Electronic Brachytherapy Source Research Articles

A Novel Approach to Centralize and Improve Source Calibrations for an Effective Electronic Brachytherapy Source Management Program at Multisite Facilities

A Novel Approach to Centralize and Improve Source Calibrations for an Effective Electronic Brachytherapy Source Management Program at Multisite Facilities

Measurement of absorbed dose to water around an electronic brachytherapy source. Comparison of two dosimetry systems: lithium formate EPR dosimeters and radiochromic EBT2 film

Interest in high dose rate (HDR) electronic brachytherapy operating at 50 kV is increasing. For quality assurance it is important to identify dosimetry systems that can measure the absorbed doses in absolute terms which is difficult in this energy region. In this work a comparison is made between two dosimetry systems, EPR lithium formate dosimeters and radiochromic EBT2 film.Both types of dosimeters were irradiated simultaneously in a PMMA phantom using the Axxent EBS. Absorbed dose to water was determined at distances of 10 mm, 30 mm and 50 mm from the EBS. Results were traceable to different primary standards as regards to absorbed dose to water (EPR) and air kerma (EBT2). Monte Carlo simulations were used in absolute terms as a third estimate of absorbed dose to water.Agreement within the estimated expanded (k = 2) uncertainties (5% (EPR), 7% (EBT2)) was found between the results at 30 mm and 50 mm from the x-ray source. The same result was obtained in 4 repetitions of irradiation, indicating high precision in the measurements with both systems. At all distances, agreement between EPR and Monte Carlo simulations was shown as was also the case for the film measurements at 30mm and 50mm. At 10mm the geometry for the film measurements caused too large uncertainty in measured values depending on the exact position (within sub-mm distances) of the EBS and the 10 mm film results were exculded from comparison.This work has demonstrated good performance of the lithium formate EPR dosimetry system in accordance with earlier experiments at higher photon energies (192Ir HDR brachytherapy). It was also highlighted that there might be issues regarding the energy dependence and intrinsic efficiency of the EBT2 film that need to be considered for measurements using low energy sources.

A modified dose calculation formalism for electronic brachytherapy sources.

To propose a modification of the current dose calculation formalism introduced in the Task Group No. 43 Report (TG-43) to accommodate an air-kerma rate standard for electronic brachytherapy sources as an alternative to an air-kerma strength standard. Electronic brachytherapy sources are miniature x-ray tubes emitting low energies with high-dose-rates. The National Institute of Standards and Technology (NIST) has introduced a new primary air-kerma rate standard for one of these sources, in contrast to air-kerma strength. A modification of the TG-43 protocol for calculation of dose-rate distributions around electronic brachytherapy sources including sources in an applicator is presented. It cannot be assumed that the perturbations from sources in an applicator are negligible, and thus, the applicator is incorporated in the formalism. The modified protocol mimics the fundamental methodology of the original TG-43 formalism, but now incorporates the new NIST-traceable source strength metric of air-kerma rate at 50cm and introduces a new subscript, i, to denote the presence of an applicator used in treatment delivery. Applications of electronic brachytherapy sources for surface brachytherapy are not addressed in this Technical Note since they are well documented in other publications. A modification of the AAPM TG-43 protocol has been developed to accommodate an air-kerma rate standard for electronic brachytherapy sources as an alternative to an air-kerma strength standard. The modified TG-43 formalism allows dose calculations to be performed using a new NIST-traceable source strength metric and introduces the concept of applicator-specific formalism parameters denoted with subscript, i.

Asymmetric dose-volume optimization with smoothness control for rotating-shield brachytherapy.

It is important to reduce fluence map complexity in rotating-shield brachytherapy (RSBT) inverse planning to improve delivery efficiency while maintaining plan quality. This study proposes an efficient and effective RSBT dose optimization method which enables to produce smooth fluence maps. Five cervical cancer patients each with a high-risk clinical-target-volume (HR-CTV) larger than 40 cm(3) were considered as the test cases. The RSBT source was a partially shielded electronic brachytherapy source (Xoft Axxent™). The anchor RSBT plans generated by the asymmetric dose-volume optimization with smoothness control (ADOS) method were compared against those produced by the dose-surface optimization (DSO) method and inverse-planning with simulated annealing (IPSA). Either L1-norm or L2-norm was used to measure the smoothness of a fluence map in the proposed ADOS method as one weighted term of the objective function. Uniform dwell-time scaling was applied to all plans such that HR-CTV D90 was maximized without violating the D2cc tolerances of the rectum, bladder, and sigmoid colon. The quality of the anchor plans was measured with HR-CTV D90 of the anchor plans. Single-shielded RSBT [(S-RSBT), RSBT with single, fix sized delivery window] and dynamic-sheilded RSBT [(D-RSBT), RSBT with dynamically varying sized delivery window] delivery plans generated based on the anchor plans were also measured, with delivery time constraints of 10, 20, and 30 min/fraction (fx). The average HR-CTV D90 values of the anchor plans achieved by the ADOS, DSO, and IPSA methods were 111.5, 94.2, and 107.4 Gy, respectively, where the weighting parameter β used in ADOS with L2-norm was set to be 100. By using S-RSBT sequencing and 20 min/fx delivery time, the corresponding D90 values were 88.8, 81.9, and 83.4 Gy; while using D-RSBT sequencing with 20 min/fx delivery time, the corresponding D90 values were 91.4, 88.3, and 78.9 Gy, respectively. The average optimization times for ADOS, DSO, and IPSA were, respectively, 77, 4, and 1800 s. By using L1-norm instead of L2-norm in the ADOS method, the optimization time was increased by 20 s, while the D90 was reduced by 6.8 Gy on average. ADOS-L1 was found to be more sensitive to the weighting parameter than ADOS-L2. If β was increased to 10 000, the D90 drop with ADOS-L1 was 38 Gy, while the drop with ADOS-L2 is 13 Gy. The ADOS method had a reasonable optimization time cost, while achieving comparable RSBT dose plans as the IPSA method, which is of much higher time complexity. Compared to the DSO and IPSA methods, ADOS is able to generate anchor plans which are more suitable for RSBT delivery while preserving the high quality of the original plans. Compared to ADOS-L1, ADOS-L2 is able to achieve better quality of anchor plans more efficiently.

New National Air-Kerma Standard for Low-Energy Electronic Brachytherapy Sources.

The new primary standard for low-energy electronic brachytherapy sources for the United States is described. These miniature x-ray tubes are inserted in catheters for interstitial radiation therapy and operate at tube potentials of up to about 50 kV. The standard is based on the realization of the air kerma produced by the x-ray beam at a reference distance in air of 50 cm.

The use of tetrahedral mesh geometries in Monte Carlo simulation of applicator based brachytherapy dose distributions

Accounting for brachytherapy applicator attenuation is part of the recommendations from the recent report of AAPM Task Group 186. To do so, model based dose calculation algorithms require accurate modelling of the applicator geometry. This can be non-trivial in the case of irregularly shaped applicators such as the Fletcher Williamson gynaecological applicator or balloon applicators with possibly irregular shapes employed in accelerated partial breast irradiation (APBI) performed using electronic brachytherapy sources (EBS). While many of these applicators can be modelled using constructive solid geometry (CSG), the latter may be difficult and time-consuming. Alternatively, these complex geometries can be modelled using tessellated geometries such as tetrahedral meshes (mesh geometries (MG)). Recent versions of Monte Carlo (MC) codes Geant4 and MCNP6 allow for the use of MG. The goal of this work was to model a series of applicators relevant to brachytherapy using MG. Applicators designed for 192Ir sources and 50 kV EBS were studied; a shielded vaginal applicator, a shielded Fletcher Williamson applicator and an APBI balloon applicator.All applicators were modelled in Geant4 and MCNP6 using MG and CSG for dose calculations. CSG derived dose distributions were considered as reference and used to validate MG models by comparing dose distribution ratios.In general agreement within 1% for the dose calculations was observed for all applicators between MG and CSG and between codes when considering volumes inside the 25% isodose surface. When compared to CSG, MG required longer computation times by a factor of at least 2 for MC simulations using the same code. MCNP6 calculation times were more than ten times shorter than Geant4 in some cases.In conclusion we presented methods allowing for high fidelity modelling with results equivalent to CSG. To the best of our knowledge MG offers the most accurate representation of an irregular APBI balloon applicator.

SU-D-19A-05: The Dosimetric Impact of Using Xoft Axxent® Electronic Brachytherapy Source TG-43 Dosimetry Parameters for Treatment with the Xoft 30 Mm Diameter Vaginal Applicator

Purpose: A full TG-43 dosimetric characterization has not been performed for the Xoft Axxent ® electronic brachytherapy source (Xoft, a subsidiary of iCAD, San Jose, CA) within the Xoft 30 mm diameter vaginal applicator. Currently, dose calculations are performed using the bare-source TG-43 parameters and do not account for the presence of the applicator. This work focuses on determining the difference between the bare-source and sourcein- applicator TG-43 parameters. Both the radial dose function (RDF) and polar anisotropy function (PAF) were computationally determined for the source-in-applicator and bare-source models to determine the impact of using the bare-source dosimetry data. Methods: MCNP5 was used to model the source and the Xoft 30 mm diameter vaginal applicator. All simulations were performed using 0.84p and 0.03e cross section libraries. All models were developed based on specifications provided by Xoft. The applicator is made of a proprietary polymer material and simulations were performed using the most conservative chemical composition. An F6 collision-kerma tally was used to determine the RDF and PAF values in water at various dwell positions. The RDF values were normalized to 2.0 cm from the source to accommodate the applicator radius. Source-in-applicator results were compared with bare-source results from this work as well as published baresource results. Results: For a 0 mm source pullback distance, the updated bare-source model and source-in-applicator RDF values differ by 2% at 3 cm and 4% at 5 cm. The largest PAF disagreements were observed at the distal end of the source and applicator with up to 17% disagreement at 2 cm and 8% at 8 cm. The bare-source model had RDF values within 2.6% of the published TG-43 data and PAF results within 7.2% at 2 cm. Conclusion: Results indicate that notable differences exist between the bare-source and source-in-applicator TG-43 simulated parameters. Xoft Inc. provided partial funding for this work

SU‐E‐T‐284: Revisiting Reference Dosimetry for the Model S700 Axxent 50 KVp Electronic Brachytherapy Source

Purpose:The model S700 Axxent electronic brachytherapy source by Xoft was characterized in 2006 by Rivard et al. The source design was modified in 2006 to include a plastic centering insert at the source tip to more accurately position the anode. The objectives of the current study were to establish an accurate Monte Carlo source model for simulation purposes, to dosimetrically characterize the new source and obtain its TG‐43 brachytherapy dosimetry parameters, and to determine dose differences between the source with and without the centering insert.Methods:Design information from dissected sources and vendor‐supplied CAD drawings were used to devise the source model for radiation transport simulations of dose distributions in a water phantom. Collision kerma was estimated as a function of radial distance, r, and polar angle, θ, for determination of reference TG‐43 dosimetry parameters. Simulations were run for 1010 histories, resulting in statistical uncertainties on the transverse plane of 0.03% at r=1 cm and 0.08% at r=10 cm.Results:The dose rate distribution the transverse plane did not change beyond 2% between the 2006 model and the current study. While differences exceeding 15% were observed near the source distal tip, these diminished to within 2% for r>1.5 cm. Differences exceeding a factor of two were observed near θ=150° and in contact with the source, but diminished to within 20% at r=10 cm.Conclusions:Changes in source design influenced the overall dose rate and distribution by more than 2% over a third of the available solid angle external from the source. For clinical applications using balloons or applicators with tissue located within 5 cm from the source, dose differences exceeding 2% were observed only for θ>110°. This study carefully examined the current source geometry and presents a modern reference TG‐43 dosimetry dataset for the model S700 source.

SU‐C‐16A‐06: Optimum Radiation Source for Radiation Therapy of Skin Cancer

Purpose:Recently, different applicators are designed for treatment of the skin cancer such as scalp and legs, using Ir‐192 HDR Brachytherapy Sources (IR‐HDRS), Miniature Electronic Brachytherapy Sources (MEBXS), and External Electron Beam Radiation Therapy (EEBRT). Although, all of these methodologies may deliver the desired radiation dose to the skin, the dose to the underlying bone may become the limiting factor for selection of the optimum treatment technique. In this project the radiation dose delivered to the underlying bone has been evaluated as a function of the radiation source and thickness of the underlying bone.Methods:MC simulations were performed using MCNP5 code. In these simulations, the mono‐energetic and non‐divergent photon beams of 30 keV, 50 keV, and 70 keV for MEBXS, 380 keV photons for IR‐HDRS, and 6 MeV mono‐energetic electron beam for EEBRT were modeled. A 0.5 cm thick soft tissue (0.3 cm skin and 0.2 cm adipose) with underlying 0.5 cm cortical bone followed by 14 cm soft tissue are utilized for simulations.Results:Dose values to bone tissue as a function of beam energy and beam type, for a delivery of 5000 cGy dose to skin, were compared. These results indicate that for delivery of 5000 cGy dose to the skin surface with 30 keV, 50 keV, 70 keV of MEBXS, IR‐HDRS, and EEBRT techniques, bone will receive 31750 cGy, 27450 cGy, 18550 cGy, 4875 cGy, and 10450 cGy, respectively.Conclusion:The results of these investigations indicate that, for delivery of the same skin dose, average doses received by the underlying bone are 5.2 and 2.2 times larger with a 50 keV MEBXS and EEBRT techniques than IR‐HDRS, respectively.

A novel high-throughput irradiator for in vitro radiation sensitivity bioassays

This paper describes the development and characterization of a fully automated in vitro cell irradiator using an electronic brachytherapy source to perform radiation sensitivity bioassays. This novel irradiator allows complex variable dose and dose rate schemes to be delivered to multiple wells of 96-well culture plates used in standard biological assays. The Xoft Axxent® eBx™ was chosen as the x-ray source due to its ability to vary tube current up to 300 µA for a 50 kVp spectrum using clinical surface applicators. Translation of the multiwell plate across the fixed radiation field is achieved using a precision motor driven computer controlled positioning system. A series of measurements was performed to characterize dosimetric performance of the system. Measurements have shown that the radiation output measured with an end window ionization chamber is stable between operating currents of 50–300 µA. In addition, radiochromic film was used to characterize the field flatness and symmetry. The average field flatness in the in-plane and cross-plane direction was 2.9 ±1.0% and 4.0 ±1.7%, respectively. The average symmetry in the in-plane and cross-plane direction was 1.8 ±0.9% and 1.6 ±0.5%, respectively. The optimal focal spot resolution at the cellular plane was determined by measuring sequential irradiations on radiochromic film for three different well spacing schemes. It was determined that the current system can irradiate every other well with negligible impact on the radiation field characteristics. Finally, a performance comparison between this system and a common cabinet irradiator is presented.

Dosimetric characterization and output verification for conical brachytherapy surface applicators. Part I. Electronic brachytherapy source.

Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR)(192)Ir sources, as well as electronic brachytherapy sources. Part I of this paper will discuss the applicators used with electronic brachytherapy sources; Part II will discuss those used with HDR (192)Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics including depth dose and surface dose distributions have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these. Existing dosimetry protocols available from the AAPM bookend the cross-over characteristics of a traditional brachytherapy source (as described by Task Group 43) being implemented as a low-energy superficial x-ray beam (as described by Task Group 61) as observed with the surface applicators of interest. This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and(192)Ir sources (Part II). Air-kerma rate measurements for the electronic brachytherapy sources were completed with an Attix Free-Air Chamber, as well as several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom. Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate. The novel output verification methods described in this work will reduce uncertainties in dose delivery for treatments with these kinds of surface applicators, ultimately improving patient care.

Dosimetric characterization and output verification for conical brachytherapy surface applicators. Part II. High dose rate 192Ir sources.

Historically, treatment of malignant surface lesions has been achieved with linear accelerator based electron beams or superficial x-ray beams. Recent developments in the field of brachytherapy now allow for the treatment of surface lesions with specialized conical applicators placed directly on the lesion. Applicators are available for use with high dose rate (HDR)(192)Ir sources, as well as electronic brachytherapy sources. Part I of this paper discussed the applicators used with electronic brachytherapy sources. Part II will discuss those used with HDR (192)Ir sources. Although the use of these applicators has gained in popularity, the dosimetric characteristics have not been independently verified. Additionally, there is no recognized method of output verification for quality assurance procedures with applicators like these. This work aims to create a cohesive method of output verification that can be used to determine the dose at the treatment surface as part of a quality assurance/commissioning process for surface applicators used with HDR electronic brachytherapy sources (Part I) and(192)Ir sources (Part II). Air-kerma rate measurements for the (192)Ir sources were completed with several models of small-volume ionization chambers to obtain an air-kerma rate at the treatment surface for each applicator. Correction factors were calculated using MCNP5 and EGSnrc Monte Carlo codes in order to determine an applicator-specific absorbed dose to water at the treatment surface from the measured air-kerma rate. Additionally, relative dose measurements of the surface dose distributions and characteristic depth dose curves were completed in-phantom. Theoretical dose distributions and depth dose curves were generated for each applicator and agreed well with the measured values. A method of output verification was created that allows users to determine the applicator-specific dose to water at the treatment surface based on a measured air-kerma rate. The novel output verification methods described in this work will reduce uncertainties in dose delivery for treatments with these kinds of surface applicators, ultimately improving patient care.

Dynamic rotating‐shield brachytherapy

To present dynamic rotating shield brachytherapy (D-RSBT), a novel form of high-dose-rate brachytherapy (HDR-BT) with electronic brachytherapy source, where the radiation shield is capable of changing emission angles during the radiation delivery process. A D-RSBT system uses two layers of independently rotating tungsten alloy shields, each with a 180° azimuthal emission angle. The D-RSBT planning is separated into two stages: anchor plan optimization and optimal sequencing. In the anchor plan optimization, anchor plans are generated by maximizing the D90 for the high-risk clinical-tumor-volume (HR-CTV) assuming a fixed azimuthal emission angle of 11.25°. In the optimal sequencing, treatment plans that most closely approximate the anchor plans under the delivery-time constraint will be efficiently computed. Treatment plans for five cervical cancer patients were generated for D-RSBT, single-shield RSBT (S-RSBT), and (192)Ir-based intracavitary brachytherapy with supplementary interstitial brachytherapy (IS + ICBT) assuming five treatment fractions. External beam radiotherapy doses of 45 Gy in 25 fractions of 1.8 Gy each were accounted for. The high-risk clinical target volume (HR-CTV) doses were escalated such that the D2cc of the rectum, sigmoid colon, or bladder reached its tolerance equivalent dose in 2 Gy fractions (EQD2 with α∕β = 3 Gy) of 75 Gy, 75 Gy, or 90 Gy, respectively. For the patients considered, IS + ICBT had an average total dwell time of 5.7 minutes∕fraction (min∕fx) assuming a 10 Ci(192)Ir source, and the average HR-CTV D90 was 78.9 Gy. In order to match the HR-CTV D90 of IS + ICBT, D-RSBT required an average of 10.1 min∕fx more delivery time, and S-RSBT required 6.7 min∕fx more. If an additional 20 min∕fx of delivery time is allowed beyond that of the IS + ICBT case, D-RSBT and S-RSBT increased the HR-CTV D90 above IS + ICBT by an average of 16.3 Gy and 9.1 Gy, respectively. For cervical cancer patients, D-RSBT can boost HR-CTV D90 over IS + ICBT and S-RSBT without violating the tolerance doses to the bladder, rectum, or sigmoid. The D90 improvements from D-RSBT depend on the patient, the delivery time budget, and the applicator structure.

SU‐C‐500‐02: Comparison of a Commercial Electronic Brachytherapy Source and Ir‐192/Co‐60 HDR Source for Endometrial and Cervical Cancers

Purpose: To evaluate the dosimetric differences between Iridium‐192, Co‐60 based HDR plans and plans generated by electronic brachytherapy source (EBS) for cervical and endometrial cancers. Material/Methods: Our institution uses volume based planning for each tandem and ovoid (T&O) or vaginal cylinder insertion and treatment. We attempt to characterize the differences between the three sources (Ir‐192, Co‐60 and EBS). For vaginal cylinder cases, patients receive 6 Gy x3 prescribed to a depth 5 mm. For T&O cases, the prescription is 8Gy x3 to point A. Organs at risk (OARs) contoured were rectum, bladder, sigmoid & small bowel. Our planning protocol requires adjustment of source dwell times and positions to keep doses to the OARs below 70% of the prescription dose. For vaginal cylinder, dose coverage is optimized to a line drawn 5mm from the vaginal cylinder surface. This is a retrospective study of previously planned Ir‐192 treated patients. Results: There is no difference between the Co‐60 and Ir‐192 plans but plans generated using the EBS offer better sparing of OARs as evaluated using the dose to highest 2cc of the organ at risk but the differences are not statistically significant. However, there are significant differences in the DVH between EBS and the other two sources. There was a reduction of up to 50% between EBS and other two sources using v15%, v25%, v50% v75% (% of volume getting 15, 50, 75 percent of the prescription dose). Conclusion: All three sources are suitable for HDR Brachytherapy but the use of EBS can reduce the organ at risk dose while achieving similar dose coverage. Even though D2cc is the recommended dose volume point for HDR plans evaluation, V15%, V25%, V50% and V75% are also useful indicator of the plan quality as they characterize the lower dose region of the DVH curves more adequately

SU‐E‐T‐454: Comparative Dosimetry of the Xoft Cervical Applicator and HDR Ir‐192 Henschke Applicator

Purpose: To evaluate HDR dosimetry for the Xoft Axxent ™cervical tandem and ovoid (T&O) applicator using the 50‐kV electronic brachytherapy (eBx) source. The titanium wall of this Henschke‐style applicator filters lower energies which flattens the radial dose function. Dose distributions around the T&O for 50‐kV‐Ti were investigated with comparison to Ir‐192. Methods: TG‐43u parameters were derived from film measurements of the 50‐kV‐Ti. To account for varying filtration effects on anisotropy from tip to barrel in the tandem, a mixed source model using the anisotropy at two dwell positions was implemented. Stylized geometry of cervical PTV, rectum and bladder were created on a CT scan of a gelatin phantom with the 15‐degree T&O. Plans for each source were optimized on BrachyVision™ using identical constraints and normalization. The GMP‐192 source was used for the Ir‐192 plans. Results: Both plans were normalized at 6‐Gy (D95) and have classic pear‐shaped doses. Doses at Points A‐H are within 5% for each plan, but are 20% lower for 50‐kV‐Ti at Point B. Along the tandem length peak Ir‐192 doses are 50% higher. The Ir‐192 dose dip at the tandem tip is absent for 50‐kV‐Ti. PTV DVHs are similar except for the effect of the peak tandem surface doses. PTV coverage (D95) for both is 6 Gy (+/− 1%) and the median dose for both was 950 cGy (+/− 2%). The peak dose region (D5) is 14% lower for 50‐kV‐Ti (18.1 Gy vs 20.7 Gy). D5 of rectum and bladder are 13% and 20% respectively for 50‐kV‐Ti and Ir‐192. Median doses to critical structures are 18–20% lower for 50‐kV‐Ti. Conclusion: PTV doses for Ir‐192 HDR and 50‐kV‐Ti eBx have similar target coverage, except at the tandem surface where doses were lower for eBx. Critical organs dose outside the PTV will be lower for eBx. Funding for this study was providing by Xoft, Inc — an iCad subsidiary

SU-D-108-07: Experimental Verification of a Compensator-Based Brachytherapy System

Purpose: To construct and experimentally validate a compensator‐based brachytherapy (CBT) prototype system for treating cervical cancer. With CBT, high‐precision 3‐D manufacturing techniques are used to generate a patient‐specific compensator with spatially‐varying thicknesses, through which an electronic brachytherapy (eBT) source travels to deliver conformal radiation dose distributions. Methods: A prototype cylindrical compensator with 8 octants, each with different thicknesses, was designed. Direct Metal Laser Sintering (DMLS) was used to construct Cobalt‐Chromium (CoCr) and Titanium (Ti) compensators, and a 4‐axis CNC milling (4‐D milling) technique was used to construct a Ti compensator. Gafchromic EBT2 films, held by an acrylic quality assurance (QA) phantom, were irradiated to approximately 125 cGy with an eBT source for both shielded and unshielded cases. The dose at each point on the films was calculated using a TG‐43 dose calculation model that was modified to account for the presence of a compensator by ray‐tracing. Results: Both 4‐D milling and DMLS methods achieved 25.4 urn and 20–40 urn spatial accuracy, respectively. Construction time is approximately 10 hours for 4‐D milling and 2 hours for DMLS. The agreement between the expected EBT2 film dose and measured dose was within 10% without a compensator, and within 15% for non‐thickness‐transition area with a compensator. The use of Monte Carlo dose calculations is expected to improve dosimetric accuracy relative to the current TG‐43‐based method. Conclusion: We expect that patient‐specific compensators can be constructed rapidly in clinical situations using DMLS. CBT is a non‐invasive alternative to supplementary interstitial BT that is expected to improve the dose conformity to bulky cervical cancer tumors relative to conventional intracavitary brachytherapy. University of Iowa Research Foundation

Rotating-shield brachytherapy for cervical cancer

In this treatment planning study, the potential benefits of a rotating shield brachytherapy (RSBT) technique based on a partially-shielded electronic brachytherapy source were assessed for treating cervical cancer. Conventional intracavitary brachytherapy (ICBT), intracavitary plus supplementary interstitial (IS+ICBT), and RSBT treatment plans for azimuthal emission angles of 180° (RSBT-180) and 45° (RSBT-45) were generated for five patients. For each patient, high-risk clinical target volume (HR-CTV) equivalent dose in 2 Gy fractions (EQD2) (α/β = 10 Gy) was escalated until bladder, rectum, or sigmoid colon tolerance EQD2 values were reached. External beam radiotherapy dose (1.8 Gy × 25) was accounted for, and brachytherapy was assumed to have been delivered in 5 fractions. IS+ICBT provided a greater HR-CTV D90 (minimum EQD2 to the hottest 90%) than ICBT. D90 was greater for RSBT-45 than IS+ICBT for all five patients, and greater for RSBT-180 than IS+ICBT for two patients. When the RSBT-45/180 plan with the lowest HR-CTV D90 that was greater than the D90 the ICBT or IS+ICBT plan was selected, the average (range) of D90 increases for RSBT over ICBT and IS+ICBT were 16.2 (6.3–27.2)and 8.5 (0.03–20.16) Gy, respectively. The average (range) treatment time increase per fraction of RSBT was 34.56 (3.68–70.41) min over ICBT and 34.59 (3.57–70.13) min over IS+ICBT. RSBT can increase D90 over ICBT and IS+ICBT without compromising organ-at-risk sparing. The D90 and treatment time improvements from RSBT depend on the patient and shield emission angle.

Rapid emission angle selection for rotating‐shield brachytherapy

The authors present a rapid emission angle selection (REAS) method that enables the efficient selection of the azimuthal shield angle for rotating shield brachytherapy (RSBT). The REAS method produces a Pareto curve from which a potential RSBT user can select a treatment plan that balances the tradeoff between delivery time and tumor dose conformity. Two cervical cancer patients were considered as test cases for the REAS method. The RSBT source considered was a Xoft Axxent(TM) electronic brachytherapy source, partially shielded with 0.5 mm of tungsten, which traveled inside a tandem intrauterine applicator. Three anchor RSBT plans were generated for each case using dose-volume optimization, with azimuthal shield emission angles of 90°, 180°, and 270°. The REAS method converts the anchor plans to treatment plans for all possible emission angles by combining neighboring beamlets to form beamlets for larger emission angles. Treatment plans based on exhaustive dose-volume optimization (ERVO) and exhaustive surface optimization (ERSO) were also generated for both cases. Uniform dwell-time scaling was applied to all plans such that that high-risk clinical target volume D90 was maximized without violating the D2cc tolerances of the rectum, bladder, and sigmoid colon. By choosing three azimuthal emission angles out of 32 potential angles, the REAS method performs about 10 times faster than the ERVO method. By setting D90 to 85-100 Gy10, the delivery times used by REAS generated plans are 21.0% and 19.5% less than exhaustive surface optimized plans used by the two clinical cases. By setting the delivery time budget to 5-25 and 10-30 min∕fx, respectively, for two the cases, the D90 contributions for REAS are improved by 5.8% and 5.1% compared to the ERSO plans. The ranges used in this comparison were selected in order to keep both D90 and the delivery time within acceptable limits. The REAS method enables efficient RSBT treatment planning and delivery and provides treatment plans with comparable quality to those generated by exhaustive replanning with dose-volume optimization.

OC-0083: Comparison of TG-43 and TG-186 in breast irradiation using a low energy electronic brachytherapy source

OC-0083: Comparison of TG-43 and TG-186 in breast irradiation using a low energy electronic brachytherapy source

Design and dosimetric characteristics of a new endocavitary contact radiotherapy system using an electronic brachytherapy source

To present design aspects and acceptance tests performed for clinical implementation of electronic brachytherapy treatment of early stage rectal adenocarcinoma. A dosimetric comparison is made between the historically used Philips RT-50 unit and the newly developed Axxent(®) Model S700 electronic brachytherapy source manufactured by Xoft (iCad, Inc.). Two proctoscope cones were manufactured by ElectroSurgical Instruments (ESI). Two custom surface applicators were manufactured by Xoft and were designed to fit and interlock with the proctoscope cones from ESI. Dose rates, half value layers (HVL), and percentage depth dose (PDD) measurements were made with the Xoft system and compared to historical RT-50 data. A description of the patient treatment approach and exposure rates during the procedure is also provided. The electronic brachytherapy system has a lower surface dose rate than the RT-50. The dose rate to water on the surface from the Xoft system is approximately 2.1 Gy∕min while the RT-50 is 10-12 Gy∕min. However, treatment times with Xoft are still reasonable. The HVLs and PDDs between the two systems were comparable resulting in similar doses to the target and to regions beyond the target. The exposure rate levels around a patient treatment were acceptable. The standard uncertainty in the dose rate to water on the surface is approximately ±5.2%. The Philips RT-50 unit is an out-of-date radiotherapy machine that is no longer manufactured with limited replacement parts. The use of a custom-designed proctoscope and Xoft surface applicators allows delivery of a well-established treatment with the ease of a modern radiotherapy device. While the dose rate is lower with the use of Xoft, the treatment times are still reasonable. Additionally, personnel may stand farther away from the Xoft radiation source, thus potentially reducing radiation exposure to the operator and other personnel.