Megavoltage Photon Beams Research Articles

Calculating Weighting Factors for Mixing Megavoltage Photon Beams to Achieve Desirable Dose Distribution in Radiotherapy.

Background: In radiotherapy, low-energy photon beams are better adapted to the treated volume, and the use of high-energy beams can reduce hot spots in the radiation therapy. Therefore, mixing low and high energies with different ratios can control the rate of hotspots, as well as the dose distribution of the target volume. Material and Methods: The percentage depth doses (PDDs) were calculated at various depths, by using a fitted double exponential equation. Then, using quality factor equation and PDD of a 10×10 cm2 field, the amount of energy equivalent to each PDD and the value of weighting factors of 6, 18 MV energies were calculated to produce different energies. To validate the mathematical model, dosimetry of 10 MV energy was used. For this purpose, PDDs and dose Profile of 10 MV obtained from the mix were compared with ones obtained from the measurement Results: The value of weighting factor of 6 MV energy required for the 10 ×10 cm2 field to create dose distribution of 15 MV energy using 6 and 18 MV energies was obtained as equal to 0.57. Comparison of percentage depth dose curves and dose profile shows good agreement with the practical measurements of 10 MV for 10×10 cm2 field using gamma index. Conclusion: The simultaneous use of high and low photon energies with different weighting factors to achieve desirable energy makes possible the treatment of tumors located at various depths without the need for different modes of energy in the accelerator leading to a decrease in the cost of the equipment and a safer treatment of the cancerous patients.

Evaluation of Gold Nanoparticle Size Effect on Dose Enhancement Factor in Megavoltage Beam Radiotherapy Using MAGICA Polymer Gel Dosimeter

Gold nanoparticles (GNPs) are among the most promising radiosensitive materials in radiotherapy. Studying the effective sensitizing factors such as nanoparticle size, concentration, surface features, radiation energy and cell type can help to optimize the effect and possible clinical application of GNPs in radiation therapy. In this study, the radiation sensitive polymer gel was used to investigate the dosimetric effect of GNP size in megavoltage (MV) photon beam radiotherapy. GNPs with the size of 30nm, 50nm and 100nm in diameter were used. Transmission electron microscope (TEM) and dynamic light scattering (DLS) were applied to analyze the size of nanoparticles. The MAGICA polymer gel was synthesized and impregnated with different sizes of GNPs. The samples were irradiated with 6MV photon beam and 24 hours after irradiation, they were read using a Magnetic Resonance Imaging (MRI) scanner. Macroscopic Dose Enhancement Factor (DEF) was measured to compare the effect of GNP size. The MAGICA response of the 6MV x-ray beam was verified comparing Percentage Depth Dose (PDD) curve extracted from polymer gel dosimetry and Treatment Planning System (TPS). MAGICA polymer gel dose response curve was linear in the range of 0 to 10 Gy. DEFs by adding 30nm, 50nm and 100nm GNPs were 1.1, 1.17 and 1.12, respectively. PDD curves of polymer gel dosimeter and treatment planning system were in good agreement. The results indicated a substantial increase in DEF uses a MV photon beam in combination with GNPs of different sizes and it was inconsistent with previous radiobiological studies. The maximum DEF was achieved for 50nm GNPs in comparison with 30nm and 100nm leading to the assumption of self-absorption effect by larger diameters. According to the outcomes of this work, MAGICA polymer gel can be recommended as a reliable dosimeter to investigate the dosimetric effect of GNP size and also a useful method to validate the current radiobiological and simulation studies.

Radiosensitizing effects of Oroxylum indicum extract in combination with megavoltage radiotherapy beams

Abstract Radiotherapy is an important treatment modality for cancer treatment, particularly for locally advanced tumours and it is known to induce cell death in tumours. Radiotherapy is limited by the total dose that can be prescribed without damaging normal tissueThe main aims of radiotherapy to optimally eradicate tumours while sparing the normal tissue could be achieved with the combination treatment using natural products based medicinal plants. Oroxylum indicum (OI) is a Malaysian consumable plant with a variety of health attributes that contains useful active biocompound that serves as anticancer agent. In this study, the potentials of OI extracts to increase cells radiosensitivity towards radiotherapy were investigated. Initially, the cytotoxic effects of OI extracts were determined in-vitro using HeLa cells. The cells with non-toxic concentration of OI extracts were irradiated with 6 MV and 10 MV photon beams. The cell survivals after irradiation were then evaluated using PrestoBlue assay. The OI extracts with maximum concentration of 0.01 mg/ml are found nontoxic to HeLa cells. The combination of OI extracts and irradiation are found to reduce the cells survival which indicates radiosensitization effects. The sensitization enhancement ratio (SER) calculated indicate higher radiosensization observed for 6 MV photon beams compare to 10 MV photon beams. SER also increased at higher OI concentration. In conclusion, the OI extracts are found to induce radiosensitization in HeLa cancer cells when irradiated with megavoltage photon beams. This study indicates the potential of plant based natural product to be applied as radiosensitizer to increase the therapeutic efficiency in radiation therapy of cancer.

Clinical implementation of brass mesh bolus for chest wall postmastectomy radiotherapy and film dosimetry for surface dose estimates.

This study presents the dosimetric data taken with radiochromic EBT3 film with brass mesh bolus using solid water and semi-breast phantoms, and its clinical implementation to analyze the surface dose estimates to the chest wall in postmastectomy radiotherapy (PMRT) patients. Water-equivalent thickness of brass bolus was estimated with solid water phantom under 6 megavoltage photon beam. Following measurements with film were taken with no bolus, 1, 2, and 3 layers of brass bolus: (a) surface doses on solid water phantom with normal incidence and on curved surface of a locally fabricated cylindrical semi-breast phantom for tangential field irradiation, (b) depth doses (in solid phantom), and (c) surface dose measurements around the scar area in six patients undergoing PMRT with prescribed dose of 50 Gy in 25 fractions. Water-equivalent thickness (per layer) of brass bolus 2.09 ± 0.13 mm was calculated. Surface dose measured by film under the bolus with solid water phantom increased from 25.2% ±0.9% without bolus to 62.5% ± 3.1%, 80.1% ± 1.5%, and 104.4% ± 1.7% with 1, 2, and 3 layers of bolus, respectively. Corresponding observations with semi-breast phantom were 32.6% ± 5.3% without bolus to 96.7% ± 9.1%, 107.3% ± 9.0%, and 110.2% ± 8.7%, respectively. A film measurement shows that the dose at depths of 3, 5, and 10 cm is nearly same with or without brass bolus and the percentage difference is <1.5% at these depths. Mean surface doses from 6 patients treated with brass bolus ranged from 79.5% to 84.9%. The bolus application was discontinued between 18th and 23rd fractions on the development of Grade 2 skin toxicity for different patients. The total skin dose to chest wall for a patient was 3699 cGy from overall treatment with and without bolus. Brass mesh bolus does not significantly change dose at depths, and the surface dose is increased. This may be used as a substitute for tissue-equivalent bolus to improve surface conformity in PMRT.

Calculation of kQ factors for Farmer-type ionization chambers following the recent recommendations on new key dosimetry data

PurposeTo calculate by Monte Carlo simulations kQ factors for Farmer-type ionization chambers in megavoltage photon beams using the new key dosimetry data recommended by the International Commission on Radiation Units and Measurements (ICRU) Report 90. MethodsMonte Carlo calculations were performed with the EGSnrc code system using both the ICRU 90 and the ICRU 37 data. Farmer-type ionization chambers with graphite and plastic walls and with graphite wall and a plastic waterproofing sleeve were considered (Nuclear Enterprise NE 2571, IBA FC65-G and FC65-P). kQ factors were calculated for photon beams in the range 6–25 MV using phase-space files as input radiation sources. The photon beam qualities in terms of TPR20,10 and %dd(10)x were established by simulating the depth-dose curves in water. Absorbed doses to the air cavity and to water were calculated using the egs_chamber user code with a target statistical uncertainty below 0.1%. ResultsThe update of key dosimetry data according to the ICRU report 90 had an impact of −0.2% in the absorbed dose to water and up to 0.5% in the absorbed dose to the air cavity. Nevertheless, changes partially offset each other when entering in kQ as ratio, and the final impact on the kQ values was below 0.3%. ConclusionsThe calculated values of kQ tend to be lower than the current values in the IAEA TRS-398 protocol with differences up to about 0.5%. A slightly better agreement (within 0.3%) is observed with the Monte-Carlo calculated values provided by the addendum to the AAPM’s TG-51 protocol.

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.

PEG coated bismuth oxide nanorods induced radiosensitization on MCF-7 breast cancer cells under irradiation of megavoltage radiotherapy beams

Gold nanoparticles have been introduced to increase the radiotherapy therapeutic efficacy. In this study, bismuth oxide nanorods coated with polyethylene glycol (Bi2O3-PEG) was applied as radiosensitizer for megavoltage photon and electron beams. Bi2O3-PEG nanorods of size 60 nm were synthesized using hydrothermal method and characterized using XRD and FESEM. Cytotoxicity evaluation was conducted on MCF-7 cells using 0.5 µMol/L of Bi2O3-PEG nanorods for 24, 48 and 72 hours of incubation time. The cells with and without Bi2O3-PEG were irradiated with 6 and 10 MV photon beams as well as electron beams of energy 6 and 12 MeV. Post-irradiation cell survival were quantified using clonogenic assays. The radiosensitization effects of Bi2O3-PEG nanorods were elucidated by extrapolating the sensitization enhancement ratio (SER) from the cell survival curves. Size and morphological characterization confirm the Bi2O3-PEG nanorods average size was 60 nm with surface modification of PEG. The cytotoxicity results show the Bi2O3-PEG nanorods are biocompactible to MCF-7 cells. The irradiation results demonstrate that Bi2O3-PEG nanorods produce radiosensitization effects when irradiated with photon beams with SER value of 1.12 for 6 MV and 1.15 for 10 MV. Meanwhile for electron beam, the SER obtained are 1.13 for 6 MeV and 1.06 for 12 MeV. In conclusion, the Bi2O3-PEG nanorods positively induced radiosensitization when irradiated with megavoltage radiotherapy beam indicating potential future clinical application as radiosensitizer.

Analysis of Tissue Phantom Ratio of the Megavoltage Photon Beams

Iso-centric beam data, phantom tissue ratios (TPR) are a dosimetric quantity commonly used to describe the change in dose with depth in tissue. Measurement of this is time-consuming and has the possibility of lose the consistency. The value of this quantity of any filed size in any depth is possible to calculate conveniently by the newly developed formula using only percentage depth dose (PDD) data of two fields. PDD for square fields ranging from 2 to 30 cm and various depths in increment of 0.4 cm up to maximum 30 cm have been measured in water at a fixed source surface distance (SSD) of 90 cm for 4, 6 and 15 MV photon beams in Ahsania Mission Cancer &amp; General Hospital (AMCGH), Dhaka, Bangladesh. TPR values calculating for these energies of the same field sizes, depths and SSD by using the developed formula compared with those determined from the measured PDD data using a standard formula and had the good agreement. Mean error less than 1% observed between these TPR values.&#x0D;

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams.

PurposeThe goal of this work is to provide a large and consistent set of data for detector‐specific output correction factors, kQclin,Qreffclin,fref, for small static fields for seven solid‐state detectors and to determine field output factors, ΩQclin,Qreffclin,fref, using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS‐483 for small‐field dosimetry were followed throughout the study.Methods ΩQclin,Qreffclin,fref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two‐dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. kQclin,Qreffclin,fref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector.ResultsField output factors and detector‐specific kQclin,Qreffclin,fref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small‐field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for kQclin,Qreffclin,fref are found to differ significantly (P < 0.05) from the corresponding data published in TRS‐483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between kQclin,Qreffclin,fref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm2 and 0.8 × 0.8 cm2. Significant differences in kQclin,Qreffclin,fref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in kQclin,Qreffclin,fref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant.ConclusionsSelection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of ΩQclin,Qreffclin,fref for small static fields in megavoltage photon beams. Large set of kQclin,Qreffclin,fref data for seven solid‐state detectors and four beam energies determined on two linacs by a single group of researchers can be considered a valuable supplement to the literature and the TRS‐483 dataset.

Determination of an Effective Wedge Angle by Combination of Two Arbitrary Universal Wedge Fields in Radiation Therapy of Cancer Patients with Megavoltage Photon Beams.

Introduction: Wedge filters are commonly used in radiation oncology for eliminating hot spots and creating a uniform dose distribution in optimizing isodose curves in the target volume for clinical aspects. These are some limited standard physical wedges (15°, 30°, 45°, 60°), or creating an arbitrary wedge angle, like motorized wedge or dynamic wedge, ... The new formulation is presented by the combination of wedge fields for determining an arbitrary effective wedge angle. The isodose curves also are derived for these wedges. Materials and Methods: we performed the dosimetry of Varian Clinac 2100C/D with Scanditronix Wellhofer water blue phantom, CU500E, OmniPro - Accept software and 0.13cc ionization chamber for 6Mv photon beam in depth of 10cm (reference depth) for universal physical wedges (15°, 30°, 45°, and 60°) and reference field 10x10cm2. By combining the isodose curve standard wedge fields with compatible weighting dose for each field, the effective isodose curve is calculated for any wedge angle. Results: The relation between a given effective wedge angle and the weighting of each combining wedge fields was derived. A good agreement was found between the measured and calculated wedge angles and the maximum deviation did not exceed 3°. The difference between the measured and calculated data decreased when the combined wedge angles were closer. The results are in agreement with the motorized single wedge appliance in the literature. Conclusion: This technique showed that the effective wedge angle that is obtained from this method is adequate for clinical applications and the motorized wedge formalism is a special case of this consideration.

Calculating weighting factors for mixing megavoltage photon beams to achieve desirable dose distribution in Radiotherapy

Introduction: Many studies have shown the effects of delivered dose distribution due to the incident photon energy on the tumor and healthy tissues. The ability to access the most appropriate radiation energy is essential to achieve the optimal treatment planning but there is a serious limitation in number of energies available on radiation therapy machines can restrict it. Materials and Methods: In this research, Electa linear accelerator with 6, 10 and 18MV photon energies and MATLAB software for analytical calculations were applied. The percentage depth dose (PDD) was measured for 6, 10 and 18MV energies in different depths with source-surface distance (SSD) of 100 cm2 for 4×4 to 40×40 cm2 fields. A double exponential function was fitted to the PDDs curves of 6 and 18MV using MATLAB software. Then, using the quality factor equation presented by LaRivere et al. and combination of 6 and 18MV energies, weighting factors to achieve desirable energies for the field of 10×10 cm2 were calculated. To verify the accuracy of this analytical method, PDD of 10MV energy was measured using ion chamber. The PDD obtained from dosimetry of 10MV energy, was compared with the results of mixing 6 and 18MV energies using the gamma index. Moreover, the dosimetric characteristics such as dmax, d50%, d80% and PDD10cm obtained from the mixed energy were compared with ones obtained from the measurement. Results: The value of the weighting factor of 6MV energy needed for the 10 ×10cm2 field to create dose distribution of 10MV energy using the mix was obtained as equal to 0.57. The obtained results from the mix of 6 and 18 MV energies and dosimetry of 10MV had good compatibility. The gamma index with two criteria of the percent dose difference (∆dM = 0.2cm) and distance to agreement (∆DM = 2%), expect at the points near the edges of the geometric fields, show values less than 1. Conclusion: The simultaneous use of the high and low photon energies with different weighting factors to achieve desirable energy makes possible the treatment of tumors located at various depths without the need for different modes of energy in the accelerator and leads to decrease in the costs of the equipment.

Biological effects of static magnetic field exposure in the context of MR-guided radiotherapy.

The clinical introduction of MRI-guided radiotherapy has prompted consideration of the potential impact of the static magnetic field on biological responses to radiation. This review provides an introduction to the mechanisms of biological interaction of radiation and magnetic fields individually, in addition to a description of the magnetic field effects on megavoltage photon beams at the macroscale, microscale and nanoscale arising from the Lorentz force on secondary charged particles. A relatively small number of scientific studies have measured the impact of combined static magnetic fields and ionising radiation on biological endpoints of relevance to radiotherapy. Approximately, half of these investigations found that static magnetic fields in combination with ionising radiation produced a significantly different outcome compared with ionising radiation alone. strength static magnetic fields appear to modestly influence the radiation response via a mechanism distinct from modification to the dose distribution. This review intends to serve as a reference for future biological studies, such that understanding of static magnetic field plus ionising radiation synergism may be improved, and if necessary, accounted for in MRI-guided radiotherapy treatment planning.

Dosimetry of small static fields used in external photon beam radiotherapy: Summary of TRS-483, the IAEA-AAPM international Code of Practice for reference and relative dose determination.

A joint IAEA/AAPM international working group has developed a Code of Practice (CoP) for the dosimetry of small static fields used in external megavoltage photon beam radiotherapy, published by the IAEA as TRS-483. This summary paper introduces and outlines the main aspects of the CoP. IAEA TRS-483 is a condensation of the wide range of different approaches that have been described in the literature for the reference dosimetry of radiotherapy machines with nominal accelerating potential up to 10MV that cannot establish the conventional 10cm ×10cm reference field, and for the determination of field output factors for relative dosimetry in small static photon fields. The formalism used is based on that developed by Alfonso etal. [Med Phys. 2008;35:5179-5186] for this modality. Three introductory sections describe the rationale and context of the CoP, the clinical use of small photon fields, and the physics of small-field dosimetry. In the fourth section, definitions of terms that are specific to the CoP (as compared to previous CoPs for broad-beam reference dosimetry, such as IAEA TRS-398 and AAPM TG-51) are given; this section includes a list of the symbols and equivalences between IAEA and AAPM nomenclature to facilitate the practical implementation of the CoP by end users of IAEA TRS-398 and AAPM TG-51. The fifth section summarizes the equations and procedures that are recommended in the CoP and the sixth section provides an overview of the methods used to derive the data provided in IAEA TRS-483. This is the first time an international Code of Practice for the dosimetry of small photon fields based on comprehensive data and correction factors has been published. This joint IAEA/AAPM CoP will ensure consistent reference dosimetry traceable to the international System of Units and enable common and internationally harmonized procedures to be followed by radiotherapy centers worldwide for the dosimetry of small static megavoltage photon fields.

Application of plastic scintillating fibres to surface dosimetry in megavoltage photon and electron beams: considerations for Cerenkov correction

This work investigated the application of plastic scintillation detectors (PSDs) in dosimetry measurements on the surface of a phantom in megavoltage photon and electron beams. Emission spectra associated with fluorescence in the PMMA optical fibre, Cerenkov in the optical fibre, and the signal from the scintillator were characterized with a spectrophotometer. The angular dependence of the PSD response was studied with the PSD both in the plane and perpendicular to the plane of gantry rotation. Two methods of stem correction were investigated: stem subtraction and full spectral correction. It was found that with the PSD in the plane of gantry rotation, the stem subtraction method overestimates the dose particularly in electrons. The magnitude of overestimation is dependent on the beam energy, scintillator type and the angle of incidence. In this study a maximum error of 12% was observed for 16 MeV with a gantry angle of 45°. It was shown that the cause of this error was a non-negligible emission of Cerenkov radiation from the scintillator itself. This error was not observed when the PSD was oriented perpendicular to the plane of rotation. Similarly, the full spectral method was effective in isolating the scintillation-only signal regardless of PSD orientation.

On Monolithic Silicon Array Detectors for Small-Field Photon Beam Dosimetry

Contemporary x-ray radiotherapy employs small radiation fields to deliver highly conformal dose distributions. Submillimeter accuracy in the measurement of the delivered dose map is a crucial requirement of detectors proposed for quality assurance applications. A 2-D monolithic silicon array detector can provide high spatial resolution by optimizing small sensitive volumes (SVs) in a large active area. They offer a stable and near energy-independent response in megavoltage photon beams, good dose linearity, and real-time read-out. The SVs are ion-implanted on a silicon wafer whose geometry and physical characteristics, such as resistivity and defects concentration, dramatically affect the detector performance. The Octa is a novel 2-D monolithic silicon array detector dedicated to small-field dosimetry. Its 512 diode SVs are arranged with a submillimeter pitch along four intersecting orthogonal linear arrays. We report on the experimental and numerical characterization (performed with Sentaurus Workbench within the Synopsys framework) of two Octa detectors manufactured, respectively, on a bulk and on an epitaxial silicon substrate. The effects of resistivity and defects concentration profiles across their large-area monolithic silicon wafers are compared and discussed in terms of the response linearity with dose, response uniformity, charge-collection efficiency, and clinical performance in the case of a small radiation field delivered with a flattening filter-free beam.

Preliminary characterization of the response of an organic field effect transistor to ionizing radiation

Organic thin film transistors (OTFT) were investigated as a novel radiation detector. The OTFTs were fabricated on a flexible PET substrate with a PMMA dielectric layer and a pentacene semiconductor. OTFTs were irradiated up to 400 Gy using kilovoltage (100 and 180 kVp) and megavoltage (6 and 18 MV) photon beams. One OTFT was irradiated to 1000 Gy using 6 MV to observe the longevity of the device. Irradiating the devices caused a positive threshold voltage shift in each device. The magnitude of the threshold voltage shift per unit dose decreased with accumulated dose until it stabilized after approximately 200 Gy. The sensitivity ranged from 2 to 10 mV/Gy at low accumulated dose and decreased to 0.5–1.5 mV/Gy after 200 Gy of accumulated dose across the various OTFTs. After 400 Gy all of the devices were still functional with a loss in mobility of about 15, 14, 12 and 9% for beam qualities of 100 kV, 180 kV, 6 MV, and 18 MV, respectively. After 1000 Gy using 6 MV the OTFT was still functional with a sensitivity of 0.8 ± 0.1 mV/Gy after 300 Gy. This study showed that an OTFT on a flexible substrate shows a measureable response to photon irradiation of various qualities.

P177] 3D Rapid manufacturing bolus vs commercial bolus – Skin dose comparison

Purpose Most of the breast treatments uses a megavoltage photon beams. This kind of energy exhibits a skin-sparing effect that is a concern in some stages of both breast conservative and post-mastectomy irradiation. To overwhelm this effect a bolus material is placed on the skin. However, when using commercial non-customized bolus, small air gaps can occur in certain regions of the treatment area where the bolus cannot establish the perfect match with the irregular skin of the patient, altering locally the expected skin dose. The objective of this study is to verify the improvement in the breast radiotherapy treatment using a customized bolus produced by 3D rapid manufacturing (3D-RM) techniques, and compare it with the commercial ones. Measurement of the surface dose and comparisons with the doses calculated by the Treatment Planning System (TPS) were also performed. Methods The customized 3D rapid manufactured bolus was prepared for an anthropomorphic female phantom breast area. 3D-CRT treatment plans for right breast were performed on CT scans of the phantom both with the customized and commercial bolus. The skin dose was measured in 5 points on the phantom breast surface using a MOSFET-based system. Results A better fitting to the surface of the phantom was observed for the customized bolus, reducing the air gaps and improving the daily positioning on the irregular breast surfaces. The average differences between calculated and measured doses in the customized and commercial bolus ranged from 2 to 10%. However, the highest dose difference (approximately 10%) was observed for the commercial bolus. Conclusions The 3D rapid manufactured bolus can reduce the uncertainty in the daily positioning and help to overcome the dose discrepancy due to unwanted air gaps affecting breast cancer radiation therapy. With the commercial bolus a higher dose difference relatively to TPS was observed. The skin dose increase is observed in the same proportion for both boluses.

The quality of megavoltage photon beams measured by the ratio of photoactivation and neutron capture yields

The activities of products of two photonuclear reactions (113In(γ,γ’)113mIn and 115In(γ,γ’)115mIn) and neutron capture (115In(n, γ)116mIn) were measured. The measurements were made to verify that obtained activities (as well as some of their ratios) change sufficiently with the change of the energy, so that the activation of the indium could be recommended as a sensitive test for the control of the endpoint energy (and beam quality) of bremsstrahlung beams in an energy region usually used in radiation therapy. Mikrotron photon beam was used for photoactivation, and the source of neutrons was the mikrotron tungsten target itself, at energies higher than threshold for (γ, n) reaction. At lower energies, neutrons were produced in Be target inserted in photon beam. It was obtained that sensitive index of endpoint energy variation can be obtained in a broad energy region from 5 MeV to 23 MeV by different combinations of saturation activities of three reaction products. The described measurements can be used to develop a very sensitive and simple method in a routine quality control of photon beams of megavoltage devices used in radiotherapy.

Characterization of spectral and intensity changes with measurement geometry in various light guides used in scintillation dosimetry.

To characterize response changes of various light guides used in megavoltage (MV) photon beam scintillation dosimetry as a function of irradiation conditions. Particular emphasis was placed on quantifying the impact of response changes on the Čerenkov light ratio (CLR). Intensity and spectral response measurements as a function of dose, depth, and fiber-beam angle were performed with a commercial scintillation detector stripped of its scintillation material and five different custom-made light guides. The core materials of the light guides investigated consisted of polymethyl methacrylate (PMMA), low- and high-hydroxyl content silica, and polystyrene. Dose levels ranging from 50monitor units (MU) to 1000MU, depths ranging from 1 to 20cm, and fiber-beam angles ranging from 10° to 90° were investigated. All measurements were performed at a photon beam energy of 6MV. The CLR was calculated by taking the ratio of the responses in the blue to green spectral regions. There was no significant change in the CLR measured with the modified commercial scintillation detector as a function of delivered dose. In addition, increases in the CLR as functions of depth and fiber-beam angle were observed where the maximum changes were 4.2% and 3.6%, respectively. The spectrum measurements showed no observable changes in spectral shape with depth except for the low-hydroxyl content silica fiber. Variations in the measured spectral shape with fiber-beam angle were observed for all fibers investigated. The magnitude of the changes in the spectral shape varied with fiber type, where the silica fibers exhibited the largest changes and the plastic fibers exhibited the smallest changes. Increases in the CLR were observed for the silica fibers with depth and for all fibers with fiber-beam angle. The plastic fibers showed no significant change in the CLR as a function of depth. Increases of 3.1% and 9.5% in the CLR were observed for the high- and low-hydroxyl content silica fibers, respectively, over the range of depths investigated. Variations of 2.3%, 6.1%, 5.1% and 11.9% were observed for the PMMA, polystyrene, high-hydroxyl, and low-hydroxyl content silica fiber CLR values as a function of fiber-beam angle, respectively. The insignificant change in the CLR with delivered dose indicates that a single CLR value over the investigated dose range is sufficient for accurate Čerenkov subtraction. Variations in the stem-effect spectrum shape can occur with changes in irradiation geometry. The magnitude of the changes are governed by the fiber construction and the optical properties of the fiber. The observed spectral shape changes can be explained by a combination of variations in optical path length through the fiber and the fiber fluorescent signal contribution to the stem-effect. These spectral shape variations directly influence the calculated CLR values. This work confirms that careful characterization of scintillation detectors is important as changes in the stem-effect spectrum can cause changes in the CLR. If the CLR changes between the reference and measurement conditions, this could result in an incorrect stem-effect subtraction and reduced measurement accuracy.

Characterization of the exradin W1 plastic scintillation detector for small field applications in proton therapy

Accurate dosimetry in small field proton therapy is challenging, particularly for applications such as ocular therapy, and suitable detectors for this purpose are sought. The Exradin W1 plastic scintillating fibre detector is known to out-perform most other detectors for determining relative dose factors for small megavoltage photon beams used in radiotherapy but its potential in small proton beams has been relatively unexplored in the literature. The 1 mm diameter cylindrical geometry and near water equivalence of the W1 makes it an attractive alternative to other detectors. This study examines the dosimetric performance of the W1 in a 74 MeV proton therapy beam with particular focus on detector response characteristics relevant to relative dose measurement in small fields suitable for ocular therapy. Quenching of the scintillation signal is characterized and demonstrated not to impede relative dose measurements at a fixed depth. The background cable-only (Čerenkov and radio-fluorescence) signal is 4 orders of magnitude less than the scintillation signal, greatly simplifying relative dose measurements. Comparison with other detectors and Monte Carlo simulations indicate that the W1 is useful for measuring relative dose factors for field sizes down to 5 mm diameter and shallow spread out Bragg peaks down to 6 mm in depth.