Electronic Brachytherapy Source Research Articles

Realization of the absorbed dose to water for 70 kV electronic brachytherapy sources for surface applications

Objective. The purpose of this work is to establish a primary standard for the absorbed dose to water for surface applications with 70 kV electronic brachytherapy sources and thereby to investigate several reference conditions with respect to applicability in metrology institutes, calibration labs and clinics. Approach. A primary standard for the absorbed dose to water for LDR-Seeds (ipFAC) was utilized. For this, the method of evaluation was modified to account for the different geometries in interstitial and surface brachytherapy and for the different energy distributions of the radiation fields, respectively. The correction factors required to determine the absorbed dose to water were evaluated using Monte Carlo (MC) simulations. MC-calculations were also used to estimate the uncertainties of this method. Main results. It could be shown that determining the absorbed dose to water in 1 cm depth below the surface of a water phantom D w (1 cm) is feasible with the ipFAC. Thereby the method to determine D w (1 cm) when the source is placed at 50 cm distance and the beam is collimated turns out to be more robust to variations in the directional emittance of the source than placing the source applicator assembly directly on the phantom. Significance. The first method features significantly lower uncertainties, 1.4% compared to 3.7% (both for k = 2), for the second one. However, a calibration based on a transfer chamber calibrated with the second method is more practical and easier to implement in clinical routine.

PO-2166 Commissioning of new rectal applicator using electronic brachytherapy source

PO-2166 Commissioning of new rectal applicator using electronic brachytherapy source

Accelerated brachytherapy with the Xoft electronic source used in association with iodine, gold, bismuth, gadolinium, and hafnium nano-radioenhancers

PurposeThe current study was designed to calculate the dose enhancement factor (DEF) of iodine (I), gold (Au), bismuth (Bi), gadolinium (Gd), and hafnium (Hf) nanoparticles (NP)s by Monte Carlo (MC) modeling of an electronic brachytherapy source in resection cavities of breast tumors. Methods and materialsThe GEANT4 MC code was used for simulation of a phantom containing a water-filled balloon and a Xoft source (50 kVp) to irradiate the margins of a resected breast tumor. NPs with a diameter of 20 nm and concentrations from 1 to 5% w/w were simulated in a tumor margin with 5 mm thickness as well as a hypothetical breast model consisting of spherical island-like residual tumor-remnants. The DEFs for all NPs were calculated in both models. ResultsIn the margin-loaded model, for the concentration of 1% w/w heavy atom, DEFs of 2.5, 2.3, 2.1, 2, and 1.7 were calculated for Bi, Au, I, Hf, and Gd NPs (descending order), which increased, almost linearly with concentration for all NPs. Moreover, normal tissue dose behind the NP-loaded margin declined significantly depending on NP type and concentration. When modeling residual tumor islands, DEF values were very close to the margin-loaded values except for Bi and I, where DEFs of 2.55 and 1.7 were seen, respectively. ConclusionsConsiderable dose enhancements were obtained for the heavy atom NPs studied in the partial breast brachytherapy with a Xoft electronic source. In addition, normal tissue doses were lowered in the points beyond the NP-loaded margin. The findings revealed promising outcomes and the probability of improved tumor control for NP-aided brachytherapy with the Xoft electronic source.

Toward 3D dose verification of an electronic brachytherapy source with a plastic scintillation detector.

BackgroundElectronic brachytherapy (eBT) is considered a safe treatment with good outcomes. However, eBT lacks standardized and independent dose verification, which could impede future use.PurposeTo validate the 3D dose‐to‐water distribution of an electronic brachytherapy (eBT) source using a small‐volume plastic scintillation detector (PSD).MethodsThe relative dose distribution of a Papillon 50 (P50) (Ariane Medical Systems, UK) eBT source was measured in water with a PSD consisting of a cylindrical scintillating BCF‐12 fiber (length: 0.5 mm, Ø: 1 mm) coupled to a photodetector via an optical fiber. The measurements were performed with the PSD mounted on a motorized stage in a water phantom (MP3) (PTW, Germany). This allowed the sensitive volume of the PSD to be moved to predetermined positions relative to the P50 applicator, which pointed vertically downward while just breaching the water surface. The percentage depth‐dose (PDD) was measured from 0 to 50 mm source‐to‐detector distance (SDD) in 1–3 mm steps. Dose profiles were measured along two perpendicular axes at five different SDDs with step sizes down to 0.5 mm. Characterization of the PSD consisted of determining the energy correction through Monte Carlo (MC) simulation and by measuring the stability and dose rate linearity using a well‐type ionization chamber as a reference. The measured PDD and profiles were validated with corresponding MC simulations.ResultsThe measured and simulated PDD curves agreed within 2% (except at 0 mm and 43 mm depth) after the PSD measurements were corrected for energy dependency. The absorbed dose decreased by a factor of 2 at 7 mm depth and by a factor of 10 at 26 mm depth. The measured dose profiles showed dose gradients at the profile edges of more than 50%/mm at 5 mm depth and 15%/mm at 50 mm depth. The measured profile widths increased 0.66 mm per 1 mm depth, while the simulated profile widths increased 0.74 mm per 1 mm depth. An azimuthal dependency of > 10% was observed in the dose at 10 mm distance from the beam center. The total uncertainty of the measured relative dose is < 2.5% with a positional uncertainty of 0.4 mm. The measurements for a full 3D dose characterization (PDD and profiles) can be carried out within 8 h, the limiting factor being cooling of the P50.ConclusionThe PSD and MP3 water phantoms provided a method to independently verify the relative 3D dose distribution in water of an eBT source.

Monte Carlo calculation of the TG-43 dosimetry parameters for the INTRABEAM source with spherical applicators

Objective: The relative TG-43 dosimetry parameters of the INTRABEAM (Carl Zeiss Meditec AG, Jena, Germany) bare probe were recently reported by Ayala Alvarez et al (2020 Phys. Med. Biol. 65 245041). The current study focuses on the dosimetry characterization of the INTRABEAM source with the eight available spherical applicators according to the TG-43 formalism using Monte Carlo (MC) simulations. Approach: This report includes the calculated dose-rate conversion coefficients that determine the absolute dose rate to water at a reference point of 10 mm from the applicator surface, based on calibration air-kerma rate measurements at 50 cm from the source on its transverse plane. Since the air-kerma rate measurements are not yet provided from a standards laboratory for the INTRABEAM, the values in the present study were calculated with MC. This approach is aligned with other works in the search for standardization of the dosimetry of electronic brachytherapy sources. As a validation of the MC model, depth dose calculations along the source axis were compared with calibration data from the source manufacturer. Main results: The calculated dose-rate conversion coefficients were 434.0 for the bare probe, and 683.5, 548.3, 449.9, 376.5, 251.0, 225.6, 202.8, and 182.6 for the source with applicators of increasing diameter from 15 to 50 mm, respectively. The radial dose and the 2D anisotropy functions of the TG-43 formalism were also obtained and tabulated in this document. Significance: This work presents the data required by a treatment planning system for the characterization of the INTRABEAM system in the context of intraoperative radiotherapy applications.

Long-term outcome after combined kyphoplasty and intraoperative radiotherapy (Kypho-IORT) for vertebral tumors

IntroductionThe spine represents the site which is most frequently affected by bone metastases in patients with systemic cancer. Of all local treatment options, combined kyphoplasty and intraoperative radiotherapy (Kypho-IORT) provides both, instantaneous stabilization and immediate pain relief. We here report on the long-term outcomes of the largest cohort treated with Kypho-IORT to date.MethodsBetween 2009 and 2019 a total of 104 patients underwent Kypho-IORT to vertebral tumors in the thoracic, lumbar, or sacral spine with transpedicular kyphoplasty and intraoperative irradiation with a needle-shaped electronic brachytherapy source at our center. Patients were treated either on trial, within the prospective Kypho-IORT studies (NCT01280032 and NCT02773966), or, after completion of the study, off trial but compliant with the study protocol. Follow-up and imaging with computed tomography (CT) or magnetic resonance imaging was scheduled after 3 and 6 months and then bi-annually.ResultsA total of 143 vertebrae (89 thoracic spine, 53 lumbar spine, and 1 sacral spine) were treated in 104 patients. The median follow-up was 14.5 months (range 0.4–109). Local progression occurred in 10 patients (10 vertebrae) after a median time of 22.3 months (range 1.5–73) resulting in local control rates of 97.1, 95.9, and 94.2% at 6, 12, and 24 months, respectively. Overall survival was 74.6, 61.7, and 50.3% at 6, 12, and 24 months, respectively. A single serious adverse event was reported.ConclusionIn addition to immediate pain reduction and stabilization, Kypho-IORT shows excellent long-term local control with minimal side effects.

Dosimetry formalism and calibration procedure for electronic brachytherapy sources in terms of absorbed dose to water

The LNE-LNHB has developed a methodology to standardize electronic brachytherapy sources in terms of absorbed dose to water. It is based on the measurement of the air-kerma rate at a given distance from the source and the Monte Carlo calculation of a conversion factor. This factor converts the air-kerma in measurement conditions into absorbed dose to water at a 1 cm reference depth in a water phantom. As a first application, the method was used to calibrate a Zeiss INTRABEAM system equipped with its 4 cm diameter spherical applicator. The absorbed-dose rate value obtained in the current study was found significantly higher than that provided by the manufacturer in line with the observations already reported by a few other teams.

On the stability of well-type ionization chamber source strength calibration coefficients.

To investigate the variability and stability of brachytherapy source strength calibration coefficients for air-communicating and pressurized well-type ionization chambers among high-dose rate (HDR), low-dose rate (LDR), and electronic brachytherapy (EBT) source qualities. These qualities of an ionization chamber are important features to assess given their role in maintaining traceability to a primary national standards laboratory and facilitating efficacious patient care in brachytherapy. The calibration records from the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UWADCL) customer well-type ionization chamber database were retrospectively analyzed for calibrations performed between 1996 and 2019. A statistical analysis was performed and differentiated among calibration type to quantify the distribution of chamber calibration coefficients among several chamber models. Distributions were quantified based on their moments and by quantile analysis. For LDR calibrations, chamber response was further differentiated by seed type to study the variability in seed dependence within and across chamber models. In addition to these metrics, the calibration lineage at the UWADCL was used to assess the stability of these calibration coefficients among chamber model and source type based on the ratio of subsequent calibration coefficients. The distribution of brachytherapy source strength calibration coefficients for a particular chamber model is not necessarily normally distributed and is sensitive to changes in the machining tolerances or design of the chamber model. Calibration source quality also influenced the distributions of calibration coefficients for a chamber model; the air-kerma rate calibration coefficients for EBT sources were the most variable followed by LDR and then HDR source types. The stability of a chamber's source strength calibration coefficient exhibited a similar dependence on the source quality. Air-communicating and pressurized chambers exhibited an average stability between subsequent calibrations of 0.2% and 3.0%, respectively, for HDR calibrations, but could exhibit more than double this variability characteristic of their leptokurtic distributions. For LDR calibrations, the spread and stability in a model's calibration ratio toward another seed type is extensive and notable. For some seed types and chamber models exhibit variations of <0.5% while others exceeded 2.0%. Furthermore, the magnitude of this dependence is outside the variability of the source's source strength due to manufacturing, which was determined from the manufacturer and NIST source strength intercomparison records at the UWADCL. As a result, establishing and disseminating calibration conversion factors among different source and chamber models is not advised as it would substantially increase the uncertainty in a clinical user's determination of source strength. The calibration of a well-type ionization chamber is unique to the chamber, source, and source holder. Attempting to generalize source strength calibration coefficients among chambers of the same model or source type is impractical given the variation in response observed across the calibration history of the UWADCL. Attempting to quantitate and transfer calibration coefficients in such a relative sense may significantly degrade the uncertainty relative to specifically calibrating a chamber for an individual source.

Dosimetric comparison of the INTRABEAM and Axxent for intraoperative breast radiotherapy

PurposeBreast intraoperative radiotherapy with electronic brachytherapy (eBT) sources, such as the Zeiss INTRABEAM and the Xoft Axxent, are used to treat the lumpectomy cavity using a stationary or a stepped 50 kVp X-ray source, respectively. For three comparable applicator sizes with volume differences <11%, we compare the dosimetry using clinical planning data. Methods and MaterialsA dosimetric comparison between the INTRABEAM and Axxent to the proximal 1.0 cm of tissue surrounding the applicator is performed using dose–volume parameters (DVPs): V90, V80, V50, Dmin, and Homogeneity Index (HI); HI was calculated as Dmax/Dmin. The dose–volume histograms of the INTRABEAM and Axxent were computed with measured percent depth dose data and with TG-43 parameters, respectively. The skin dose of 0.7–1.0 cm from the applicator surface was also computed. ResultsThe mean DVPs were 5.5 ± 0.8% V90, 12.1 ± 1.5% V80, 47.5 ± 5.8% V50, 6.4 ± 0.6 Gy Dmin, and 3.2 ± 0.3 HI for the INTRABEAM applicators compared with 7.4 ± 0.3% V90, 14.7 ± 0.8% V80, 55.2 ± 4.7% V50, 4.0 ± 0.6 Gy Dmin, and 6.4 ± 1.1 HI for the Axxent applicators. INTRABEAM skin doses ranged from 7.7 to 9.0 Gy at 0.7 cm to 5.5–6.8 Gy at 1.0 cm, whereas Axxent skin doses ranged from 10.7 to 13.0 Gy at 0.7 cm to 7.8 to 9.3 Gy at 1.0 cm. ConclusionsWe demonstrated ±5% comparable dosimetric coverage for tissue ≤0.5 cm from the cavity and higher skin dose for Axxent plans. The DVPs increased with applicator size and with stepped treatment delivery.

Dose measurements nearby low energy electronic brachytherapy sources using radiochromic film

PurposeWe investigate the effect of the GafChromic™ film EBT3 model absorbed dose energy response when used for dose measurements around low-energy photon sources. Monte Carlo based correction procedure in synergy with appropriate calibration curves was shown to provide more accurate absorbed dose (either relative or absolute). An assessment was made of possible dose errors that might be encountered if such energy dependent response is ignored. MethodsWe measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared to PDD measurements acquired with a PTW-TN34013 parallel-plate ionization chamber. For the x-ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Measurement strips of EBT3 film were positioned at distances of 2–6 cm from the Xoft source in a water phantom using a custom-made holder and irradiated simultaneously. ResultsOur results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 5%. However, if the calibration curve was established in an MV beam quality, the error in absorbed dose could be as large as 20%. ConclusionAccurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at appropriate corresponding low energies, as large absorbed dose errors are expected when film calibration is performed in MV beam qualities.

Evaluation of Ge-doped silica fibre TLDs for in vivo dosimetry during intraoperative radiotherapy

Ge-doped silica fibre (GDSF) thermoluminescence dosimeters (TLD) are non-hygroscopic spatially high-resolution radiation sensors with demonstrated potential for radiotherapy dosimetry applications. The INTRABEAM® system with spherical applicators, one of a number of recent electronic brachytherapy sources designed for intraoperative radiotherapy (IORT), presents a representative challenging dosimetry situation, with a low keV photon beam and a desired rapid dose-rate fall-off close-up to the applicator surface. In this study, using the INTRABEAM® system, investigations were made into the potential application of GDSF TLDs for in vivo IORT dosimetry. The GDSFs were calibrated over the respective dose- and depth-range 1 to 20 Gy and 3 to 45 mm from the x-ray probe. The effect of different sizes of spherical applicator on TL response of the fibres was also investigated. The results show the GDSF TLDs to be applicable for IORT dose assessment, with the important incorporated correction for beam quality effects using different spherical applicator sizes. The total uncertainty in use of this type of GDSF for dosimetry has been found to range between 9.5% to 12.4%. Subsequent in vivo measurement of skin dose for three breast patients undergoing IORT were performed, the measured doses being below the tolerance level for acute radiation toxicity.

195. Electronic Brachytherapy in clinical practice: Implementation and comparison against conventional techniques and electron montecarlo algorithm

Purpose Electronic Brachytherapy (EBX), a technique that utilizes miniaturized X-Rays source, is gaining ground in radiotherapy treatment in presence of small superficial lesions. This work focuses on acceptance test, commissioning, clinical implementation and dosimetric features of the novel EBX system Esteya® (Elekta AB, Stockholm, Sweden). Methods Flatness and symmetry of X-ray beams have been evaluated according IEC60976 using a high definition 2D array equipped of liquid filled ionization chambers (SRS 1000, PTW, Germany). Half Value Layer (HVL), PDD and absolute dose have been measured for each applicator with a soft X-ray parallel plate chamber (T34013 PTW, Germany) and solid water according to IAEA TRS 398 [1] . Dose distributions have been compared with the ones calculated for conventional electron treatments. Calculations have been carried out in a virtual water phantom by electron montecarlo (eMc) algorithm implemented in a commercial TPS (Monaco v. 5.11, Elekta) for each applicator. Results Flatness, symmetry and penumbra showed excellent performance even if compared with eMC plans. Build up absence and PDDs slopes allow good homogeneity for coverage in superficial targets. Conclusions Shielding requirements, patient compliance and global management of Electronic brachytherapy sources are well known advantages [2] . This study demonstrated that EBX could be a valid alternative to electron treatment in case of small skin lesions also from a dosimetric point of view.

Postoperative endometrial cancer treatments with electronic brachytherapy source

AbstractPurposeThis study is a dosimetric and acute toxicity comparison of endometrial cancer patients treated with either Axxent (Xoft, Inc., San José, CA, USA) electronic and interstitial brachytherapy versus interstitial high dose rate brachytherapy (HDRBT).Materials and MethodsBetween 2015 and 2017, 94 patients with postoperative endometrial cancer were treated in our centre with the Axxent electronic brachytherapy (eBT) system. The V150 and V200 are evaluated prospectively for each plan. The mean age of patients was 65.9 years (age range 33–84 years), with different tumour staging. Of the 94 patients, 37 received exclusive adjuvant brachytherapy (25 Gy in five sessions); the remaining patients received external beam radiotherapy (EBRT) with a regimen of 23 sessions of 2 Gy each to the entire pelvis, followed by eBT (15 Gy in three sessions). Additionally, the absorbed doses received by the organs at risk (OAR), urinary bladder, rectum and sigmoid colon were compared with HDRBT plans, evaluating D2cc, V50% and V35%. Median follow-up was done for each of the 94 patients to assess the toxicity of the treatment: vaginal mucosa toxicity, rectal and urinary toxicity; and results are presented for acute toxicity, toxicity at 1 month after the end of treatment and follow-up after 12 months for a portion of patients according to the Radiation Therapy Oncology Group (RTOG) toxicity criteria.ResultsThe doses in OAR for eBT plans were lower than that for HDRBT plans, both Ir-192 and Co-60 plans, whose doses were similar. The dose in bladder with eBT was 63.8% of the prescribed dose for D2cc versus 70.1% for HDRBT Ir-192, for V50% was 7.2% versus 12.7% and for V35% was 15.2% versus 28.2%. In rectum the D2cc was 61.2% versus 68.4%, for V50% was 7.9% versus 14.3% and for V35% was 16.7% versus 32%. Results demonstrated lower doses to OAR in all eBT plans. Acute toxicity in eBT was very low in cases of mucositis, with only one case of toxicity greater than grade 1, rectal toxicity and urinary toxicity; results at 1 month are equally good, toxicity symptoms disappeared and no relapses have occurred to date.ConclusionsThe results of treatment with the Axxent eBT unit for 94 patients are very good, as no recurrence has been observed and the toxicity of the treatment is very low. The increase in V150 and V200 has not produced an increase in vaginal mucosa toxicity, and the doses in the OAR are lower than in the plans implemented for HDRBT with Ir-192 or Co-60. eBT is a good alternative to treat endometrial cancer in centres without conventional HDR availability. To date, there are limited published studies reporting on outcomes from patients treated with eBT.

Investigation of a source model for a new electronic brachytherapy tandem by film measurement.

PurposeTo investigate the accuracy of a vendor‐supplied source model for a new Xoft Axxent 0‐degree titanium tandem by film measurement.MethodsWe measured the anisotropy factors at varying distances and angles from the tandem in water using radiochromic film (Gafchromic EBT3) and an Epson Perfection v750 desktop flatbed scanner (US Epson, Long Beach, CA). A 0‐degree tandem was placed vertically in a water phantom. Four pieces of film, each at varying depths, were positioned orthogonal to the longitudinal axis of the tandem for azimuthal anisotropy measurements. Polar anisotropy measurements were taken with the film aligned parallel to the tandem. An absolute dose calibration for the film was verified with a PTW 34013 Soft X‐Ray Chamber. The film measurements were analyzed using different color channels. The measured polar anisotropy for varying source positions was compared to the vendor's data. Azimuthal anisotropy was measured as a function of the radius and angle, and normalized to the mean value over all angles at the specified radius.ResultsThe azimuthal anisotropy of the tandem and source was found to be consistent for different positions along the tandem's longitudinal axis and at varying distances from the tandem. Absolute dose using a calibrated parallel plate chamber showed agreement to within 2% of expected TPS values. The custom tandem, which has a thicker tip than the wall, was attenuating the 50 kV photons more than expected, at the angles where the photons had more wall material to traverse. This discrepancy was verified at different distances from the tandem and with different measurement techniques. As distance increased, anisotropy values had better agreement.ConclusionsWe quantified the agreement between the measured and provided anisotropy factors for a new Xoft Axxent 0‐degree titanium tandem. Radiochromic film response at low kV energy was also investigated. Our results showed that vendor‐supplied TG‐43 values were appropriate for clinical use at majority of the angles. A rigorous quality assurance method for new electronic brachytherapy sources and applicators, along with complete knowledge of all dosimetric measuring tools, should be implemented for all parts of the verification and commissioning process.

PO-0995: Development of cylindrical applicator for CNT based miniature electronic brachytherapy source

PO-0995: Development of cylindrical applicator for CNT based miniature electronic brachytherapy source

Determination of absorbed dose to water from a miniature kilovoltage x-ray source using a parallel-plate ionization chamber

Electronic brachytherapy sources are widely accepted as alternatives to radionuclide-based systems. Yet, formal dosimetry standards for these devices to independently complement the dose protocol provided by the manufacturer are lacking. This article presents a formalism for calculating and independently verifying the absorbed dose to water from a kV x-ray source (The INTRABEAM System) measured in a water phantom with an ionization chamber calibrated in terms of air-kerma. This formalism uses a Monte Carlo (MC) calculated chamber conversion factor, , to convert air-kerma in a reference beam to absorbed dose to water in the measurement beam. In this work was determined for a PTW 34013 parallel-plate ionization chamber. Our results show that was sensitive to the chamber plate separation tolerance, with differences of up to 15%. was also found to have a depth dependence which varied with chamber plate separation (0 to 10% variation for the smallest and largest cavity height, over 3 to 30 mm depth). However for all chamber dimensions investigated, was found to be significantly larger than the manufacturer reported value, suggesting that the manufacturer recommended method of dose calculation could be underestimating the dose to water.

Dosimetric effects of saline- versus water-filled balloon applicators for IORT using the model S700 electronic brachytherapy source

PurposeThe Xoft Axxent Electronic Brachytherapy System (Xoft, Inc., San Jose, CA) is a viable option for intraoperative radiation therapy (IORT) treatment of early-stage breast cancer. The low-energy (50-kVp) X-ray source simplifies shielding and increases relative biological effectiveness but increases dose distribution sensitivity to medium composition. Treatment planning systems typically assume homogenous water for brachytherapy dose calculations, including precalculated atlas plans for Xoft IORT. However, Xoft recommends saline for balloon applicator filling. This study investigates dosimetric differences due to increased effective atomic number (Zeff) for saline (Zeff = 7.56) versus water (Zeff = 7.42). MethodsBalloon applicator diameters range from 3 to 6 cm. Monte Carlo N-Particle software is used to calculate dose at the surface (Ds) of and 1 cm away (D1cm) from the water-/saline-filled balloon applicator using a single dwell at the applicator center as a simple estimation of the dosimetry and multiple dwells simulating the clinical dose distributions for the atlas plans. ResultsSingle-dwell plans show a 4.4–6.1% decrease in Ds for the 3- to 6-cm diameter applicators due to the saline. Multidwell plans show similar results: 4.9% and 6.4% Ds decrease, for 4-cm and 6-cm diameter applicators, respectively. For the single-dwell plans, D1cm decreases 3.6–5.2% for the 3- to 6-cm diameter applicators. For the multidwell plans, D1cm decreases 3.3% and 5.3% for the 4-cm and 6-cm applicators, respectively. ConclusionsThe dosimetric effect introduced by saline versus water filling for Xoft balloon applicator–based IORT treatments is ∼5%. Users should be aware of this in the context of both treatment planning and patient outcome studies.

Dosimetric optimization of a truncated conical-shaped transmission target for electronic brachytherapy X-ray source: A Monte Carlo study

The purpose of this study is to determine the optimal target configurations to improve the uniformity of dose distribution for the electronic brachytherapy source. A truncated conical-shaped transmission type was designed. Monte Carlo simulation technique was used to investigate the target thickness, geometry of electron beam, and target angle of a truncated conical-shaped target for the electronic brachytherapy X-ray source. Dosimetric parameters recommended by TG-43U1 protocol were used to determine the optimal target design of electronic brachytherapy source. The target thicknesses for maximizing the transmitted X-ray intensity were approximately 1.2 to 1.5 μm for 0° and 90°. In a range of optimal thickness, transmitted X-ray intensity at 90° was approximately 92% of maximum photon intensity. The effects of electron beam shapes on 2D anisotropy functions were investigated at radial distances of 0.5, 1.0, 2.0, 3.0, and 5.0 cm. Minimum variations for all radiation distances and angular ranges were observed for uniform cylindrical electron beam with a radius of 2.0 mm. Anisotropy functions at 0°, F(r, 0° ), were close to unity and slightly more than unity for non-uniform cylindrical (R = 1.0 - 2.0 mm) electron beam and uniform cylindrical electron beam with a radius of 2.0 mm. The angles of target anode between 45° and 50° show minimum fluctuations in the anisotropy functions and are close to unity for F(r, 0° ). The optimal target configurations are a truncated conical-shaped target having an angle between 45° and 50°. It is concluded that tungsten target having the thickness of 1.2 to 1.5 μm and uniform circular with a radius of 2.0 mm as electron beam produces optimal dosimetric characteristics for electronic brachytherapy X-ray source.

PO-0939: Comparison of brachytherapy sources of endometrial cancer: Electronic brachytherapy source and 192Ir

PO-0939: Comparison of brachytherapy sources of endometrial cancer: Electronic brachytherapy source and 192Ir

SU‐F‐T‐58: Dosimetric Evaluation of Breast Tissue Composition for Electronic Brachytherapy (BET) Source In High Dose Rate Accelerated Partial Breast (APBI) Irradiation

Purpose:To quantitatively evaluate the dosimetric impact of differing breast tissue compositions for electronic brachytherapy source for high dose rate accelerated partial breast irradiation.Methods:A series of Monte Carlo Simulation were created using the GEANT4 toolkit (version 10.0). The breast phantom was modeled as a semi‐circle with a radius of 5.0 cm. A water balloon with a radius of 1.5 cm was located in the phantom with the Xoft AxxentTM EBT source placed at center as a point source. A mixed of two tissue types (adipose and glandular tissue) was assigned as the materials for the breast phantom with different weight ratios. The proportionality of glandular and adipose tissue was simulated in four different fashions, 80/20, 70/30, 50/50 and 30/70 respectively. The custom energy spectrum for the 50 kVp XOFT source was provided via the manufacturer and used to generate incident photons. The dose distributions were recorded using a parallel three dimensional mesh with a size of 30 × 30 × 30 cm3 with 1 × 1 × 1 mm3 voxels. The simulated doses absorbed along the transverse axis were normalized at the distance of 1 cm and then compared with the calculations using standard TG‐43 formalism.Results:All simulations showed underestimation of dose beyond balloon surface compared to standard TG‐43 calculations. The maximum percentage differences within 2 cm distance from balloon surface were found to be 18%, 11%, 10% and 8% for the fat breast (30/70), standard breast (50/50), dense breast (70/30 and 80/20), respectively.Conclusion:The accuracy of dose calculations for low energy EBT source was limited when considering tissue heterogeneous composition. The impact of atomic number on photo‐electric effect for lower energy Brachytherapy source is not accounted for and resulting in significant errors in dose calculation.