enhancement for portal imaging can be increased when a higher atomic number of NP, higher nanoparticle concentration, lower photon beam energy and no flattening filter of photon beam are used in nanoparticle-enhanced radiotherapy.

Study of the a1 (1269) resonance in the γp →π+π+π-n reaction * * Supported by the National Natural Science Foundation of China (11475227, 11735003), and the Youth Innovation Promotion Association CAS (2016367)

Within an effective Lagrangian approach and resonance model, we study the and reactions via the -exchange mechanism. For the reaction, we perform a calculation of the differential and total cross-sections by considering the contributions of the intermediate resonance decaying into and then into . Besides, the non-resonance process is also considered. With a lower mass of , the experimental data for the invariant mass distributions can be fairly well reproduced. For the reaction, with the model parameters, the total cross-section is of the order of 10 μb at the photon beam energy ~2.5 GeV. It is expected that the model calculations in this work could be tested by future experiments.

Measurement of the contralateral breast photon and neutron dose in breast cancer radiotherapy: A Monte Carlo study.

This study aimed to calculate the photon and neutron doses received to the contralateral breast (CB) during breast cancer radiotherapy for various field sizes in the presence of a physical wedge. Varian 2100 C/D linear accelerator was simulated using a MCNP4C Monte Carlo code. Then, a phantom of real female chest was simulated and the treatment planning was carried out on tumoral breast (left breast). Finally, the received photon and neutron doses to CB (right breast) were calculated in the presence of a physical wedge for 18 MV photon beam energy. These calculations were performed for different field sizes including 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm. The findings showed that the received doses (both of the photon and neutron) to CB in the presence of a physical wedge for 11 cm × 13 cm, 11 cm × 17 cm, and 11 cm × 21 cm field sizes were 9.87%, 12.91%, and 27.37% of the prescribed dose, respectively. In addition, the results showed that the received photon and neutron doses to CB increased with increment in the field size. From the results of this study, it is concluded that the received photon and neutron doses to CB in the presence of a physical wedge is relatively more, and therefore, they should be reduced to as low as possible. Therefore, using a dynamic wedge instead of a physical wedge or field-in-field technique is suggested.

Dosimetric Evaluation of Dose calculation algorithms of Monaco Treatment Planning System in the heterogeneities area

Introduction: In radiation therapy, the accuracy of dose calculations by a treatment planning system (TPS) is important to achieve tumor control and to spare normal tissue. Treatment planning system calculations in the heterogeneous situation may present significant inaccuracies. In this study, three different dose calculation algorithms, pencil beam (PB), collapsed cone (CC), and Monte-Carlo (MC), provided by our planning system were compared to assess their impact on the three-dimensional planning of lung region. Materials and Methods: The methodology was based on IAEA TEC-DOC 1583. The phantom resembling the human thorax are used and all tests were planned on three- dimensional treatment planning systems (TPSs) and irradiated with photon beams of 6, and 18 MV X-ray energies. The doses in specific points were measured with an ionization chamber. The differences between the measured and calculated doses were reported. This study was tested using different algorithms/inhomogeneity correction methods implemented in Monaco treatment planning system. Results: The measurements were conducted for all test case datasets for two photon beam energies and calculation algorithms. The deviation between the measured and calculated values for all test cases made with advanced algorithms (MC and CC) were within the agreement criteria, while the larger deviations were observed for PB algorithm. Subsequently, there were discovered dose differences greater than 20% for some simple algorithms and high energy X-ray beams. The number of measurements with results outside the agreement criteria increased with the increase of the beam energy and decreased with TPS calculation algorithm sophistication. Also, a few errors in the basic dosimetry data in TPS were detected and corrected. Conclusion: Differences were found when comparing the calculation algorithms. The PB algorithm actually overestimated the dose compared with those calculated by the CC and MC algorithms. The MC algorithm showed better accuracy than the other algorithms. Therefore advanced dose calculation algorithms are suitable and should be used in clinical practice

Investigating the effects of different kernels used for CT image reconstruction on dose distributions in treatment planning of kidney cancer radiotherapy

Introduction: The quality of CT images used for treatment planning of cancer patients is an important issue in accurate outlining of the tumor volume and organs at risk. Different kernels in CT scanner systems are available for improving the image quality. Applying these kernels on CT images will change the CT numbers and electron density of tissues, consequently. Therefore, the aim of this study was to assess effects of different CT kernels on Hounsfield unit variations, the related computed electron densities and the calculated dose distributions in radiation treatment planning system. Materials and Methods: The 16 slice Siemens CT scanner used in this work. The B30, B35, B41 and B50 kernels applied on abdomen CT images for a kidney cancer patient. The ISOgray treatment planning system was used for radiation treatment planning and calculating dose of 6MV photon beam energy. The dose volume histogram (DVH) of left kidney as target volume and spinal cord as organ at risk were calculated. Results: The B50 kernel, had the greatest effect on calculated CT numbers for considered reference points relative to standard kernel B30, among the applied kernels for image reconstruction (16-19 HU variations). The average of calculated percent dose in target volume for 3 reference points obtained %99.19, %100.36T %99.89 and %100.87 for standard kernel B30 and other B35, B41 and B50 kernels, respectively. Mean dose in DVH was 2.03 for B30 and 2.05 for other kernels. Conclusion: The Hounsfield units of the selected reference points, in the target volume of reconstructed CT images by various kernels had relatively high variations respect to B30 kernel. Despite these variations, electron density and consequently the average of calculated percent dose in the target volume did not show considerable changes. Hence, it can be concluded that the use of image reconstruction kernels to improve the quality of CT images will help to determine the edges and outlines of tumors and organs at risk more precisely. While, applying these kernels does not significantly affect the amount of calculated doses in the treatment planning system

Evaluation of Thyroid Gland Irradiation in Women with Breast Cancer during Radiotherapy with different Radiation Energies at Supraclavicular Region

Introduction: During treatment of breast cancer, Radiotherapy to the Supraclavicular fossa region results in absorption of radiation by the thyroid gland and consequently leads to hypothyroidism in 40% of patients. The aim of this study was to compare thyroid gland radiation absorption during radiotherapy with different anterior-posterior beam radiation of 6-15 and 15-15 MV photon beam energies. Materials and Methods: 29 patients with breast cancer were recruited to this study. Adjuvant radiotherapy with a total dose of 50 Gy was performed for each participant. Thyroid gland dosimetric measurements were evaluated including, mean dose, minimum and maximum dose, and V20, V30, V40, and V50 (percentage of thyroid volume receiving ≥20 Gy, ≥30 Gy, ≥40 Gy, and ≥50 Gy, respectively). The irradiation delivered doses were measured using Prowess Panther treatment planning system (Version 5.5). All data were evaluated using SPSS software. Results: 29 subjects with mean age of 53.4±9.4 were studied. According to the obtained results, at 15- 15 MV energies, a significantly lower dose was absorbed by the thyroid gland, was observed in contrast to their counterparts who were treated with 6-15 MV photon beam energies. Conclusion: Using 15-15 MV photon beam energies field can significantly reduce the absorbed dose to the thyroid gland and consequently can reduce the risk of developing hypothyroidism in breast cancer patients treated with radiotherapy.

Empirical Law to Evaluate the Skin Dose with Photon Beam Energy and Irradiation Field Size

The objective of this study is to evaluate mathematically an empirical law governs the skin dose with photon beam energy and irradiation field size based on experiment data of dose distributions. The dose distributions were expressed on percentage depth dose (PDD). The measurements of PDDs were done as a function of irradiation field size with an uncertainty of 2% as recommended by IAEA protocols. The skin dose corresponds to PDD at depth of 0 mm for photon beam energy of 6 and 18 MV. For these both photon beam energies, the skin dose increased linearly with irradiation field size and the skin dose rate decreased in power law as a function of irradiation field size with mathematical error under to 10%. This law allows us to predict the skin dose with irradiation field size for high radiotherapy quality and high protection of organ at risks.

ANALISIS NILAI PERCENTAGE DEPTH DOSE(PDD) TERHADAP VARIASI KEDALAMAN TARGET DAN LUAS LAPANGAN PENYINARAN MENGGUNAKAN PESAWAT LINAC-CX

Percentage Depth Dose (PDD) value measurement is done by varying the target depth and field size. The study was carried out with variations in target depth of 0 cm to 30 cm with 1 cm intervals and fields size ranging from (5x5) cm2 to (39x39) cm2 with intervals (2x2) cm2. PDD is a comparison of the radiation dose at a depth to the radiation dose at the maximum depth normalized by percentage. This study was also conducted to determine the value of radiation doses received by phantoms from the PDD measurement value. This research used 2 photon energy beam that is 6 MV and 10 MV. This study uses water phantom as a substitute for patients with radiation source technique Source Surface Distance (SSD). PDD measurements were performed using a chamber ionization detector. The results obtained showed that the number of radiation doses received by fantom increased to the maximum depth (zmax). After passing zmaks, the radiation dose received by the phantom has decreased. Likewise along with the increase in the amount of radiation field, the value of radiation doses received by phantoms will also increase.

Use of radiochromic film dosimetry in radiobiology experiments

In radiobiology experiments, that usually entail unique irradiation geometries, it is necessary to know properly the absorbed doses involved. It is difficult to find them out, due to the differences between these irradiation conditions and the reference conditions in which the absorbed dose is measured by the usual ionometric procedures.The aim of this study is to check the EBT3 radiochromic film as a dosimetric system for x-ray beams used in radiobiology experiments. The film has been calibrated in different energy photon beams, so its energy independence has been proved. Afterwards it has been used to evaluate the absorbed dose in two radiobiology experiments: first one required the irradiation of flowers, while in the second, mice with implanted tumors were irradiated. In both cases EBT3 radiochromic film has shown to be a suitable tool for measuring the absorbed dose in this kind of experiments.

Error detection during VMAT delivery using EPID-based 3D transit dosimetry

PurposeTo investigate the effectiveness of an EPID-based 3D transit dosimetry system in detecting deliberately introduced errors during VMAT delivery. MethodsAn Alderson phantom was irradiated using four VMAT treatment plans (one prostate, two head-and-neck and one lung case) in which delivery, thickness and setup errors were introduced. EPID measurements were performed to reconstruct 3D dose distributions of “error” plans, which were compared with “no-error” plans using the mean gamma (γmean), near-maximum gamma (γ1%) and the difference in isocenter dose (ΔDisoc) as metrics. ResultsOut of a total of 42 serious errors, the number of errors detected was 33 (79%), and 27 out of 30 (90%) if setup errors are not included. The system was able to pick up errors of 5 mm movement of a leaf bank, a wrong collimator rotation angle and a wrong photon beam energy. A change in phantom thickness of 1 cm was detected for all cases, while only for the head-and-neck plans a 2 cm horizontal and vertical shift of the phantom were alerted. A single leaf error of 5 mm could be detected for the lung plan only. ConclusionAlthough performed for a limited number of cases and error types, this study shows that EPID-based 3D transit dosimetry is able to detect a number of serious errors in dose delivery, leaf bank position and patient thickness during VMAT delivery. Errors in patient setup and single leaf position can only be detected in specific cases.

Angular Dependence of η Photoproduction in Photon-Induced Reaction

Photoproduction of η mesons from nucleons can provide valuable information about the excitation spectrum of the nucleons. The angular dependence of η photoproduction in the photon-induced reaction is investigated in the multisource thermal model. The results are compared with experimental data from the η→3π0→6γ decay mode. They are in good agreement with the experimental data. It is shown that the movement factor increases linearly with the photon beam energies. And the deformation and translation of emission sources are visually given in the formalism.

Analysis of stabilization of photon beam softening with off-axis distance for filtration system enhancement to increase dosimetry in radiotherapy

Beam softening is depending on the material atomic number and photon beam energy for photon beam filtration; the filtration quality is essential to producing a clinical beam in external radiotherapy. Beam softening analysis is done using Monte Carlo method. The Monte Carlo model was performed for 6 MV photon beam produced by Varian Clinac 2100 with flattening filter thereafter the flattening filter was removed and replaced by slab of aluminum and copper with different thickness. The purpose of this study is to analyze the photon beam softening with off-axis distance for the reference field size of 10 × 10 cm2; the beam softening is studied in terms of tow coefficients a1 and a2.Inside the irradiation field, for aluminum slab, the coefficient a1 varied in interval from −1.2 cm−1 to 1.7 cm−1 and it has big fluctuations with off-axis distance. For copper, a1 varied from 0 cm−1 to 1.5 cm−1 and it has small fluctuations with off-axis distance. The coefficient a2, for aluminum slab, varied from −1 cm−2 to 0.4 cm−2 with big fluctuations. For copper slab, a2 varied from −0.9 cm−2 to 0 cm−2 with small fluctuations. We conclude that the beam softening is very stabilized for copper than aluminum in despite of copper attenuate more photons than aluminum for same slab thickness when they are used in photon beam filtration. To increase dosimetry in radiotherapy, the flattening filter should be constructed based on low atomic number materials but with high stabilization of beam softening with off-axis distance.

Correction factors for diode and diamond detectors in the measurement of small field output factors, using film dosimetry as reference

The accurate measurement of field output factors at small field sizes typically encountered in SRS and IMRT is challenging. In this study we measured detector output ratios for small fields using a range of detectors and orientations, we then calculated correction factors for each detector using film dosimetry as reference to obtain field output factors. The calculated correction factors were compared to the ones from TRS 483. Detectors used were: EBT3 Gafchromic film, PTW semiflex 31010 chamber, PTW pinpoint 31016 chamber, PTW diamond (used in two different orientations) and PTW SRS 60018 diode. Photon beam energies were 6 MV flat and 7 MV FFF with square field sizes from 0.5 to 10.0 cm side. Semiflex and pinpoint chambers agreed well with film down to 2.0 cm field size but under-responded by up to 40% at 0.5 cm field size. At 0.5 cm field size, diamond with stem parallel to beam axis under-estimated field output factors by up to 6.2%; diamond with stem perpendicular to beam axis over-estimated field output factors by up to 1.5% and diode over-estimated field output factors by up to 6.5%. Calculated correction factors, in general, agreed with and confirmed those given in TRS 483 film is an acceptable alternative to Monte Carlo simulation for deriving correction factors for solid state detectors. For field sizes smaller than 1.0 cm, diamond detector with stem perpendicular to beam axis was the most adequate for field output factor measurement, although not recommended in TRS 483.

Characterisation of two new radiochromic gel dosimeters TruView™ and ClearView™ in combination with the vista™ optical CT scanner: A feasibility study

PurposeThis study aims at characterising the properties of TruView™ and ClearView™ two new gel dosimeters (Modus Medical Devices Inc.) and at studying the feasibility of relative dosimetry using these dosimeters and the Vista™ Optical CT scanner to accurately evaluate dose. MethodsIn this work, we investigated key dosimetric aspects (dose response, energy and dose rate dependence) and stability of these radiochromic gels initiated in preliminary works (Huet et al., 2017; Colnot et al., 2017) using spectrophotometric measurements. Moreover, by mean of optical CT scanning (Vista™), their performances to measure relative depth dose (PDD) and cross profiles were analysed. ResultsTruView™ and ClearView™ present a linear dose response up to 20 Gy and up to 80 Gy respectively, independent of both photon beam energy (4–18 MV) and dose rate (up to 9.9 Gy/min) (Huet et al., 2017; Colnot et al., 2017). ClearView™ response proves to be stable for a week post-irradiation and uniform within the batch whereas TruView™ presents an unstable but uniform response. Optical CT scanning generates errors due to stray light that need to be corrected in order to use these gels; ClearView™ scanning particularly requires important precautions. After corrections, those gels used in combination with the Vista™ scanner show promising spatial and dosimetric precision (dose difference <5%). Finally, TruView™ is reusable and presents excellent reproducible response (maximum 3% difference) and the ClearView™ dosimeter presents good spatial stability (0.5% difference after 6 days). ConclusionThis study provides important knowledge about two gel dosimeters presenting interesting dosimetric properties. A study is ongoing to benchmark those promising candidates for clinical dose verification.

OA240] Comparison between four routine control devices detecting photon beam energy changes

Purpose The quality assurance (QA) program of a LINAC includes the control of the stability of the photon beam energy, through a control variable, the beam quality BQ. Our department owns four devices for this task. In this work a method to compare the performance of these QA devices is designed and applied. Methods Two photon beams with energies 6 and 15 MV, produced by a Siemens Artiste LINAC, were used to analyse the response of the following QA devices: StarTrack (Iba Dosimetry), QuickCheck and StarCheck (PTW) and IC Profiler (Sun Nuclear). They determine the BQ indirectly through their own control variable, BQ dev which shall be related to the BQ measured in reference conditions. To determine this relationship, the photon energy was modified by varying the bending magnet intensity I BM For each value of I BM , three PDD curves in water and three measurements for each device were taken. Through the measurements taken before and assuming that they follow a normal distribution, pairs of values of BQ and BQ dev were generated using Monte Carlo simulation methods. The pairs of simulated values are related between them by the relationship obtained previously. By comparing if the pair of values are inside or outside the tolerance interval, each simulation is classified as true positive TP, false positive FP, true negative TN or false negative FN. The tolerance interval chosen was 0.5%. Summing all the TP, FP, TN and FN, two parameters can be calculated: sensitivity and specificity. These parameters will allow to compare the performance between devices. Results For all devices, the relationship between BQ dev and BQ is linear, with a determination coefficient r 2 ≥ 0.9 . The QuickCheck and IC Profiler devices showed the best performance, with a sensitivity and a specificity higher than 95%. The StarTrack device had a good sensitivity, but a specificity of around 85%, for both energies. The StarCheck device showed a decent sensitivity, but a specificity of 67.7% and 78.7% for each energy, respectively. Conclusions The QuickCheck and IC Profiler devices showed the best performance, followed by StarTrack. The StarCheck device had a specificity much lower than the other three devices.

A Feasibility Study of IMRT of Lung Cancer Using Gafchromic EBT3 Film

Intensity modulated radiation therapy (IMRT) is an advanced method for delivery of three dimensional therapies, which provides optimal dose distribution with giving multiple nonuniform fluency to the patient. The complex dose distribution of IMRT should be checked to ensure that the accurate dose is delivered. Today, film dosimetry is a powerful tool for radiotherapy treatment Quality Assurance (QA) and a good method to verify dose distribution in phantoms. This study aimed to evaluate the accuracy of IMRT treatment planning system, Prowess Panther® software, with Gafchromic EBT3 films in a inhomogeneity phantom. The IMRT plan was generated by Prowess Panther® treatment planning system (TPS) version 5.2 on a inhomogeneity phantom, then it was irradiated by ONCOR linear accelerator (Linac) with 6 (MV) photon beam energy. The Gafchromic EBT3 film located between the phantom has measured the dose distribution. To compare between TPS calculated doses and film measured doses, Gamma criteria 3%/3 mm, 4%/4 mm, 5%/5 mm, 6%/6 mm and 7%/7 mm Dose Difference (DD) and Distance to Agreement (DTA), respectively were used. Gammas passing rates for PTV are obtained 67.5% for 3%/3mm, 78.8% for 4%/4mm, 86.3% for 5%/5mm, 91.2% for 6%/6mm and 94.3% for 7%/7mm and for organs at risk is 72.4% for 3%/3mm, 82.8% for 4%/4mm, 89.8% for 5%/5mm, 93.3% for 6%/6mm and 95.4% for 7%/7mm (respectively DD/DTA). By increasing the range of criteria the capability increased. The results show that the use of EBT3 film in a inhomogeneity phantoms allows us to evaluate the dose differences between the EBT3 measured dose distribution and TPS calculated dose distribution .Hence, a result Prowess Panther® TPS can be used for IMRT technique treatment.

Evaluating dose to thyroid gland in women with breast cancer during radiotherapy with different radiation energies at supraclavicular fossa region

AbstractBackground and aimDuring the treatment of breast cancer, radiotherapy to the supraclavicular fossa region results in absorption of radiation by the thyroid gland and consequently leads to hypothyroidism in 40% of patients. The aim of this study was to compare thyroid gland radiation absorption during radiotherapy with different anterioposterior beam radiation of 6–15 and 15–15 MV photon beam energies.Materials and methodsIn total, 29 patients with breast cancer were recruited to this study. Adjuvant radiotherapy with a total dose of 50 Gy was performed for each participant. Thyroid gland dosimetric measurements were evaluated including, mean dose, minimum and maximum dose, and V20, V30, V40 and V50 (percentage of thyroid volume receiving ≥20, ≥30, ≥40 and ≥50 Gy, respectively). The irradiation delivered doses were measured using Prowess Panther treatment planning system (Version 5.5). All data were evaluated using SPSS software.ResultsIn total, 29 subjects with mean age of 53·4±9·4 were studied. According to the obtained results, at 15–15 MV energies, a significantly lower dose was absorbed by the thyroid gland, was observed in contrast to their counterparts who were treated with 6–15 MV photon beam energies.FindingsUsing 15–15 MV photon beam energies field can significantly reduce the absorbed dose to the thyroid gland and consequently can reduce the risk of developing hypothyroidism in breast cancer patients treated with radiotherapy.

Energy optimization in gold nanoparticle enhanced radiation therapy

Gold nanoparticles (GNPs) have been demonstrated as radiation dose enhancing agents. Kilovoltage external photon beams have been shown to yield the largest enhancement due to the high interaction probability with gold. While orthovoltage irradiations are feasible and promising, they suffer from a reduced tissue penetrating power. This study quantifies the effect of varying photon beam energies on various beam arrangements, body, tumor, and cellular GNP uptake geometries. Cell survival was modeled based on our previously developed GNP-local effect model with radial doses calculated using the TOPAS-nBio Monte Carlo code. Cell survival curves calculated for tumor sites with GNPs were used to calculate the relative biological effectiveness (RBE)-weighted dose. In order to evaluate the plan quality, the ratio of the mean dose between the tumor and normal tissue for 50–250 kVp beams with GNPs was compared to the standard of care using 6 MV photon beams without GNPs for breast and brain tumors. For breast using a single photon beam, kV + GNP was found to yield up to 2.73 times higher mean RBE-weighted dose to the tumor than two tangential megavoltage beams while delivering the same dose to healthy tissue. For irradiation of brain tumors using multiple photon beams, the GNP dose enhancement was found to be effective for energies above 50 keV. A small tumor at shallow depths was found to be the most effective treatment conditions for GNP enhanced radiation therapy. GNP uptake distributions in the cell (with or without nuclear uptake) and the beam arrangement were found to be important factors in determining the optimal photon beam energy.

A mathematical deconvolution formulation for superficial dose distribution measurement by Cerenkov light dosimetry.

Cerenkov photons are created by high-energy radiation beams used for radiation therapy. In this study, we developed a Cerenkov light dosimetry technique to obtain a two-dimensional dose distribution in a superficial region of medium from the images of Cerenkov photons by using a deconvolution method. An integral equationwas derived to represent the Cerenkov photon image acquired by a camera for a given incident high-energy photon beam by using convolution kernels. Subsequently, an equationrelating the planar dose at a depth to a Cerenkov photon image using the well-known relationship between the incident beam fluence and the dose distribution in a medium was obtained. The final equationcontained a convolution kernel called the Cerenkov dose scatter function (CDSF). The CDSF function was obtained by deconvolving the Cerenkov scatter function (CSF) with the dose scatter function (DSF). The GAMOS (Geant4-based Architecture for Medicine-Oriented Simulations) Monte Carlo particle simulation software was used to obtain the CSF and DSF. The dose distribution was calculated from the Cerenkov photon intensity data using an iterative deconvolution method with the CDSF. The theoretical formulation was experimentally evaluated by using an optical phantom irradiated by high-energy photon beams. The intensity of the deconvolved Cerenkov photon image showed linear dependence on the dose rate and the photon beam energy. The relative intensity showed a field size dependence similar to the beam output factor. Deconvolved Cerenkov images showed improvement in dose profiles compared with the raw image data. In particular, the deconvolution significantly improved the agreement in the high dose gradient region, such as in the penumbra. Deconvolution with a single iteration was found to provide the most accurate solution of the dose. Two-dimensional dose distributions of the deconvolved Cerenkov images agreed well with the reference distributions for both square fields and a multileaf collimator (MLC) defined, irregularly shaped field. The proposed technique improved the accuracy of the Cerenkov photon dosimetry in the penumbra region. The results of this study showed initial validation of the deconvolution method for beam profile measurements in a homogeneous media. The new formulation accounted for the physical processes of Cerenkov photon transport in the medium more accurately than previously published methods.