Dose Buildup Research Articles

Computation of Neutron Dose Buildup Factors for Ordinary Concrete in the Fast Range of Energy: A Comparative Study

In this work, we calculated neutron dose buildup factor at energy interval 80–120 MeV for the calcareous type of ordinary concrete (i.e., one made of sand and aggregate composed basically of limestone) using the data from the literature and an improved empirical fitting formula for estimating neutron buildup factors. We could estimate the removal macroscopic cross section of neutron (Σ R ) in this range of energy (80–120 MeV) at various mass thicknesses (0 < X < 550) for this type of concrete. The results are compared with the Monte Carlo method.

Pair production contribution effect on the calculation of gamma ray dose buildup factor for water and graphite in the energy range (4–10) MeV

Gamma ray buildup factor for two shielding materials, water (Zeff = 7.42) and graphite (Z = 6) for isotropic plane source normally incident within the energy range 4–10 MeV and up to 5 thickness mean free path has been studied. This research has been achieved by writing a simulation computer program in (Visual Basic language version G) and applied depending on Monte Carlo simulation method. The contribution of annihilation effect on the calculation of gamma ray dose buildup factor has been studied for both graphite and water. The results indicated that the calculated values of dose buildup for plane source, when ignoring the contribution of pair production, is less in comparison with the case when the contribution of pair production is taken into consideration. However, the contribution of Compton effect is still more predominant than the effect of annihilation within the range of energy (4–10) MeV.

Goals and Motivations of Families Pursuing Oral Immunotherapy for Food Allergy

We understand little about motivations of families who choose oral immunotherapy (OIT), particularly outside of the research setting. Understanding these goals may guide development of future therapies. Parents of children treated with OIT were recruited for a web survey using social media and food allergy advocacy list serves. 123 parents of children aged 1-19 years participated. 33% were in OIT maintenance, 34% in dose build-up, 22% had finished maintenance, and 11% discontinued OIT prematurely. The majority of subjects received non-research based OIT (65%). Sixty-five percent agreed with a definition of success as “avoiding the food but having a lower rate of reaction than prior to treatment,” while only 19% agreed that success was “eating the allergenic food but having a higher rate of reaction.” Asked about their primary goal for starting OIT, 75% of respondents chose “Reducing the risk of a fatal food reaction,” 12% “Reducing the hassle of strict avoidance,” and 11% “Being able to incorporate the food into the diet normally.” A significant minority thought there was a high likelihood that their child would have a fatal reaction prior to the initiation of OIT: 28% thought the risk was at least 1 in 100, 18% 1 in 1000, and 54% 1 in 10,000 or less. Families who pursue OIT are driven strongly by anxiety about fatal reactions and define success as lowering the rate of allergic reactions to a food. It appears that some families may be motivated by inflated assessment of the danger of untreated food allergy.

Dosimetry Investigation and Evaluation for Removing Flattening Filter Configuration of Linac: Monte Carlo Study

The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible and then to investigate the removing of flattening filter impact on dosimetry for a high radiotherapy efficiency. Monte Carlo codes used in this work were BEAMnrc code to simulate photons beam and DOSXYZnrc code to examinate absorbed dose in water phantom. PDDs and beam profiles were calculated for 6 × 6 cm2 and 10 × 10 cm2 field sizes. Good agreement was found between calculated PDD and beam profile compared to measurements. Gamma index acceptance rate was more than 98% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc, DOSXYZnrc and BEAMDP codes package. Varian Clinac 2100 with removing flattening filer could increase the dose by a gain was approximately 80% for 6 × 6 cm2 field size and it was approximately 110% for 10 × 10 cm2 at the build-up dose region but for all depth in water phantom, the dose of without FF configuration of linac was increased by more than 40% of dose of with FF configuration of linac.

Abstract ID: 198 Monte Carlo simulation and simple model of dose distribution in synchrotron radiation rotational radiotherapy of breast cancer: An experimental phantom study

External beam rotational radiotherapy of breast cancer with kilovoltage photons (kV-EBRT), with the patient in prone position, has been proposed [1] as a possible alternative to conventional radiotherapy with two tangential 6-MV X-ray beams produced by a medical linac with a supine patient. kV-EBRT uses a dedicated setup initially developed for breast CT, with a 320-kVp beam from an orthovoltage X-ray tube rotating in full circles around the breast; CT for tumour localization can be performed by using the same setup. Though dose build-up is not present, rotational summation of dose delivery allows a skin sparing comparable to conventional radiotherapy. We proposed the use of a monoenergetic synchrotron radiation (SR) collimated beam for image-guided rotational radiotherapy for the pendant breast (SR3T) [2] . The high flux of SR permits dose rates even greater to that of conventional radiotherapy, while the optimal photon energy can be selected for the treatment. In this work, we present Monte Carlo simulation and a simple model for evaluation of the 3D dose distribution in SR3T. The MC code was based on GEANT4 toolkit ver.10.00. We validated these simulations via measurements on breast phantoms performed at the Imaging and Medical Beamline of the Australian Synchrotron. We measured the dose distribution in cylindrical PMMA and polyethylene phantoms at 60 keV, using an ionization chamber, thermoluminescent dosimeters and radiochromic films. This study indicated that for a tumour localized at the centre of a mid-sized breast (14-cm diameter at chest wall), a ”skin-to-tumour” dose ratio of 14% can be reached at 60 keV with a collimated SR beam. Non uniform dose distribution to the target (dose painting) can be performed with multiple rotations around the organ. Kilovoltage SR3T with suitable radiosensitizers may exploit the dose enhancement factor due to increased photoelectric absorption, at SR beam energies in the 60–110 keV range.

Comparison of the dosimetric response of 4-elements OSL and TL passive personal dosimeters

IntroductionThermoluminescent dosimeters (TLD) made from LiF:Mg,Ti (manufacturer: Thermo Fisher Scientific, USA, brand name: TLD-100) and optically stimulated luminescence detectors (OSLD) made from BeO (manufacturer: Dosimetrics GmbH, Germany, brand name: BeOSL) are used as passive personal detectors. Although differences exist between them, mainly in terms of handling, reading, material response and dose algorithms, both systems have to satisfy some specific requirements. The aim of this study is to verify that the 4-element version of both dosimeters match the IEC 62387 requirements for photon fields, and to compare them. Materials and methodsThe LiF-TLDs tested were those provided by the Spanish National Dosimetry Centre (CND), which makes use of an in-house dose calculation algorithm. The BeO-OSLDs were those distributed by Dosimetrics. The following features were evaluated for photon radiation: coefficient of variation, non-linearity, photon angular and energy dependence, reusability, dose build-up, fading, self-irradiation and response to natural radiation, as well as accuracy of energy estimation. For the BeO-OSLDs the consistency between consecutive re-readings was also determined. ResultsBoth systems satisfy the IEC 62387 requirements for the tests described above. One of the main advantages of the BeO-OSLDs over the LiF-TLDs is their lower energy dependency and higher reusability. They are also easier to manipulate, read and prepare. On the other hand, advantages of the LiF-TLD system over the BeO-OSLD system are the more accurate energy estimations provided by the CND dose algorithm, which are used to correct the higher energy dependency, and the glow curves provided by the TL readers. ConclusionThe LiF-TLDs and BeO-OSLDs tested satisfy the dosimetric requirements for their use as passive personal dosimeters in photon fields. For this reason, a study of the simplicity, reusability, amount of dosimeters read each month or automation in the handling and reading process can be helpful to decide the best option. The choice between one of them from a dosimetric point of view might also depend on the importance given to the different criteria here studied.

Dosimetric effects of air cavity on target volume and organs at risk during intensity-modulated radiation therapy for nasopharyngeal carcinoma

Objective To evaluate the dosimetric effects of air cavity on primary tumor and organs at risk (OARs) during intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma patients. Methods Nine patients with nasopharyngeal carcinoma had CT simulation before treatment and on the 25 fraction of radiotherapy. Radiotherapy plan1 was first created by delineating the target volumes and OARs on the first CT image, which was then copied and merged with the second CT image. Air cavity that was formed following tumor volume regression was delineated on the first CT image, and the density of air cavity was set to zero. A new gross target volume (GTV) was formed by subtracting the air cavity from the original GTV, and the new CT image was named CTAir. Plan2 was then created by calculating the dose distribution on CTAir using the same portals and parameters as Plan1. Assuming that Plan1 and Plan2 were both used throughout the course of radiotherapy, the dosimetric parameters of nasopharyngeal tumor and OARs in the absence and presence of air cavity were then compared using the paired t-test. Results Dmean, D95, D90, D10, and D5 of tumor were significantly higher in plan2 than in plan1(P=0.000, 0.001, 0.001, 0.001, and 0.005, respectively), with a<0.5 Gy increase in dose. A dose build-up effect was observed within the tumor region posterior to the air cavity, where the highest dose was 1.4 cm posterior to the air cavity, resulting in a dose difference of 0.36 Gy. In addition, D2cc and D1cc of the brain stem were significantly higher in plan2 than in plan1(P=0.036 and 0.044, respectively). Dmax of the optical chiasm, left optical nerve, and right optical nerve were also increased in the presence of air cavity (P=0.438, 0.434, and 0.477, respectively), but the change in dose was<0.12 Gy. Conclusions Air cavity induces a small but negligible increase in the tumor and OARs dose in patients with nasopharyngeal carcinoma during IMRT. However, closer monitoring should be conducted for patients with OARs that is close to or has surpassed tolerance prior to radiotherapy. Key words: Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Air cavity effects; Organs at risk

The accuracy of treatment planning system dose modelling in the presence of brass mesh bolus.

This work assesses the dosimetric accuracy of three commercial treatment planning system (TPS) photon dose calculation algorithms in the presence of brass mesh used as a bolus. Bolus material is used in radiotherapy to provide dose build-up where superficial tissues require irradiation. They are generally water equivalent but high density materials can also be used. Dose calculations were performed on Monaco and Masterplan TPS (Elekta AB, Sweden) using phantoms defined by the three DICOM CT image sets of water equivalent blocks (no bolus, 1 layer and 2 layers of brass mesh) exported from the CT scanner. The effect of the mesh on monitor units, build-up dose, phantom exit dose and beam penumbra were compared to measured data. Dose calculations for 6 and 15MV photon beams on plain water equivalent phantoms were seen to agree well with measurement validating the basic planning system algorithms and models. Dose in the build-up region, phantom exit dose and beam penumbra were poorly modelled in the presence of the brass mesh. The beam attenuation created by the bolus material was overestimated by all three calculation algorithms, at both photon energies, e.g. 1.6% for one layer and up to 3.1% for two layers at 6MV. The poor modelling of the physical situation in the build-up region is in part a consequence of the high HU artefact caused by the mesh in the CT image. CT imaging is not recommended with the brass mesh bolus in situ due to the poor accuracy of the subsequent TPS modelling.

Experimental verification of EGSnrc Monte Carlo calculated depth doses within a realistic parallel magnetic field in a polystyrene phantom.

Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth doses (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field. Two cylindrical electromagnets were used to produce a 0.207T magnetic field parallel to the central axis of a 6MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth dose in polystyrene was simulated using DOSXYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros. The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2mm γ acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to be approximately 10% (relative to no field Dmax ) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (>0.13T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these dose increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison. Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately calculate the PDDs with these surface and buildup dose modifications in a homogeneous phantom.

B-FAHF-2 plus oral immunotherapy (OIT) is safer and more effective than OIT alone in a murine model of concurrent peanut/tree nut allergy.

Concurrent sensitization to peanut (PN) and tree nuts (TN), the most dangerous food allergies, is common. Current oral immunotherapy (OIT) is not fully satisfactory. To determine whether the herbal formula B-FAHF-2 (BF2) ameliorates PN/TN OIT adverse reactions and enhances persistence of a tolerant state. Concurrently sensitized PN-, walnut- (WN) and cashew (CSH)-allergic mice received 1-day PN/WN/CSH rush OIT plus 3weeks of maintenance dosing, with or without 3weeks prior and 3weeks BF2 co-treatment. Anaphylactic symptom scores, core body temperatures, plasma histamine levels, basophil numbers, antigen-specific IgE, cytokine levels, and IL-4, INF-γ and Foxp3 gene promoter DNA methylation status, and their correlation with final challenge symptom scores were determined. BF2+OIT-treated mice experienced significantly fewer and less severe adverse reactions than OIT-only-treated mice (P<.01) during the 1-day rush OIT build-up dose phase. Both OIT-only and BF2+OIT mice showed significant desensitization (P<.01 and .001, respectively) at 1week post-therapy challenge, being greater in BF2+OIT mice. All sham-treated and 91% of OIT-treated mice experienced anaphylaxis whereas only 21% of BF2+OIT-treated mice exhibited reactions during 5-6weeks of dose escalation single PN and TN challenges. Greater and more persistent protection in BF2+OIT mice was associated with significantly lower plasma histamine and IgE levels, increased IFN-γ/IL-4 and IL-10/IL-4 ratios, DNA remethylation at the IL-4 promoter and demethylation at IFN-γ and Foxp3 promoters. Final challenge symptom scores were inversely correlated with IL-4 DNA methylation levels (P<.0002) and positively correlated with IFN-γ and Foxp3 gene promoter methylation levels (P<.0011) (P<.0165). Combined BF2/OIT therapy was safer and produced longer post-treatment protection and more tolerance-prone immunological and epigenetic modifications than OIT alone. BF2/OIT may provide an additional OIT option for patients with concurrent PN/TN and other food allergies.

Comparison of surface dose delivered by 7MV-unflattened and 6MV-flattened photon beams.

The purpose of this study is to determine the central-axis dose in the buildup region and the surface dose delivered by a 6MV flattened photon beam (6MV-FB) and a higher energy unflattened (7MV-FFF) therapeutic photon beam for different-sized square fields with open fields and modifying filters. The beams are produced by a Siemens Artiste linear accelerator with a NACP-02 ionization chamber and the dose is measured by using GafChromic film and two different, commonly used, dosimeters: a p-type photon semiconductor dosimeter (PFD) and a cylindrical ionization chamber (CC13). The results indicate that the surface dose increases linearly with FS for both open and wedged fields for the 6MV-FB and 7MV-FFF beams. The surface dose delivered by the 7MV-UFB beam is consistent with that delivered by the 6MV-FB beam for field sizes up to 10cm×10cm, after which the surface dose decreases. The buildup dose for the 7MV-UFB beam is slightly less than that for the 6MV-FB beam for field sizes ranging from 5cm×5cm to 15cm×15cm. For both the 6MV-FB and 7MV-FFF beams, the measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement. The surface dose measured with the PFD dosimeter and the NACP-02 and CC13 chambers differ significantly from the results obtained when using GafChromic film. The 7MV-FFF beam results in a slightly smaller surface dose in the buildup region compared with the 6MV-FB beam. The surface dose delivered by the higher energy 7MV-FFF beam is less than that delivered by the energy-unmatched FFF beam in previously published works.

46. Real-time MR image guided radiotherapy: The time is near!

Introduction Despite the fact that several studies has shown that dose escalation improves treatment outcome, there are currently very limited clinical indications treated with high dose-high precision radiotherapy. Due to the inability to visualize tumor and critical organs sufficiently during delivery, a normal tissue margin around the tumor needs to be included in the target volume, which leads to toxicity and thereby limiting the maximum dose. If high quality (contrast) real-time MRI images of the tumor and surroundings are available during treatment, it is expected that smaller normal tissue margins are necessary (what you see is what you treat), and consequently increasing the possibility for dose escalation. Methods MR guided radiotherapy is rapidly evolving from research feasibilities towards real clinical treatment machines. Cobalt systems are currently on the market and soon the first clinical MR-linac based on a 7 MV linear accelerator and a 1.5T MRI scanner will become operational. It is a misconception to consider the MR-guided radiotherapy systems as the next generation CBCT-linacs. The combination of a radiotherapy device with an MRI scanner enables high dose-high precision radiotherapy for many complex indications, bringing a new paradigm for radiotherapy. This enables new possibilities of on-line treatment plan adaptation, including gating and tumor tracking. However, the implications on reference dosimetry, machine QA and treatment planning due to the changed dose deposition by the magnetic field must be taken into account. This opens a whole new field of radiotherapy physics. In this talk the different MR radiotherapy solutions currently clinically available or under development will be discussed. Furthermore, a short description of the clinical MR guided workflow is given. The impact of the magnetic field on dose deposition is large due to the Lorentz force on the electrons released in matter. This results in skewed dose distributions and different build-up and exit doses compared to distributions in conventional radiotherapy. Since the trajectories of the electrons are now curved, readings in ionization chambers will also differ. My talk will address these effects on dose based on the Utrecht experience but these implications also apply to the other MR radiotherapy solutions. Conclusions The use of MRI in radiotherapy is fast growing. To explore the patient benefits of MR guided radiotherapy almost all conventional aspects of current radiotherapy needs an overhaul, from the clinical workflow to dosimetry physics and machine QA. A short overview will be given.

A feasibility study of 2-mm bolus for postmastectomy radiation therapy

PurposeThe purpose of this study was to prospectively evaluate the use of daily 2-mm bolus in patients undergoing postmastectomy radiation without reconstruction using optically stimulated luminescence dosimetry and weekly assessment of skin toxicity. Methods and materialsWe prospectively collected data from the first 49 women treated with a daily 2-mm Superflab bolus during their postmastectomy radiation therapy from 2013 to 2016 at The University of Chicago Comprehensive Cancer Center at Silver Cross. Within the first 3 days of starting radiation therapy, we measured the surface dose in vivo at 5 anatomical locations under the 2-mm bolus on the chest wall. We assessed weekly the acute skin toxicity during radiation using the National Cancer Institute Common Toxicity Criteria. Patients with reconstruction before radiation therapy were excluded. ResultsForty-nine women with a mean age of 54.3 years were treated with daily 2-mm bolus to the chest wall following mastectomy. Median follow-up was 32.7 weeks. The mean percentages of prescribed dose (standard deviation) for the median, central, lateral, superior, and inferior optically stimulated luminescence dosimeters were 100.1% (5.6%), 108.1% (6.7%), 98.1% (6.5%), 102.6% (8.9%), and 106.3% (6.6%), respectively. The majority (71.4%) of women experienced a maximum acute National Cancer Institute Common Toxicity Criteria skin toxicity score of 2, with only 12.2% experiencing a score of 3. There were no grade 4 toxicities. There were no local recurrences during our follow-up period. ConclusionsA daily 2-mm bolus is a feasible regimen for chest wall bolus during postmastectomy radiation therapy with acceptable dose buildup and skin toxicity.

Influence of standard RF coil materials on surface and buildup dose from a 6 MV photon beam in magnetic field.

Magnetic resonance guided teletherapy systems aspire to image the patient concurrently with the radiation delivery. Thus, the radiofrequency (RF) coils used for magnetic resonance imaging, placed on or close to patient skin and in close proximity to the treatment volume, would be irradiated leading to modifications of radiation dose to the skin and in the buildup region. The purpose of this work is to measure and assess these dose modifications due to standard off-the-shelf RF coil materials. A typical surface coil was approximated as layered sheets of polycarbonate, copper tape, and Teflon to emulate the base, conductor, and cover, respectively. A separate investigation used additional coil materials, such as copper pipe, plastic coil housing, a typical coil padding material, and a thin copper conductor. The materials were placed in the path of a 6 MV photon beam at various distances from polystyrene phantoms in which the surface and buildup doses were measured. The experiments were performed on a clinical Varian linac with no magnetic field and with a 0.21 T electromagnet producing a magnetic field parallel to the beam central axis. The authors repeated similar experiments in the presence of a 0.22 T magnetic field oriented perpendicular to the beam central axis using an earlier linac-MR prototype, with a biplanar permanent magnet. The radiation detectors used for the measurements were two different parallel plate ion chambers and GAFChromic films. A typical open beam surface dose of 20% (relative to open beam Dmax) was increased to 63% by the coil padding material and to >74% by all other materials when placed in direct contact with the phantom, irrespective of magnetic field presence or orientation. Without a magnetic field, the surface dose decreased as the test materials were moved away from the phantom surface toward the radiation source, reaching between 30% and 40% at 10 cm gap between the phantom and the test materials. In the presence of the transverse magnetic field, the surface dose reduction was more rapid reaching a dose level of 30%-40% with only 3-4 cm gap. In the presence of the parallel magnetic field, as expected, the surface dose did not decrease considerably as the gap between the phantom surface and test materials was increased; the surface dose remained >60% at 10 cm gap for all tested materials except for the thin copper conductor. As expected, placing coil materials in direct contact with the phantom surface increases the surface dose considerably. The surface dose is reduced by creating a gap between the coil materials and phantom surface. This dose reduction happens more rapidly in the presence of a transverse magnetic field. However, the surface dose stays relatively large irrespective of the gap in the presence of a parallel magnetic field. Thus, the standard, off-the-shelf RF coils should be used with caution in integrated linac-MR systems, especially those using a parallel magnetic field orientation in which case the RF coils will probably need to be reconfigured to create open ports for the radiation beam.

Evaluation of the dosimetric characteristics of 6 MV flattened and unflattened photon beam

This work aims to compare and evaluate the dosimetric properties of 6MV flattening (FF) and flattening filter free (FFF) photon beams which generated from a Varian 2100 medical accelerator. These properties include percentage depth dose, dose rate, beam profile, out-of-field, energy spectra, scatter factor, and surface dose. This study has been effected by using BEAMnrc and DOSXYZnrc user’s code based on EGSnrc Monte Carlo method.The results obtained showed that the unflattened beams have a dose rate of 2.46 times higher than the flattened beams that would reduce the treatment time. The out-of-field dose of FFF beams at 3.5cm from the field edge was less than the flattened one due to the reduction of head scatter. The scatter factor and penumbra dose of unflattened beam were found less than that of the flattened one for all field sizes. The unflattened beam has a higher surface dose and build-up dose compared to the flattened beams. Replacing the air column under the jaws by helium leads to the reduction of the surface dose ratio from 1.23 to 1.13, build-up doses and dmax restored as in flattened case. The results of this study have demonstrated that, unflattened beams are very useful for treating cancer cells and sparing the adjacent healthy tissue.

A METHOD TO IMPROVE DOSE ASSESSMENT BY RECONSTRUCTION OF THE COMPLETE ISOTOPES INVENTORY.

Radiation shielding assessments may underestimate the expected dose if some isotopes at trace level are not considered in the isotopes inventory of the shielded radioactive materials. Indeed, information about traces is not often available. Nevertheless, the activation of some minor isotopic traces may significantly contribute to the dose build-up. This paper presents a new method (Isotopes Inventory Reconstruction-IIR) estimating the concentration of the minor isotopes in the irradiated material at the beginning of the cooling period. The method requires the solution of the inverse problem describing the irradiated material's decay. In a mixture of an irradiated uranium-plutonium oxide shielded by a set-up made of stainless-steel, porous polyethylene plaster and lead methyl methacrylate, the comparison between different methods proves that the IIR-method allows better assessment of the dose than other approximate methods.

Translucent poly(vinyl alcohol) cryogel dosimeters for simultaneous dose buildup and monitoring during chest wall radiation therapy.

Chest wall radiation therapy treatment delivery was monitored using a 5 mm thick radiochromic poly(vinyl alcohol) cryogel that also provided buildup material. The cryogels were used to detect positioning errors and measure the impact of shifts for a chest wall treatment that was delivered to a RANDO phantom. The phantom was shifted by ±2,±3, and ±5 mm from the planned position in the anterior/posterior (A/P) direction; these shifts represent setup errors and the uncertainty associated with lung filling during breath‐hold. The two‐dimensional absolute dose distributions measured in the cryogel at the planned position were compared with the distributions at all shifts from this position using gamma analysis (3%/3 mm, 10% threshold). For shifts of ±2,±3, and ±5 mm the passing rates ranged from 94.3% to 95.6%, 74.0% to 78.8%, and 17.5% to 22.5%, respectively. These results are consistent with the same gamma analysis performed on dose planes calculated in the middle of the cryogel and on the phantom surface using our treatment planning system, which ranged from 94.3% to 95.0%, 76.8% to 77.9%, and 23.5% to 24.3%, respectively. The Pinnacle dose planes were then scaled empirically and compared to the cryogel measurements. Using the same gamma metric, the pass rates ranged from 97.0% to 98.4%. The results of this study suggest that cryogels may be used as both a buildup material and to evaluate errors in chest wall treatment positioning during deep‐inspiration breath‐hold delivery. The cryogels are sensitive to A/P chest wall shifts of less than 3 mm, which potentially allows for the detection of clinically relevant errors.PACS number(s): 87.55.km, 87.57.uq

SU‐F‐J‐146: Experimental Validation of 6 MV Photon PDD in Parallel Magnetic Field Calculated by EGSnrc

Purpose:Integrated linac‐MR systems are uniquely suited for real time tumor tracking during radiation treatment. Understanding the magnetic field dose effects and incorporating them in treatment planning is paramount for linac‐MR clinical implementation. We experimentally validated the EGSnrc dose calculations in the presence of a magnetic field parallel to the radiation beam travel.Methods:Two cylindrical bore electromagnets produced a 0.21 T magnetic field parallel to the central axis of a 6 MV photon beam. A parallel plate ion chamber was used to measure the PDD in a polystyrene phantom, placed inside the bore in two setups: phantom top surface coinciding with the magnet bore center (183 cm SSD), and with the magnet bore's top surface (170 cm SSD). We measured the field of the magnet at several points and included the exact dimensions of the coils to generate a 3D magnetic field map in a finite element model. BEAMnrc and DOSXYZnrc simulated the PDD experiments in parallel magnetic field (i.e. 3D magnetic field included) and with no magnetic field.Results:With the phantom surface at the top of the electromagnet, the surface dose increased by 10% (compared to no‐magnetic field), due to electrons being focused by the smaller fringe fields of the electromagnet. With the phantom surface at the bore center, the surface dose increased by 30% since extra 13 cm of air column was in relatively higher magnetic field (&gt;0.13T) in the magnet bore. EGSnrc Monte Carlo code correctly calculated the radiation dose with and without the magnetic field, and all points passed the 2%, 2 mm Gamma criterion when the ion chamber's entrance window and air cavity were included in the simulated phantom.Conclusion:A parallel magnetic field increases the surface and buildup dose during irradiation. The EGSnrc package can model these magnetic field dose effects accurately.Dr. Fallone is a co‐founder and CEO of MagnetTx Oncology Solutions (under discussions to license Alberta bi‐planar linac MR for commercialization).

A patient immobilization device for prone breast radiotherapy: Dosimetric effects and inclusion in the treatment planning system

PurposeTo assess the dosimetric impact of a patient positioning device for prone breast radiotherapy and assess the accuracy of a treatment planning system (TPS) in predicting this impact. MethodsBeam attenuation and build-up dose perturbations, quantified by ionization chamber and radiochromic film dosimetry, were evaluated for 3 components of the patient positioning device: the carbon fiber baseplate, the support cushions and the support wedge for the contralateral breast. Dose calculations were performed using the XVMC dose engine implemented in the Monaco TPS. All components were included during planning CT acquisition. ResultsBeam attenuation amounted to 7.57% (6MV) and 5.33% (15MV) for beams obliquely intersecting the couchtop–baseplate combination. Beams traversing large sections of the support wedge were attenuated by 12.28% (6MV) and 9.37% (15MV). For the support cushion foam, beam attenuation remained limited to 0.11% (6MV) and 0.08% (15MV) per centimeter thickness. A substantial loss of dose build-up was detected when irradiating through any of the investigated components. TPS dose calculations accurately predicted beam attenuation by the baseplate and support wedge. A manual density overwrite was needed to model attenuation by the support cushion foam. TPS dose calculations in build-up regions differed considerably from measurements for both open beams and beams traversing the device components. ConclusionsIrradiating through the components of the positioning device resulted in a considerable degradation of skin sparing. Inclusion of the device components in the treatment planning CT allowed to accurately model the most important attenuation effect, but failed to accurately predict build-up doses.

EP-1567: Investigation of dose buildup region from electron beam by of polymer films and ionization chamber

EP-1567: Investigation of dose buildup region from electron beam by of polymer films and ionization chamber