MV X-ray Beams Research Articles

Energy Dependence of the New Gafchromic EBT3 Film:Dose Response Curves for 50 KV, 6 and 15 MV X-Ray Beams

Purpose: To study and compare the dose response curves of the new GafChromic EBT3 film for megavoltage and kilovoltage x-ray beams, with different spatial resolutions. Methods: EBT3 films (lot#A101711-02) were exposed to each x-ray beam (6 MV, 15 MV, and 50 kV) at 7 dose values (50-3200 cGy). Each film piece was scanned three consecutive times in the center of Epson 10000XL flatbed scanner in 48-bit color at two separate spatial resolutions of 75 and 300 dpi. The data were analyzed using ImageJ and, for each scanned image, a region of interest (ROI) of 2 × 2 cm2 at the field center was selected to obtain the mean pixel value with its standard deviation in the ROI. For each energy, dose value and spatial resolution, the average net optical density (netOD) and its associated uncertainty were determined. The Student’s t-test was performed to evaluate the statistical differences between the net OD/dose values of the three energy modalities, with different color channels and spatial resolutions. Results and Discussion: The dose response curves for the three energy modalities were compared in three color channels. Weak energy dependence was found. For doses above 100 cGy, no statistical differences were observed between 6 and 15 MV beams, regardless of spatial resolution and color channel. However, statistical differences were observed between 50 kV and the megavoltage beams. The degree of energy dependence (from MV to 50 kV) was found to be a function of color channel, dose level, and spatial resolution. Conclusions: The dose response curves for GafChromic EBT3 films were found to be weakly dependent on the energy of the photon beams from 6 MV to 15 MV. For very low energy photon (e.g. 50 kV), variation of more than 11% due to the energy-dependence is observed, depending on the absorbed dose, spatial resolution and color channel used.

Dosimetry and fast neutron energies characterization of photoneutrons produced in some medical linear accelerators

This work focusses on the estimation of induced photoneutrons energy, fluence, and strength using nuclear track detector (NTD) (CR-39). Photoneutron energy was estimated for three different linear accelerators, LINACs as an example for the commonly used accelerators. For high-energy linear accelerators, neutrons are produced as a consequence of photonuclear reactions in the target nuclei, accelerator head, field-flattening filters and beam collimators, and other irradiated objects. NTD (CR-39) is used to evaluate energy and fluence of the fast neutron. Track length is used to estimate fast photoneutrons energy for linear accelerators (Elekta 10 MV, Elekta 15 MV, and Varian 15 MV). Results show that the estimated neutron energies for the three chosen examples of LINACs reveals neutron energies in the range of 1–2 MeV for 10 and 15 MV X-ray beams. The fluence of neutrons at the isocenter (Φtotal) is found to be (4×106 n cm2 Gy−1) for Elekta machine 10 MV. The neutron source strengths Q are calculated. It was found to be 0.2×1012 n Gy−1 X-ray at the isocenter. This work represents simple, low cost, and accurate methods of measuring fast neutrons dose and energies.

Evaluation of the Toxicity of Total Body Irradation As a Separate Treatment in Preparation for Hematopoietic Cell Transplantation

Abstract 4579 BACKGROUND:Myeloablative regimens used for allogeneic or autologous hematopoietic cell transplantation (HCT) may be based on either chemotherapy alone or total body irradiation (TBI). TBI is generally considered highly toxic, however, it is usually combined with high-doses of cyclophosphamide or other cytostatics, which does not allow reliable assessment of its tolerance. In Maria Sklodowska-Curie Cancer Center and Institute of Oncology in Gliwice TBI is used a single treatment in the preparation for the first autologous HCT in patients with multiple myeloma (MM). Second HCT in a tandem procedure is performed after 4 months interval with high-dose melphalan as conditioning. The protocol gives opportunity to separately assess early toxicity of TBI, which is a goal of this study. PATIENTS AND METHODS:The clinical course of 30 patients with MM, aged 58 years (range, 50–66), including 14 men and 16 women was evaluated. The disease status prior to autoHCT was CR (n=9), VGPR (n=7), or PR (n=14). 47% patients were pre-treated with 2 or more lines of chemotherapy and the median number of preceding cycles was 8. TBI was administered at the dose of 12 Gy divided in 3 fractions on days -3, -2, -1. 6 MV X-ray beam was used with lung shielding after 6 Gy and additional irradiation of chest wall using electrons 9–15 MeV to the planned total dose. Peripheral blood was used as a source of stem cells. The incidence of adverse events was assessed using ‘Common Toxicity Criteria v4.03'. RESULTS:Within 100 days follow-up all patients were alive. Median time to neutrophil >0.5 ×10e9/L and platelet >50 ×10e9/L recovery was 12 days (11–21) and 14 days (9–18), respectively. Patients required the median of 1 (0–3) platelet and 0 (0–6) red blood cell transfusions. The median hospital stay since HCT was 16 days (14–21). The most frequent grade 3 or 4 adverse events were infections (43%), including neutropenic fever (23%). Other severe toxicities were rare. In particular no patient experienced hepatic or renal complications (Table 1). CONCLUSION:TBI as a separate treatment is well tolerated and may be safely used for conditioning in a population of patients up to 66 years old. Our observation may potentially be applied to both autologous and allogeneic transplantations.EventGrade 2Grade 3Grade 4Neutropenic fever—23%—Other infections3%17%3%Nausea/vomiting27%——Diarrhea7%——Mucositis7%3%—Hypotension—3%— Disclosures:No relevant conflicts of interest to declare.

Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system

To apply the dual ionization chamber method for mixed radiation fields to an accurate comparison of the secondary neutron dose arising from the use of a tungsten alloy multileaf collimator (MLC) as opposed to a brass collimator system for defining the shape of a therapeutic proton field. Hydrogenous and nonhydrogenous ionization chambers were constructed with large volumes to enable measurements of absorbed doses below 10(-4) Gy in mixed radiation fields using the dual ionization chamber method for mixed-field dosimetry. Neutron dose measurements were made with a nominal 230 MeV proton beam incident on a closed tungsten alloy MLC and a solid brass block. The chambers were cross-calibrated against a (60)Co-calibrated Farmer chamber in water using a 6 MV x-ray beam and Monte Carlo simulations were performed to account for variations in ionization chamber response due to differences in secondary neutron energy spectra. The neutron and combined proton plus γ-ray absorbed doses are shown to be nearly equivalent downstream from either a closed tungsten alloy MLC or a solid brass block. At 10 cm downstream from the distal edge of the collimating material the neutron dose from the closed MLC was (5.3 ± 0.4) × 10(- 5) Gy/Gy. The neutron dose with brass was (6.4 ± 0.7) × 10(- 5) Gy/Gy. Further from the secondary neutron source, at 50 cm, the neutron doses remain close for both the MLC and brass block at (6.9 ± 0.6) × 10(- 6) Gy/Gy and (6.3 ± 0.7) × 10(- 6) Gy/Gy, respectively. The dual ionization chamber method is suitable for measuring secondary neutron doses resulting from proton irradiation. The results of measurements downstream from a closed tungsten alloy MLC and a brass block indicate that, even in an overly pessimistic worst-case scenario, secondary neutron production in a tungsten alloy MLC leads to absorbed doses that are nearly equivalent to those seen from brass collimators. Therefore, the choice of tungsten alloy in constructing the leaves of a proton MLC is appropriate, and does not lead to a substantial increase in the secondary neutron dose to the patient compared to that generated in a brass collimator.

Effects of Secondary Neutron Beam Generated in Radiotherapy on Electronic Medical Devices

In recent years, the administration of radiotherapy to patients bearing an implantable electronic device, such as a pacemaker, has been increasing. Although the effects of primary radiation on electronic devices have been studied, research on the effects of the secondary radiation beam is limited. Because semiconductors constitute the main parts of an electronic device, the effects of the secondary neutrons in radiation should be considered. We performed a Monte Carlo simulation of neutron generation during radiotherapy, and considered the influence of neutrons on the electronic medical devices. Monte Carlo simulations were performed using the Particle and Heavy Ion Transport code System (PHITS), developed by the Japan Atomic Energy Agency. Nuclear data was taken from ENDF/B-VII. We irradiated a water phantom with a 10 MV X-ray beam and a 200 MeV proton beam. Then, the neutron dose was evaluated by using PHITS. Moreover, we also measured the dose with a CR-39 dosimeter. In the case of the 10 MV X-ray beam, although fast neutrons (> 100 keV) were generated in the upstream part of the beam, thermal neutrons were scarcely generated. Inside the water phantom, which simulated a human body, the generation of fast neutrons decreased rapidly as a result of thermalization. In the case of the 200 MeV proton beam, fast neutrons were distributed along the beam axis inside the water phantom, and were generated inside the water phantom. Thermal neutrons (< 0.5 eV) were uniformly distributed throughout the whole phantom. In the evaluation of secondary neutrons generated during radiotherapy, we consider that using Monte Carlo calculations in addition to experimental measurements will be useful for clinical applications.

In vivodosimetry with optically stimulated luminescent dosimeters, OSLDs, compared to diodes; the effects of buildup cap thickness and fabrication material

For external beam in vivo measurements, the dosimeter is normally placed on the patient's skin, and the dose to a point of interest inside the patient is derived from surface measurements. In order to obtain accurate and reliable measurements, which correlate with the dose values predicted by a treatment planning system, a dosimeter needs to be at a point of electronic equilibrium. This equilibrium is accomplished by adding material (buildup) above the detector. This paper examines the use of buildup caps in a clinical setting for two common detector types: OSLDs and diodes. Clinically built buildup-caps and commercially available hemispherical caps are investigated. The effects of buildup cap thickness and fabrication material on field-size correction factors, C(FS), are reported, and differences between the effects of thickness and fabrication material are explained based on physical parameters. Measurements are made on solid water phantoms for 6 and 15 MV x-ray beams. Two types of dosimeters are used: OSLDs, InLight∕OSL Nanodot dosimeters (Landauer, Inc., Glenwood, IL) and a P-type surface diode (Standard Imaging, Madison, WI). Buildup caps for these detectors were fabricated out of M3, a water-equivalent material, and sheet-metal stock of Al, Cu, and Pb. Also, commercially available hemispherical buildup caps made of plastic water and brass (Landauer, Inc., Glenwood, IL) were used with Nanodots. OSLDs were read with an InLight microStar reader (Landauer, Inc., Glenwood, IL). Dose calculations were carried out with the XiO treatment planning system (CMS∕Elekta, Stockholm) with tissue heterogeneity corrections. For OSLDs and diodes, when measurements are made with no buildup cap a change in C(FS) of 200% occurs for a field-size change from 3 cm × 3 cm to 30 cm × 30 cm. The change in C(FS) is reduced to about 4% when a buildup cap with wall thickness equal to the depth of maximum dose is used. Buildup caps with larger wall thickness do not cause further reduction in C(FS). The buildup cap fabrication material has little or no effect on C(FS). The perturbation to the delivered dose caused by placing a detector with a buildup cap on the surface of a patient is measured to be 4%-7%. A comparison between calculated dose and dose measured with a Nanodot and a diode for 6 and 15 MV x-rays is made. When C(FS) factors are carefully determined and applied to measurements made on a phantom, the differences between measured and calculated doses were found to be between ±1.3%. OSLDs and diodes with appropriate buildup caps can be used to measure dose on the surface of a patient and predict the delivered dose to depth dmax in a range of ±1.3% for 100 cGy. The buildup cap: can be fabricated from any material examined in this work, is best with wall thickness dmax, and causes a perturbation to the delivered dose of 4%-7% when the wall thickness is dmax. OSLDs and diodes with buildup caps can both give accurate measurements of delivered dose.

In vivo measurement of radiation dose during radiotherapy in breast cancer patients using MOSFET dosimeter

Objective The purpose of the study was to observe and analysis the actual dosage of patients with breast cancer using metal oxide semiconductor field effect transistor (MOSFET) detector.Methods First, Phantom measurements were performed to investigate dose distribution in the area of the junction in a half-field matching method and the influence of factors related to the accelerator. In vivo dose measurements were performed for patients with breast cancer to investigate the skin dose and the junction of supraclavicular-axillary field and tangential field in 6 MV X-ray beams. Results Phantom measurements showed that the relative deviation in the junction were within + 3％, and the dose distributions in the junction area depended on the matching field direction (x or y). In vivo measurement of tangential region for patients showed that, the maximum dose deviation between measurement and calculation was -30. 39％,the minimum deviation was - 18. 85％, the average dose deviation was -24. 76％. The dose deviation of tangential fields for patients with breast-conserving surgery was larger than that patients with radical surgery (t =2. 40 ,P＜0. 05), while dose deviation of supraclavicular-axillary fields was not significantly different. The average values of 15 fraction in the junction area showed more stable than one individual measurement.Conclusions It is important to real-time, in vivo measurement of radiation dose during radiotherapy in patients with breast cancer, and change treatment plan in time, to ensure the accuracy of target dose. Key words: Metal oxide semiconductor field effect transistor detector; Dosimetry; Adjacent half-fields; Phantom; Breast neoplasm

Dosimetric Evaluation of Linac Photon Small Fields using MAGIC Polymer Gels

Introduction: In radiotherapy, methods of treatment planning are becoming increasingly more complicated. This requires verification of the doses delivered to increasingly smaller and more precise regions. Radiotherapy techniques are continuously employing smaller and smaller field sizes to deliver tighter radiation doses with higher therapeutic ratios, generating interest among researchers to provide reliable dosimetry for beams and treatment plans collimated to small field sizes. In this study, the dosimetry of these fields was evaluated in clinical applications by using polymer gel dosimetry. Material and Methods: The MAGIC polymer gel was used in this study. The gel samples were manufactured and poured into phantoms and calibration vials and were irradiated with a 6 MV x-ray beam. The R2 maps of the dose distributions were obtained from the gel MR images. The depth dose distributions and dose profile measurements were measured in different fields at a depth of 5 cm in gel, and were compared against another technique using a pinpoint chamber. Results: Comparison of the results of gel and pinpoint chamber measurements showed largest differences between the dose profile measurements in the low dose regions (near the edges). In these regions, the pinpoint-chamber measured penumbra width was at most 3.2 mm wider than those given by gel dosimeters. For a 30 × 30 mm2 field, maximum difference between gel and pinpoint chamber was 2 mm within the depth of maximum dose region, and were 2 mm, 3 mm and 2 mm for 20 × 20 mm2, 10 × 10 mm2 and 5 × 5 mm2 fields, respectively. The maximum differences within the buildup region for 30 × 30 mm2, 20 × 20 mm2, 10 × 10 mm2 and 5 × 5 mm2 fields were 9%, 4.5%, 2% and 9%, respectively. Discussion and Conclusion: The differences in the depth dose distributions and dose profile measurements between MAGIC polymer gel and pinpoint chamber are attributable to different factors. The dosimetry of these fields using ionization chambers has limitations regarding uncertainty, due to the relatively large sizes of ionization chambers with respect to small field sizes and the lack of charged-particle equilibrium in such small fields. Pinpoint chambers are too large for very small photon fields and smaller dosimeters should be used. Fewer limitations in the dosimetry of small fields exist when using polymer gel dosimeters. Hence polymer gel dosimeters are suitable for dosimetry of photon small fields.

A convolution/superposition method using primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance: comparison of calculated and measured 10-MV X-ray doses in thorax-like phantoms

We performed experimental studies on the convolution/superposition method reported in the former companion paper (Iwasaki in Radiol Phys Technol 4, 2011) using 10-MV X-ray beams from open-jaw-collimated fields. The method uses primary and scatter dose kernels formed for energy bins of X-ray spectra reconstructed as a function of off-axis distance. We made a comparison of calculations and measurements in water phantoms and thorax-like phantoms with respect to percentage depth dose curves, tissue-phantom ratio curves, and dose profiles. We made the dose calculation by taking into account the beam-hardening effect with depth and the off-axis radiation-softening effect. We found that the method could be used, in general, for performing accurate dose calculations.

SU-E-T-106: Experimental Characterization of Al2O3:C Optically Stimulated Luminescence Detector (OSLD) Exposed to 6 MV X-Ray Beams

Purposes: To characterize the response of Al2O3:C optically stimulated luminescencedetectors (OSLDs) to 6 MV x‐ray beams and to determine the optimal bleaching method that would allow the re‐use of the OSLDs. Methods: OSLDs were exposed to a 6 MV x‐ray linac beam in the dose range from 50 mGy–10 Gy. Readouts were performed with a commercial OSLD reader (microStarTM, Landauer Inc.). For 50 mGy and 100 mGy doses, OSLDs were readout using the reader's low‐dose mode, which provides higher stimulation power compared to the high‐dose mode. All other readouts were performed using the high‐dose mode. The OSLD response to dose, bleaching time and repeated readouts (depletion) were determined. Bleaching of the OSL signal was performed using a 250 W halogen lamp with two Methods: (a) direct exposure to light; and (b) exposure using a long‐pass optical filter to block wavelengths shorter than 495 nm. Results: The OSLD dose‐response was linear for the investigated dose range. After 100 readouts, the OSL signal was depleted by (24.5 ± 0.7) % and (3.16 ± 0.07) % with a depletion rate of (0.251 ± 0.002) and (0.023 ± 0.002) %/readout for low‐ and high‐dose modes, respectively. After a 5 min bleaching time, (85.1 ± 1.4) and (70.5 ± 2.0) % reductions in the OSL signal for all doses was attained using methods (a) and (b), respectively. After a 100 min bleaching time, (99.5 ± 0.2) % reduction was attained. Conclusions: We observed linearity of the OSLD's dose response for the investigated dose range. A 100 min bleaching time was sufficient to bleach 99.5 % of the OSL signal for both bleaching methods. The depletion rate using the low‐dose mode is 11 times higher than using the high‐dose mode. for the high‐dose mode of the reader, the depletion rate is independent on dose.

SU-E-T-163: Characterization of the Response of Active Detectors and Passive Dosemeters Used for Dose Measurement in Small Photon Beams

Purpose: To characterize the response of detectors used for the dosimetry in small photon beams, especially for stereotactic radiation therapy. Methods: Measurements were performed with active detectors dedicated to the dosimetry in small fields (high resolution diodes IBA SFD, Sun Nuclear EDGE, PTW 60016 and PTW 60017; PTW 31014 0.015 cm3 PinPoint chamber, PTW 31018 microLion liquid chamber and PTW 60003 natural diamond) and with passive dosemeters suitable for small fields (TLD micro-cubes, EBT2 radiochromic films). They were irradiated in a Cobalt 60 beam for investigation of reproducibility and drift and for study of the response as a function of the dose, in X-ray linac beams ranging from 4 to 18 MV for study of the response as a function of the beam energy and as a function of the dose rate, and in small and large 6 MV X-ray beams of different irradiating systems for study of the influence of beam size. Results: Regarding the reproducibility of the detector response, diodes give the best results, especially the EDGE and the PTW 60016 diodes with reproducibility better than 0.3%. The SFD diode reveals problem of stability, with sudden unexplainable large drift of the response. Both the PTW diodes undergo a slight variation of the response as a function of the dose rate. The detectors showing the highest variation of the response as a function of beam energy are the unshielded diodes. On the contrary, shielded diodes reveal more important impact of their non tissue equivalent composition on the measurement of output factors in very small fields (Huet et al., 2011; Bassinet et al., 2011). Correction factors depend on the measurement depth. Conclusions: This study will help to determine an optimized methodology for the measurement of the data characterizing the small photon beams used in radiotherapy.

SU-E-T-726: Validation of ProACTive, an Irreducible Vector Space Method of Dose Calculation

Purpose: Monte Carlo dose calculations are considered to be the gold standard by which other techniques are compared, particularly for complex geometries with regions of electronic disequilibrium. The major disadvantage of Monte Carlo is the associated long calculation times. This necessitates limiting the number of histories and/or using interpolative techniques, potentially reducing accuracy. ProACTive is an irreducible vector space method of dose calculation with computation times significantly faster than Monte Carlo. The purpose of the work is to validate ProACTiveˈs accuracy.Method: ProACTive calculations were compared to measurement and/or EGS4/PRESTA Monte Carlo for both homogenous and heterogeneous geometries for 6MV and 18MV. Sufficient Monte Carlo histories were run to minimize statistical uncertainty. Depth dose and profile measurements were taken from an average of measurements obtained on 40 Varian linear accelerators commissioned in the last several years. Heterogeneous phantoms included a complex phantom consisting of water, aluminum and lung irradiated by a 2×2 cm2 18 MV x-ray beam as described by Mohan and Rogers (ICCR2000 Proceedings). Results: ProACTive calculation times were significantly faster than Monte Carlo run with a large number of histories. For homogenous geometries, ProACTive calculations showed good agreement with measurement for a wide range of depths and beam sizes, both on and off axis. For the ICCR2000 phantom, ProACTive and Monte Carlo showed overall good agreement. Both calculation methods showed some difference compared to measurement, especially at a density boundary. It is not clear whether the differences are real or due to uncertainty in the measurements and their interpretation for this extreme geometry. Conclusions: For the geometries tested, ProACTive shows good agreement with measurement and similar computational accuracy to Monte Carlo when the latter is run with sufficient histories to minimize statistical uncertainty. ProACTiveˈs accuracy and computational time make it a feasible clinical solution to accurate dose calculation.

Establishment of references in terms of absorbed dose to water in MV X-ray beams for small radiation fields

Establishment of references in terms of absorbed dose to water in MV X-ray beams for small radiation fields

Simple Proposal for Dosimetry with an Elekta iViewGTTM Electronic Portal Imaging Device (EPID) Using Commercial Software Modules

An electronic portal imaging device (EPID) is used to control for patient setup and positioning during fractionated radiotherapy. Due to the rising complexity and conformity of irradiation techniques, the demand for an accurate verification of the dose delivered to the patient has also increased. The purpose of this study was to investigate a simple guidance for dosimetry with an Elekta iViewGT™ EPID using commercial software modules. EPID measurements were performed using an Elekta iViewGT™ EPID on a linear accelerator with 6 MV x-ray beam. The EPID signal was studied for reproducibility, as well as characteristics as a function of dose, dose rate, and field size. A series of experiments, comparing the response of the flat panel imager and ionization chamber measurements of dose, determine the parameters for the calibration model. EPID measurements were also compared with calculations of the treatment planning system. We found a stable response of the EPID signal over a period of 14 months. It showed nonlinearity depending on dose up to 6.8%. There were low oscillations up to 1.2% depending on dose rate. For all fields, the calibrated flat panel profiles match the measured and calculated dose profiles with maximum deviation of 2-3% for the in-field region. In the high gradient areas, higher differences up to 6% were found. The gamma evaluation indicates good correlation between predicted and acquired EPID images. The EPID-based pretreatment IMRT verification method will help to improve the quality assurance procedure.

Thermal neutron fluence in a treatment room with a Varian linear accelerator at a medical university hospital

The indium foil activation technique has been employed to measure thermal neutron fluences ( Φ th) among various locations in the treatment room with a 20×20 cm 2 field size and a 15 and 10 MV X-ray beam. Spatial Φ th are visualized using colored three-dimensional graphical representations; intensities are up to (1.97±0.13)×10 5 and (1.46±0.13)×10 4 n cm −2/Gy-X at isocenter, respectively. The Φ th is found to increase with the X-ray energy of the LINAC and decreases as it moves away from the beam center. However, thermal neutron exposure is not assessed in routine dosimetry planning and radiation assessment of patients since neutron dose contributes <1% of the given therapy dose. However, unlike the accelerated beam limited within the gantry window, photoneutrons are widely spread in the treatment room. Distributions of Φ th were measured in water phantom irradiated with 15 MV X-ray beams. The shielding effect of the maze was also evaluated. The experimentally estimated Φ th along the maze distance was fitted explicate and the tenth-value layer (TVL) was calculated and discussed. Use of a 10 cm-thick polyethylene door placed at the maze was suitable for radiation shielding.

Investigation of radiological properties and water equivalency of PRESAGE® dosimeters

PRESAGE is a dosimeter made of polyurethane, which is suitable for 3D dosimetry in modern radiation treatment techniques. Since an ideal dosimeter is radiologically water equivalent, the authors investigated water equivalency and the radiological properties of three different PRESAGE formulations that differ primarily in their elemental compositions. Two of the formulations are new and have lower halogen content than the original formulation. The radiological water equivalence was assessed by comparing the densities, interaction probabilities, and radiation dosimetry properties of the three different PRESAGE formulations to the corresponding values for water. The relative depth doses were calculated using Monte Carlo methods for 50, 100, 200, and 350 kVp and 6 MV x-ray beams. The mass densities of the three PRESAGE formulations varied from 5.3% higher than that of water to as much as 10% higher than that of water for the original formulation. The probability of photoelectric absorption in the three different PRESAGE formulations varied from 2.2 times greater than that of water for the new formulations to 3.5 times greater than that of water for the original formulation. The mass attenuation coefficient for the three formulations is 12%-50% higher than the value for water. These differences occur over an energy range (10-100 keV) in which the photoelectric effect is the dominant interaction. The collision mass stopping powers of the relatively lower halogen-containing PRESAGE formulations also exhibit marginally better water equivalency than the original higher halogen-containing PRESAGE formulation. Furthermore, the depth dose curves for the lower halogen-containing PRESAGE formulations are slightly closer to that of water for a 6 MV beam. In the kilovoltage energy range, the depth dose curves for the lower halogen-containing PRESAGE formulations are in better agreement with water than the original PRESAGE formulation. Based on the results of this study, the new PRESAGE formulations with lower halogen content are more radiologically water equivalent overall than the original formulation. This indicates that the new PRESAGE formulations are better suited to clinical applications and are more accurate dosimeters and phantoms than the original PRESAGE formulation. While correction factors are still needed to convert the dose measured by the dosimeter to an absorbed dose in water in the kilovoltage energy range, these correction factors are considerably smaller for the new PRESAGE formulations compared to the original PRESAGE and the existing polymer gel dosimeters.

Secondary Cancer Risks in Out-of-field Organs for 3-D Conformal Radiation Therapy

The risk of secondary cancers after radiation therapy has become an important issue, mainly because advanced treatment techniques such as 3-D conformal radiation therapy (CRT) and intensity-modulated radiation therapy (IMRT) tend to increase the radiation dose to out-of-field organs. Moreover, cancer patients’ average survival period, following those advances in treatment techniques, also has increased, effectively raising the risks for the same levels of radiation exposure. In the present study, the secondary cancer risks to out-of-field organs were determined for some representative cases of 3-D CRT by measuring organ doses with LiF thermoluminescence (TL) dosimeters and an ATOM TM male phantom. Our results show that the secondary cancer risks are similar (difference: < 20%) for 6 and 10 MV X-ray beams for the same prostate treatment case. The total secondary cancer risks of the out-of-field organs were on the order of 2-4% for the treatment cases considered in the present study.

Analysis of output trends from Varian 2100C/D and 600C/D accelerators

Analysis of Varian linear accelerator output trends is reported. Two groups, consisting of four matched Varian 2100C/D and four matched Varian 600C/D accelerators, with different designs of monitor chamber, have been investigated and the data acquired from regular calibrated ion chamber/electrometer measurements of the output performance of the eight accelerators analysed. The trend of machine output with time, having removed the effect of adjusting the monitor chamber response, was compared on a monthly and annual basis for monitor chambers with ages ranging between 1 year and 7 years. The results indicate that the response is generally consistent within each set of accelerators with different monitor chamber designs. Those used in a Varian 600C/D machine result in a reduction in measured output over time, with an average monthly reduction of 0.35 ± 0.09% over the course of the first 4 years of use. The chambers used in a 2100C/D accelerator result in an increase in measured output over time, with an average monthly increase of 0.26 ± 0.09% over the course of the first 4 years of use. The output increase then reduces towards the end of this period of time, with the average monthly change falling to −0.03 ± 0.02% for the following 3 years. The output response trend was similar for all clinical energies used on the 2100C/D accelerators-–6, 15 MV x-ray beams, and 4, 6, 9, 12, 16 and 20 MeV electron beams. By tracking these changes it has been possible to predict the response over time to allow appropriate adjustments in monitor chamber response to maintain a measured accelerator output within tolerance and give confidence in performance. It has also provided data to indicate the need for planned preventative intervention and indicate if the monitor chamber response is behaving as expected.

Study of dosimetric water equivalency of PRESAGE® for megavoltage and kilovoltage x-ray beams

PRESAGE is a dosimeter that is suitable for 3D dosimetry. To be used as an ideal dosimeter, however, it should present radiologically water equivalent properties. In this work, we have investigated the radiological properties of three different PRESAGE® formulations. The radiological water equivalence was assessed by comparing the photon cross sections and radiation dosimetry properties of the three different PRESAGE® formulations with the corresponding values for water. Relative depth doses were calculated using Monte Carlo methods for 75, 125, 180 and 280 kVp and 6 MV x-ray beams. Based on the results of this study, the PRESAGE® formulations with lower halogen content are more dosimetrically water equivalent.

High resolution polymer gel dosimetry for small beam irradiation using a 7T micro-MRI scanner

The use of small field radiation beams has greatly increased with advanced radiation therapy techniques such as IMRT, rotational IMRT, and stereotactic body radiotherapy. In this work small field 3D dose distributions have been measured with high spatial resolution using polymer gels and 7T micro-MR imaging. A MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) polymer gel [1] phantom was used to capture the 3D dose distributions for two small field (5 × 5 mm2 and 10 × 10 mm2) for a 6MV x-ray beam. High resolution 3D T2 maps were obtained with 7T micro-MRI (0.156mm × 0.156mm × 1mm, MSME pulse sequence). For comparison T2 maps, the gel phantom was scanned in a 3T MRI clinical scanner (0.254mm × 0.254mm × 2mm, FSE pulse sequence). Normalized 3D dose maps were calculated in Matlab. Results show that 7T micro-MRI 3D gel dosimetry measurements are much more stable, less noisy, and have higher spatial resolution than those obtained using a 3T clinical scanner for the same amount of scan time. In general, 3D gel dosimetry results also agree with simultaneously-obtained radiochromic film dosimetry. This study indicates that the MAGIC polymer gel with 7T micro-MRI for 3D dose readout could potentially be used for small radiation beams, including measurements for micro-beams (field size ∼ 100um).