Dosimetry for 131Cs and 125I seeds in solid water phantom using radiochromic EBT film

PurposeTo measure the 2D dose distributions with submillimeter resolution for ¹³¹Cs (model CS-1 Rev2) and ¹²⁵I (model 6711) seeds in a Solid Water phantom using radiochromic EBT film for radial distances from 0.06 cm to 5 cm. To determine the TG-43 dosimetry parameters in water by applying Solid Water to liquid water correction factors generated from Monte Carlo simulations.MethodsEach film piece was positioned horizontally above and in close contact with a ¹³¹Cs or ¹²⁵I seed oriented horizontally in a machined groove at the center of a Solid Water phantom, one film at a time. A total of 74 and 50 films were exposed to the ¹³¹Cs and ¹²⁵I seeds, respectively. Different film sizes were utilized to gather data in different distance ranges. The exposure time varied according to the seed air-kerma strength and film size in order to deliver doses in the range covered by the film calibration curve. Small films were exposed for shorter times to assess the near field, while larger films were exposed for longer times in order to assess the far field. For calibration, films were exposed to either 40 kV (M40) or 50 kV (M50) x-rays in air at 100.0 cm SSD with doses ranging from 0.2 Gy to 40 Gy. All experimental, calibration and background films were scanned at a 0.02 cm pixel resolution using a CCD camera-based microdensitometer with a green light source. Data acquisition and scanner uniformity correction were achieved with Microd3 software. Data analysis was performed using ImageJ, FV, IDL and Excel software packages. 2D dose distributions were based on the calibration curve established for 50 kV x-rays. The Solid Water to liquid water medium correction was calculated using the MCNP5 Monte Carlo code. Subsequently, the TG-43 dosimetry parameters in liquid water medium were determined.ResultsValues for the dose-rate constants using EBT film were 1.069±0.036 and 0.923±0.031 cGy U⁻¹ h⁻¹ for ¹³¹Cs and ¹²⁵I seed, respectively. The corresponding values determined using the Monte Carlo method were 1.053±0.014 and 0.924±0.016 cGy U⁻¹ h⁻¹ for ¹³¹Cs and ¹²⁵I seed, respectively. The radial dose functions obtained with EBT film measurements and Monte Carlo simulations were plotted for radial distances up to 5 cm, and agreed within the uncertainty of the two methods. The 2D anisotropy functions obtained with both methods also agreed within their uncertainties.ConclusionEBT film dosimetry in a Solid Water phantom is a viable method for measuring ¹³¹Cs (model CS-1 Rev2) and ¹²⁵I (model 6711) brachytherapy seed dose distributions with submillimeter resolution. With the Solid Water to liquid water correction factors generated from Monte Carlo simulations, the measured TG-43 dosimetry parameters in liquid water for these two seed models were found to be in good agreement with those in the literature.     

A Monte Carlo study on neutron and electron contamination of an unflattened 18-MV photon beam

Recent studies on flattening filter (FF) free beams have shown increased dose rate and less out-of-field dose for unflattened photon beams. On the other hand, changes in contamination electrons and neutron spectra produced through photon (E>10 MV) interactions with linac components have not been completely studied for FF free beams. The objective of this study was to investigate the effect of removing FF on contamination electron and neutron spectra for an 18-MV photon beam using Monte Carlo (MC) method. The 18-MV photon beam of Elekta SL-25 linac was simulated using MCNPX MC code. The photon, electron and neutron spectra at a distance of 100 cm from target and on the central axis of beam were scored for 10×10 and 30×30 cm² fields. Our results showed increase in contamination electron fluence (normalized to photon fluence) up to 1.6 times for FF free beam, which causes more skin dose for patients. Neuron fluence reduction of 54% was observed for unflattened beams. Our study confirmed the previous measurement results, which showed neutron dose reduction for unflattened beams. This feature can lead to less neutron dose for patients treated with unflattened high-energy photon beams.     

N-isopropylacrylamide gel dosimeter to evaluate clinical photon beam characteristics

The introduction of beam intensity control concept in current radiotherapy techniques has increased treatment planning complexity. Thus, small-field dose measurement has become increasingly vital. Polymer gel dosimetry method is widely studied. It is the only dose measurement tool that provides 3D dose distribution. This study aims to use an N-isopropylacrylamide (NIPAM) gel dosimeter to conduct beam performance measurements of percentage depth dose (PDD), beam flatness, and symmetry for photon beams with field sizes of 3×3 and 4×4 cm². Computed tomography scans were used to readout the gel dosimeters. In the PDD measurement, the NIPAM gel dosimeter and Gafchromic^TM EBT3 radiochromic film displayed high consistency in the region deeper than the build-up region. The gel dosimeter dose profile had 3% lower flatness and symmetry measurement at 5 cm depth for different fields compared with that of the Gafchromic^TM EBT3 film. During gamma evaluation under 3%/3 mm dose difference/distance-to-agreement standard, the pass rates of the polymer gel dosimeter to the TPS and EBT3 film were both higher than 96%. Given that the gel is tissue equivalent, it did not exhibit the energy dependence problems of radiochromic films. Therefore, the practical use of NIPAM polymer gel dosimeters is enhanced in clinical dose verification.     

Distribution of spatial photoneutrons inside a 70 kg water phantom via neutron activation analysis

This study evaluated spatial Φ_th inside a 70 kg water phantom using the NAA method. Fifty indium foils were placed inside the water phantom and exposed under 15 MV LINAC for 2.5 min to yield the 10 Gy X-ray dose. The Φ_th value at the isocenter of the water was 1.03×10⁶ n cm^?2/Gy-X, and the maximum quantity of Φ_th appeared at the water surface along the z-axis, 3.99×10⁶ n cm^?2/Gy-X. The thermal neutron dose at isocenter of the water phantom occupied approximately 0.151% of the whole photo and neutron dose.     

Concrete enclosure for shielding a neutron source

In the aim to design a shielding for a 0.185 TBq ²³⁹PuBe isotopic neutron source several Monte Carlo calculations were carried out using MCNP5 code. First, a point-like source was modeled in vacuum and the neutron spectrum and ambient dose equivalent were calculated at several distances ranging from 5 cm up to 150 cm, these calculations were repeated modeling a real source, including air, and a 1×1×1 m³ enclosure with 5, 15, 20, 25, 30, 50 and 80 cm-thick Portland type concrete walls. At all the points located inside the enclosure neutron spectra from 10⁻⁸ up to 0.5 MeV were the same regardless the distance from the source showing the room-return effect in the enclosure, for energies larger than 0.5 MeV neutron spectra are diminished as the distance increases. Outside the enclosure it was noticed that neutron spectra becomes “softer” as the concrete thickness increases due to reduction of mean neutron energy. With the ambient dose values the attenuation curve in terms of concrete thickness was calculated.     

Effect of wedge filter and field size on photoneutron dose equivalent for an 18 MV photon beam of a medical linear accelerator

Photoneutrons produced during radiation therapy with high energy photons is the main source of unwanted out-of-field received doses of patients. To analyze the neutron dose equivalent (NDE) for wedged beams and its variation with field size, Monte Carlo (MC) modeling of an 18 MV photon beam was performed using MCNPX MC code. The results revealed that the NDE is on average 6.5 times higher for wedged beams. For open beams, the NDE decreased with increasing field size especially for field sizes >20×20 cm². While, for wedged beams, the NDE increased with field size. It was suggested that the increase of NDE for wedged beams should be taken into account in radiation-induced secondary cancer risk estimations and radiation protection calculations.     

Electronic brachytherapy for postsurgical adjuvant vaginal cuff irradiation therapy in endometrial and cervical cancer: A retrospective study

PurposeA new platform for brachytherapy called electronic brachytherapy (EBT) has been developed, which uses a miniature X-ray source to generate low-energy radiation. A retrospective study of adverse events and clinical outcomes in patients treated with EBT to the vaginal cuff, either as monotherapy or in combination with external beam radiation therapy (EBRT), was conducted.Methods and MaterialsMedical records were reviewed from 16 patients treated with postoperative EBT for endometrial (n = 13) or cervical cancer (n = 3) between February 2009 and November 2010. Patients received either intracavitary vaginal EBT alone or EBT in combination with EBRT. The radiobiologic effectiveness of EBT was assumed to be one.ResultsMedian follow-up was 20.5 months (range, 7–36 months). When EBT was used alone (n = 5), the median dose per fraction, number of fractions, and total dose delivered were: 6 Gy (range, 5.5–6.2 Gy), 5 fractions (range, 5–6), and 30 Gy (range, 30–34 Gy), respectively. When EBT was combined with EBRT, the EBT component median dose per fraction, number of fractions, and total dose delivered were: 5 Gy (range, 4.5–7 Gy), 2 fractions (range, 2–4), and 14 Gy (range, 9–20 Gy), respectively. The median EBRT dose was 45 Gy (range, 45–49.2 Gy). Our local control rate, locoregional (pelvic) control rate, and overall survival rate were 94%, 94%, and 88%, respectively. Of the 16 patients, 4 patients reported Grade 2 or greater toxicity (25%); however, there were no Grade 4–5 adverse events. Gynecologic, genitourinary, and gastrointestinal adverse events accounted for 57% (n = 4), 43% (n = 3), and 0% (n = 0) of all Grade 2 or greater side effects. No Grade 2 or higher toxicities were noted in patients treated with EBT alone.ConclusionEBT is an acceptable means of delivering postoperative vaginal brachytherapy and appears comparable with other methods; as the sole method of treatment, the toxicity rates of EBT are low.     

Spectrally average conversion coefficients for air kerma to ambient dose equivalent for clinical linear accelerator

This work aims to calculate the conversion coefficients from air kerma to ambient dose equivalent, H*(10)/K_air for photon beams produced by linear accelerators, such as the Clinac-4, Clinac-6, Clinac-18 and Clinac-2500, after transmission through primary barriers of radiotherapy treatment rooms. Concrete walls of thickness 1.0, 1.5 and 2.0 m were irradiated with 30 cm×30 cm primary beam spectra. The transmitted spectra were calculated to obtain the conversion coefficients for beams found in radiotherapy services. The calculations were done using the MCNP-4B Monte Carlo code. The results indicate the need to use a factor of about 1.20 to obtain the ambient dose equivalent for radiation surveys near primary barriers using instruments calibrated in air kerma.     

Evaluation of the incident directional dependence of radiochromic film by use of Monte Carlo simulation and measurement

In high-precision radiotherapy, absolute and relative doses are evaluated for patient-specific intensity-modulated radiation therapy (IMRT) quality assurance (QA). In our institution, we use GAFCHROMIC EBT3 (EBT3) for relative dose evaluation in IMRT QA. We usually use two directional film configurations, which are in the axial and sagittal planes. The QA in our institution shows some differences between the gamma pass rates in the axial and sagittal directions. The purpose of this study was to evaluate the incident directional dependence of EBT3 by using the percent depth dose (PDD) and the off-center ratio (OCR) between EBT3 films positioned perpendicular to the beam axis and along the beam axis. Furthermore, we compared the PDD in EBT3 films positioned perpendicular to the beam axis and the PDD by using an ionization chamber. In addition, PDDs in water phantoms with and without EBT3 films were calculated by Monte Carlo simulation. The results showed that the PDD in EBT3 films positioned perpendicular to the beam axis increased with the depth from the phantom surface. Monte Carlo simulation showed the same trend as did the film measurements. The OCR results were slightly different at dose levels below 20 %. The OCR in EBT3 films positioned along the beam axis was higher than that perpendicular to the beam axis. Thus, we conclude that EBT3 film has incident directional dependence. In IMRT QA, the gamma analysis results may be affected by the incident directional dependence of EBT3 film.     

Study of the IsoAid ADVANTAGE™ 125I brachytherapy source dosimetric parameters using Monte Carlo simulation

Monte Carlo calculations have been performed using the MCNP4C code for an iodine seed design. As the ADVANTAGE? I-125, Model IAI-125 source is commercially available for interstitial brachytherapy treatment, dosimetric characteristics (dose rate constant, radial dose function and anisotropy function) of this source were theoretically determined following the updated AAPM task group 43 (TG-43U1) recommendations. The dose distribution around the seed was calculated with Monte Carlo simulation in liquid water. The Monte Carlo calculated dose rate constant of this source in water was found to be 0.986 cGy h^?1 U^?1, with an approximate uncertainty of 0.4%. The obtained result has been compared with the previous study. Comparison of the calculated dose rate constant with the value presented by Meigooni et al. shows a very good agreement. Also the anisotropy function and the radial dose function for this source are graphically compared.     

A comparison of small-field tissue phantom ratio data generation methods for an Elekta Agility 6 MV photon beam

Tissue-phantom ratios (TPRs) are a common dosimetric quantity used to describe the change in dose with depth in tissue. These can be challenging and time consuming to measure. The conversion of percentage depth dose (PDD) data using standard formulae is widely employed as an alternative method in generating TPR. However, the applicability of these formulae for small fields has been questioned in the literature. Functional representation has also been proposed for small-field TPR production. This article compares measured TPR data for small 6 MV photon fields against that generated by conversion of PDD using standard formulae to assess the efficacy of the conversion data. By functionally fitting the measured TPR data for square fields greater than 4 cm in length, the TPR curves for smaller fields are generated and compared with measurements. TPRs and PDDs were measured in a water tank for a range of square field sizes. The PDDs were converted to TPRs using standard formulae. TPRs for fields of 4 × 4 cm² and larger were used to create functional fits. The parameterization coefficients were used to construct extrapolated TPR curves for 1 × 1 cm², 2 × 2-cm², and 3 × 3-cm² fields. The TPR data generated using standard formulae were in excellent agreement with direct TPR measurements. The TPR data for 1 × 1-cm², 2 × 2-cm², and 3 × 3-cm² fields created by extrapolation of the larger field functional fits gave inaccurate initial results. The corresponding mean differences for the 3 fields were 4.0%, 2.0%, and 0.9%. Generation of TPR data using a standard PDD-conversion methodology has been shown to give good agreement with our directly measured data for small fields. However, extrapolation of TPR data using the functional fit to fields of 4 × 4 cm² or larger resulted in generation of TPR curves that did not compare well with the measured data.     

Study of self-absorption for the determination of gross alpha and beta activities in water and soil samples

Source thickness is important in determinations of gross alpha and beta activities in soil or water samples because of self-absorption and energy losses. Its influence was studied theoretically, experimentally, and with Monte Carlo simulations. No problems were found for gross beta activity determinations using sources with mass thicknesses up to 25 mg/cm². However, to simultaneously determine gross alpha activity, the results appear to indicate that about 10 mg/cm² should be the upper limit to avoid problems.     

Design and construction of a thermal neutron beam for BNCT at Tehran Research Reactor

An irradiation facility has been designed and constructed at Tehran Research Reactor (TRR) for the treatment of shallow tumors using Boron Neutron Capture Therapy (BNCT). TRR has a thermal column which is about 3 m in length with a wide square cross section of 1.2×1.2 m². This facility is filled with removable graphite blocks. The aim of this work is to perform the necessary modifications in the thermal column structure to meet thermal BNCT beam criteria recommended by International Atomic Energy Agency. The main modifications consist of rearranging graphite blocks and reducing the gamma dose rate at the beam exit. Activation foils and TLD700 dosimeter have been used to measure in-air characteristics of the neutron beam. According to the measurements, a thermal flux is 5.6×10⁸ (n cm⁻² s⁻¹), a cadmium ratio is 186 for gold foils and a gamma dose rate is 0.57 Gy h⁻¹.     

Thermoluminescent chip detector for in vivo dosimetry in pelvis and head & neck cancer treatment

Our aim is to show the TL dosimetry as a confident QA method for radiotherapy treatments. Before in vivo entrance dose measurements using TLD-100 chips, ECLIPSE TPS-simulated treatments for a Rando anthropomorphic phantom, two for pelvis and one head & neck. In Vivo measurements results with ⁶⁰Co beam remained within ±5% limits. Results for 6 and 15 MV are in conclusion. This is a National Cancer Institute/RJ/Brazil study under the 13.111—IAEA Coordinated Research Project.     

Effective dose evaluation for BNCT brain tumor treatment based on voxel phantoms

For BNCT treatments, in addition to tumor target doses, non-negligible doses will result in all the remaining organs of the body. This work aims to evaluate the effective dose as well as the average absorbed doses of each of organs of patients with brain tumor treated in the BNCT epithermal neutron beam at THOR. The effective doses were evaluated according to the definitions of ICRP Publications 60 and 103 for the reference male and female computational phantoms developed in ICRP Publication 110 by using the MCNP5 Monte Carlo code with the THOR-Y09 beam source. The effective dose acquired in this work was compared with the results of our previous work calculated for an adult hermaphrodite mathematical phantom. It was found that the effective dose for the female voxel phantom is larger than that for the male voxel phantom by a factor of 1.2–1.5 and the effective dose for the voxel phantom is larger than that for the mathematical phantom by a factor of 1.3–1.6. For a typical brain tumor BNCT, the effective dose was calculated to be 1.51 Sv and the average absorbed dose for eye lenses was 1.07 Gy.     

Primary standardization of 57Co

This work describes the method developed by the Nuclear Metrology Laboratory in IPEN, São Paulo, Brazil, for the standardization of a ⁵⁷Co radioactive solution. Cobalt-57 is a radionuclide used for calibrating gamma-ray and X-ray spectrometers, as well as a gamma reference source for dose calibrators used in nuclear medicine services. Two 4πβ–γ coincidence systems were used to perform the standardization, the first used a 4π(PC) counter coupled to a pair of 76 mm×76 mm NaI(Tl) scintillators for detecting gamma-rays, the other one used a HPGe spectrometer for gamma detection. The measurements were performed by selecting a gamma-ray window comprising the (122 keV+136 keV) total absorption energy peaks in the NaI(Tl) and selecting the total absorption peak of 122 keV in the germanium detector. The electronic system used the TAC method developed at LMN for registering the observed events. The methodology recently developed by the LMN for simulating all detection processes in a 4πβ–γ coincidence system, by means of the Monte Carlo technique, was applied and the behavior of extrapolation curve compared to experimental data. The final activity obtained by the Monte Carlo calculation agrees with the experimental results within the experimental uncertainty.     

Local control in advanced cancer of the nasopharynx: Is a boost dose by endocavitary brachytherapy of prognostic significance?

PurposeTo analyze whether local tumor control in advanced nasopharyngeal cancer (NPC) can be optimized by boosting the primary dose by endocavitary brachytherapy (EBT).Methods and MaterialsTo study the role of EBT, three data sets on NPC, that is, the “Vienna”, “Rotterdam,” and “Amsterdam” series, with a total number of 411 advanced NPC patients, were available. The Rotterdam series consisted of 72 patients (34 T1,2N+ and 38 T3,4N0,+) and were treated with neoadjuvant chemotherapy followed by external beam radiotherapy (dose 70/2 Gy). After 70/2 Gy, a boost was applied by EBT (in case of T1,2N+) or stereotactic radiation (in case of T3,4 tumors). The Amsterdam (Antoni van Leeuwenhoek Hospital/The Netherlands Cancer Institute) series consisted of 76 patients (40 T1,2N+ and 36 T3,4N0,+) and were irradiated to a dose of 70/2 Gy with concomitant chemotherapy. No second boost by EBT was applied.ResultsIn the case of T1,2N+ tumors, the local relapse rate (LRR) was significantly smaller if a boost was applied, that is, 0% (0/34, EBT boost) vs. 14% (14/102, no EBT boost) (p = 0.023). For the T3,4 tumors, an LRR of 10% (4/38, EBT or stereotactic radiation boost) vs. 15% (17/111, no boost) was found (p = 0.463).ConclusionsIn the case of advanced NPC (T1,2N+ vs. T3,4N+,0), for early T-stages (T1,2N+), an EBT boost seems an excellent way to deliver highly conformal high doses of radiation to the nasopharynx, with high local control rates. For advanced T-stages (T3,4N+,0), the reduction in LRR (10% vs. 15%) was not significant (p = 0.463).     

Dose characterization of the rad sourceTM 2400 X-ray irradiator for oyster pasteurization

The RS 2400's cylindrical X-ray source yields dose rates high enough to allow the irradiator to replace widely used gamma irradiators. Except for the leftmost 5 cm, beam uniformity is within 10% at the tube surface. At maximum operating parameters, the beam has HVL₁=13.66 mm aluminum, HC=0.47, and hν_eq=88.5 keV. Maximum dose rates to tissue are 65 Gy min⁻¹±3.1% at tube surface, 37 Gy min⁻¹±3.1% at center of canisters, 14.1 Gy min⁻¹±6.5% for thin-shelled oysters, and 12.3 Gy min⁻¹±6.2% for thick-shelled oysters.     

Volumetric-modulated arc radiotherapy for pancreatic malignancies: Dosimetric comparison with sliding-window intensity-modulated radiotherapy and 3-dimensional conformal radiotherapy

Volumetric-modulated arc radiotherapy (VMAT) is an iteration of intensity-modulated radiotherapy (IMRT), both of which deliver highly conformal dose distributions. Studies have shown the superiority of VMAT and IMRT in comparison with 3-dimensional conformal radiotherapy (3D-CRT) in planning target volume (PTV) coverage and organs-at-risk (OARs) sparing. This is the first study examining the benefits of VMAT in pancreatic cancer for doses more than 55.8 Gy. A planning study comparing 3D-CRT, IMRT, and VMAT was performed in 20 patients with pancreatic cancer. Treatments were planned for a 25-fraction delivery of 45 Gy to a large field followed by a reduced-volume 8-fraction external beam boost to 59.4 Gy in total. OARs and PTV doses, conformality index (CI) deviations from 1.0, monitor units (MUs) delivered, and isodose volumes were compared. IMRT and VMAT CI deviations from 1.0 for the large-field and the boost plans were equivalent (large field: 0.032 and 0.046, respectively; boost: 0.042 and 0.037, respectively; p > 0.05 for all comparisons). Both IMRT and VMAT CI deviations from 1.0 were statistically superior to 3D-CRT (large field: 0.217, boost: 0.177; p < 0.05 for all comparisons). VMAT showed reduction of the mean dose to the boost PTV (VMAT: 61.4 Gy, IMRT: 62.4 Gy, and 3D-CRT: 62.3 Gy; p < 0.05). The mean number of MUs per fraction was significantly lower for VMAT for both the large-field and the boost plans. VMAT delivery time was less than 3 minutes compared with 8 minutes for IMRT. Although no statistically significant dose reduction to the OARs was identified when comparing VMAT with IMRT, VMAT showed a reduction in the volumes of the 100% isodose line for the large-field plans. Dose escalation to 59.4 Gy in pancreatic cancer is dosimetrically feasible with shorter treatment times, fewer MUs delivered, and comparable CIs for VMAT when compared with IMRT.