Xoft Axxent Electronic Brachytherapy Research Articles

Measurement of dose enhancement factor for Xoft Axxent electronic brachytherapy device using nanoparticle-embedded alginate film and radiochromic film.

Inw nanoparticles-aided radiotherapy, the radiation sensitivity of tumor is increased with the infusion of nanoparticles in tumor. This therapeutic modality is capable of delivering enhanced dose to tumor, without exceeding the normal tissue tolerance dose. Further, the quantification of the enhanced dose using suitable dosimeter is important. The present study is aimed at measuring the dose enhancement factors (DEFs) using the combination of nanoparticles-embedded alginate (Alg) film and unlaminated Gafchromic EBT3 film. Gold nanoparticles (AuNPs)- and silver nanoparticles (AgNPs)-embedded Alg polymer films were synthesized and characterized using standard techniques. Further, a customized version of the Gafchromic EBT3 film, i.e., unlaminated EBT3 film, was specially fabricated. The DEFs were measured using Xoft Axxent electronic brachytherapy device. The surface plasmon resonance (SPR) and particle size of AuNPs were found to be 550 and 15 ± 2 nm, respectively. In the case of AgNPs, the SPR and particle size were recorded as 400 and 13 ± 2 nm, respectively. The DEFs measured, using unlaminated EBT3 film, for Xoft Axxent electronic brachytherapy using AuNPs and AgNPs were 1.35 ± 0.02 and 1.20 ± 0.01, respectively. The increase in dose enhancement during nanoparticles-aided electronic brachytherapy can be attributed to dominance of photoelectric effect, due to the presence of low-energy X-rays. The investigation indicates that the Xoft Axxent electronic brachytherapy device is suitable for nanoparticles-aided brachytherapy.

Abstract PS14-24: Multi-center trial in the treatment of ductal carcinoma in situ of breast using intra-operative electronic brachytherapy technique, results

Abstract Purpose: To report on the technique of intra-operative electronic brachytherapy, local control, side effects and complications after a single fraction of 2000 cGy at the time of partial mastectomy. Material & Methods: 1200 patients with early stage invasive ductal carcinoma and carcinoma in situ were enrolled in the Multi-Center Trial. 246 (20.5%) patients among these 1200 patients had carcinoma in situ and were treated by a single fraction of 2000 cGy dose by intra-operative irradiation to the lumpectomy cavity using Xoft system of electronic brachytherapy at the time of partial mastectomy. 80% of the patients were Caucasian, 7% African American, 7% Hispanic and 6% others. Estrogen receptor was positive in 211 patients, negative in 28 patients and unknown/not assessed in 7 patients. Progesterone receptor was positive in 190 patients, negative in 45 patients and unknown/not assessed in 11 patients. All patients were BRCA negative. 34 (14%) patients had low grade tumors, 109 (44%) intermediate grade tumors, and 103 (42%) high grade tumors. Seroma occurred in 9.8%, induration in 7.6%, and significant fibrosis in 3.5% of the patients. 2/246 (1%) developed ipsilateral recurrence and one patient developed a gastrointestinal cancer. 118 (48%) patients had characteristics of ASTRO criteria being suitable, 108 (44%) cautionary and 20 (8%) unsuitable. Conclusion: The relatively early 2.3 year results of a single fraction of 2000 cGy by IORT to the lumpectomy cavity at the time of partial mastectomy using Xoft Axxent electronic brachytherapy in the treatment of carcinoma in situ of the breast is safe and associated with low morbidity and local recurrence with good to excellent cosmesis in 80% of the patients. Citation Format: Alam M.Nisar Syed. Multi-center trial in the treatment of ductal carcinoma in situ of breast using intra-operative electronic brachytherapy technique, results [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS14-24.

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.

Physics aspects of the Papillon technique—Five decades later

PurposeThe Papillon technique using 50-kVp soft X-rays to treat rectal adenocarcinomas was developed and clinically implemented in the 1960s. We describe differences between accurate dosimetry and clinical implementation of this technique that is extending from its very inception to date. Methods and MaterialsA renaissance of the Papillon technique occurred with two recently introduced 50-kVp systems: Papillon+ by Ariane and a custom-made rectal applicator (consisting of a surface applicator inserted into a proctoscope) by iCAD's Xoft Axxent Electronic Brachytherapy (eBT) System (iCad, Inc., Sunnyvale, CA). In contrast to the initial design, we investigated the impact of introducing a plastic lid, which would provide more reproducible and more accurate dose delivery across the rectal adenocarcinoma patient population. We use both parallel-plate chamber and radiochromic film dosimeters to determine differences in basic dosimetry characteristics (beam half-value layers, outputs, percent depth doses, and profiles) between the Xoft Electronic Brachytherapy rectal applicator system with and without the plastic lid in place. ResultsCompared to the open-cone applicator, the proposed applicator with the plastic lid produces a slightly harder (more penetrating) beam quality (half-value layer of 1.4 vs. 1.3-mm Al), but with reduced output (by 33%), and a slightly broader beam with flatness not worse than 3% and symmetry not worse than 2%. ConclusionsIn addition to characterizing beam properties modified by the possible introduction of the plastic cap, we also pointed out and addressed misconceptions in the use of radiochromic films for dose measurements at low-energy photon beams.

Acute and Chronic Complications in Breast Cancer Patients Treated with Intraoperative Radiation Therapy.

Intraoperative radiation therapy (IORT) permits the delivery of radiation therapy directly to the tumor bed at the time of surgery. Minimal data are available about the complications associated with this modality of treatment using the Xoft(®) Axxent Electronic Brachytherapy (Axxent) System. A total of 702 patients who received IORT using the Xoft(®) Axxent System at Hoag Memorial Hospital Presbyterian between June 2010-February 2016 were accrued in an IORT data registry study. The prospective and retrospective protocols were approved by the institutional review board and met the guidelines of their responsible governmental agency. Data were collected at 1week, 1month, 3months, 6months, 1year, and thereafter yearly. Acute complications were defined as those occurring within the first month. Chronic complications were those that persisted beyond 6months. Acute complications were observed in 21% of patients and included hematomas that required drainage, seromas requiring drainage more than 3 times, infections treated with antibiotics or surgery, necrosis requiring surgery, and erythema. Chronic complications were observed in 13% of patients and included seromas, fibrosis, and hyperpigmentation. The majority of acute and chronic problems from IORT were mild. If grade I erythema, fibrosis, and hyperpigmentation were removed, only 32 of 702 (4.6%) had significant complications. Our complication rates were comparable to those of the TARGIT trial. IORT is a modality that safely delivers radiation therapy to patients diagnosed with breast cancer. This technique allows women who cannot (or decline to) undergo whole breast radiation to consider breast-conserving therapy rather than mastectomy.

SU‐F‐T‐45: Dosimetric Effects of Saline Filled Balloons During IORT Using Xoft Electronic Brachytherapy

Purpose:The portability of Xoft Axxent Electronic Brachytherapy (EBx) System has made it a viable option for intraoperative radiation therapy (IORT) treatment of early‐stage breast cancer. The low energy (50kVp) of the X‐ray source makes the shielding easy, but also means its dose distribution is sensitive to the medium's composition. Current treatment planning systems (TPS) typically assume homogenous water for brachytherapy dose calculations, including the pre‐calculated atlas plans for the Xoft IORT cases. However, Xoft recommends using saline to fill the balloon applicator. This study investigates the dosimetric difference due to the increased effective atomic number (Zeff) from water (7.42) to saline (7.56).Methods:The diameter of the balloon applicators ranges from 3–6cm, with 4cm being most frequently used. For the 4‐cm and 6‐cm diameter applicators, MCNP Monte Carlo program was used to calculate the dose at the surface (Ds) of the middle section of the balloon and 1 cm away (D1cm) for water‐ and saline‐filled balloons: one plan with a single dwell at the center and another with multiple dwells as in the atlas plans. The single dwell plan is a simple estimation of the dosimetry, while the atlas plan is representative of the actual dose distribution.Results:The single‐dwell plan showed a 5.1% and 6.1% decrease in Ds for the 4‐ and 6‐cm applicators, respectively, due to the saline. The atlas plan showed similarResults:4.8% and 6.4% decrease, respectively. The decrease in D1cm is 4.3%–5.2% and 3.3%–5.3s% in the single‐dwell and atlas plans, respectively, for the 4‐ and 6‐cm applicator.Conclusion:The dosimetric effect introduced by saline is on the order of 5%. This effect should be taken into account during both treatment planning and patient outcome studies.

SU‐E‐T‐421: Failure Mode and Effects Analysis (FMEA) of Xoft Electronic Brachytherapy for the Treatment of Superficial Skin Cancers

Purpose:To perform a failure mode and effects analysis (FMEA) of the process for treating superficial skin cancers with the Xoft Axxent electronic brachytherapy (eBx) system, given the recent introduction of expanded quality control (QC) initiatives at our institution.Methods:A process map was developed listing all steps in superficial treatments with Xoft eBx, from the initial patient consult to the completion of the treatment course. The process map guided the FMEA to identify the failure modes for each step in the treatment workflow and assign Risk Priority Numbers (RPN), calculated as the product of the failure mode's probability of occurrence (O), severity (S) and lack of detectability (D). FMEA was done with and without the inclusion of recent QC initiatives such as increased staffing, physics oversight, standardized source calibration, treatment planning and documentation. The failure modes with the highest RPNs were identified and contrasted before and after introduction of the QC initiatives.Results:Based on the FMEA, the failure modes with the highest RPN were related to source calibration, treatment planning, and patient setup/treatment delivery (Fig. 1). The introduction of additional physics oversight, standardized planning and safety initiatives such as checklists and time‐outs reduced the RPNs of these failure modes. High‐risk failure modes that could be mitigated with improved hardware and software interlocks were identified.Conclusion:The FMEA analysis identified the steps in the treatment process presenting the highest risk. The introduction of enhanced QC initiatives mitigated the risk of some of these failure modes by decreasing their probability of occurrence and increasing their detectability. This analysis demonstrates the importance of well‐designed QC policies, procedures and oversight in a Xoft eBx programme for treatment of superficial skin cancers. Unresolved high risk failure modes highlight the need for non‐procedural quality initiatives such as improved planning software and more robust hardware interlock systems.

SU‐E‐T‐171: Characterization of the New Xoft Axxent Electronic Brachytherapy Source Using PRESAGE Dosimeters

Purpose:To characterize the Xoft Axxent electronic brachytherapy source using PRESAGE™ dosimeters to obtain independent confirmation of TG‐43U1 dosimetry values from previous studies and ascertain its reproducibility in HDR brachytherapy.Methods:PRESAGE™ dosimeters are solid, polyurethane‐based dosimeters doped with radiochromic leucodyes that produce a linear optical‐density response when exposed to radiation. Eight 1‐kg dosimeters were scanned prior to irradiation on an optical‐CT scanner to eliminate background signal and any optical imperfections from each dosimeter. To quantify potential imaging artifacts due to oversaturated responses in the immediate range of the source, half of the eight dosimeters were cast with a smaller channel diameter of 5.4 mm, and the other half were cast with a larger channel diameter of 15mm. During irradiation, the catheters were placed in the center of each channel. Catheters fit the 5.4mm diameters channels whereas polyurethane plugs were inserted into the larger channels to create a sturdy, immobile catheter which allowed uniform dose distributions. Two dosimeters of each 5.4mm and 15mm were irradiated at either 1517.3 cGy or 2017.5 cGy. Post‐irradiation scans were performed within 48 hours of irradiation. A 3D reconstruction based on subtraction of these two images and the relative dose measurements were made using in‐house software.Results:Comparing measured radial dose rates with previous results revealed smaller percent errors when PRESAGE™ irradiations were at lower maximum dose. The dosimeters showed small deviations in radial dose function, gp (r), from previous studies. Among the dosimeters irradiated at 1517.3 cGy, the gp(r) compared to previous studies fluctuated from 0.0043 to 0.3922. This suggests small fluctuations can drastically change radial dose calculations.Conclusion:The subtraction of pre‐irradiation and post‐irradiation scans of PRESAGE™ dosimeters using an optical‐CT scanner shows promising results in determining 3D dosimetry for Xoft Axxent devices; however, further research is recommended.NIH Grant#: 5‐U24‐CA081647‐13; ROI Grant#: 5R01CA100835

SU-E-T-85: Ionization Chamber Measured Dose-Rate Constant of the Xoft Axxent Electronic Brachytherapy Source

Purpose: To measure the dose‐rate constant of the Xoft Axxent electronic brachytherapy source with an air‐kerma calibrated ionization chamber. Methods: An air‐kerma calibrated chamber was used in TG‐43 geometry (source and chamber in liquid water) to measure the dose rate of the Axxent source at several distances. The air‐kerma strength was determined with the University of Wisconsin Attix free‐air chamber. Since there is no national standard for absorbed dose to water at Axxent energies, this work followed the formalism of AAPM's TG‐61 protocol to obtain dose to water from the air‐kerma calibrated chamber. A Monte Carlo model of the chamber was created to calculate the necessary corrections to use the air‐kerma calibrated chamber to obtain dose to water from chamber measurements. The chamber model was benchmarked with energy dependence simulations. The dose‐rate constant was calculated from chamber measured dose rates and free‐air chamber measured air‐kerma strengths. Results: The simulated energy dependence using the PinPoint chamber model showed agreement to within 4.2% of the measured values. The chamber‐measured dose‐rate constant was 4.6% higher than the Monte Carlo dose‐rate constant and 13% higher than the TLD value, but the difference between the chamber‐measured values and TLD‐measured values decreased as the distance increased. Conclusions: Using an accurate chamber model to calculate corrections allows an air‐kerma calibrated chamber to be used for dose‐rate measurements of a brachytherapy source. The chamber values agree with TLD values as distance increases implying that positioning uncertainties may cause greater discrepancies at shorter distances and the spectrum used for simulations may be more accurate at the larger distances. An air‐kerma calibrated chamber was used to measure the dose‐rate constant of the Axxent source provided corrections were calculated with an accurate chamber model. This work was partially sponsored by Xoft, Inc.

The Initial Experience of Electronic Brachytherapy for the Treatment of Non-melanomatous Cutaneous Malignancies

To evaluate the acute adverse effects and early cosmetic outcomes of patients treated with electronic brachytherapy for the treatment of cutaneous malignancies. From July 2009 till March 2010, 34 patients with 40 cutaneous malignancies were treated with Xoft Axxent Electronic Brachytherapy system using surface applicators to a dose of 40.0 Gy in 8 fractions, 5 Gy per fraction, delivered twice weekly with minimum of 48 hour interval. The appropriately sized surface applicator, 10mm, 20mm, 35mm, or 50mm, was selected to allow for complete coverage of target lesion with acceptable margin. CT scan was performed to assess skin depth. The prescription depth was 3-7mm, 5mm for 83% of the lesions, 3mm for 15% of the fractions, and 7mm for the patient with T-cell lymphoma. All patients were treated outside of a linear accelerator vault in CT simulator room. Photographs were taken for all patients at initial consultation and follow-up visits. Acute safety outcomes and cosmetic results were analyzed. Cosmetic results were documented in the patient record using a standardized cosmesis scale. There were 34 T1 lesions and 6 T2 lesions; 38 were primary cancers, and 2 were recurrences. 26 skin lesions were basal cell carcinoma, 12 were squamous cell carcinoma, 1 was Merkle cell carcinoma, and 1 was T-cell lymphoma. 17 lesions were located on the nose, 5 on the ear, 10 on the face, 4 on the scalp, and 4 on the extremities. Median follow-up was 3.2 months (range, 1-7.3). Average treatment time was 6.8 minutes per fraction. 91% developed radiation dermatitis grade 1 or 2 during treatment and resolved by the 1 month, 3 month or 6 month visit. There were no severe (Grade 3 or higher) acute toxicities. 1 patient developed moist desquamation during treatment which resolved at 1-month follow-up. One patient developed hypopigmentation at 3 months, and 3 patients developed hypopigmentation at 6 months. Cosmetic assessments of the treated areas done at various intervals from the last fraction are grouped for reporting: 25 were assessed at 1 month, 17 at 3 months, 8 at 6 months, and 1 at 7 months. The cosmetic results showed 88.0%, 94.1% 71.4%, 100.0% excellent at 1, 3, 6 and 7 months respectively and 12%, 5.9%, and 28.6% showed good results at 1, 3, and 6 months respectively. There was no evidence of tumor recurrence for all lesions. The early outcomes of electronic brachytherapy for the treatment of non-melanomatous cutaneous malignancies appear to show acceptable acute toxicities and favorable early cosmesis. The early results seem comparable to High Dose Rate Brachytherapy. The hypofractionated approach provides patient convenience approach with effective early outcomes. Long-term follow-up is being collected to further assess efficacy and cosmesis.

Initial Results of a Multi-center Trial Utilizing Xoft Axxent Electronic Brachytherapy to Deliver Intraoperative Radiation Therapy in the Treatment of Early-stage Breast Cancer

CONCLUSION Post-operative whole breast radiation therapy (WBRT) has been shown to decrease local cancer recurrence rates by 30%, as well as to improve overall survival compared to surgery alone. However, delivered over a period of 6 to 7 weeks, WBRT can become a burdensome undertaking due to required commitment of time and resources, thus necessitating many women to either elect mastectomy or omit post-operative radiation altogether.

SU-GG-T-38: Dosimetry and Inverse Treatment Planning for 3D Intensity Modulated Brachytherapy

Purpose: To propose a dosimetry algorithm for three‐dimensional (3D) intensity modulated brachytherapy (IMBT) and to develop an inverse treatment planning method applying the dosimetry algorithm. Method and Materials: A Matlab based prototype 3D IMBT treatment planning system was developed. The system consisted of three main components: (1) a comprehensive source commissioning method for intensity modulated sources based on Monte Carlo (EGSnrc) simulations; (2) a “modified TG43” (mTG43) dose calculation algorithm for IMBT dosimetry; (3) an inverse IMBT treatment planning method based on Dose Volume Histogram (DVH) or DoseSurface Histogram (DSH) constraints and simulated annealingoptimization algorithm. The system was applied for planning of an intracavitary accelerated partial breast irradiation (APBI) case treated with Xoft Axxent electronic brachytherapy. Plan quality, planning and delivery time of the IMBT plan were compared with the original plan used for the patient's treatment. Results: For the patient studied, IMBT plan showed better plan quality compared with the original plan. With similar coverage to the target, high dose region V200 was decrease by 16.1%. Maximum doses to skin and ribs were reduced by 56 cGy and 104 cGy in one fraction respectively. Mean dose to ipsilateral and contralateral breasts and lungs were also slightly reduced by IMBT. Conclusion: Application of three‐dimensional intensity modulation in brachytherapytreatment planning is both feasible and promising. 3D IMBT improves the quality of APBI brachytherapy treatment plan, increasing dose uniformity in target and reducing the dose to critical structures.

SU-GG-T-492: Determining RBE Values for a New Miniaturized X-Ray Source

Purpose: To quantify the Relative Biological Effectiveness (RBE) of the Xoft Axxent Electronic Brachytherapy® (XAEB) system for MCF7 human breast cancercells in vitro. Method and Materials: MCF7 cells were irradiated with 0, 2, 4 and 6Gy using a Theratron1000 Co‐60 machine or the XAEB system (at 3cm in air). After irradiation, cells were plated in triplicate and cultured in complete medium. Three weeks later, the cultures were stained and viable colonies enumerated. A survival curve for cellstreated with each radiation source was generated. Data from two separate experiments were combined and RBE values were calculated. A third experiment is currently underway. Results: XAEB system RBE values were calculated using Co‐60 as the standard. The RBE for 37% survival was determined to be 2.2 based upon 37% survival doses of 2.8Gy versus 1.3Gy for Co‐60 versus the XAEB source, respectively. The RBE for 10% survival was determined to be 1.6 (8.8 Gy vs. 5.5 Gy, respectively). The RBE for 1% survival was determined to be 1.6 (8.8 Gy vs. 5.5 Gy, respectively). The current clinical recommendation for the XAEB system lists the theoretical RBE as 1.2 based on Monte Carlo simulations.Conclusion: Our results demonstrate that the RBE of the Xoft Axxent Electronic Brachytherapy® system can be up to 83% higher than the theoretical value quoted in the system's literature. Due to significant low‐energy components of the 50kVp XAEB system output, treatment filtration has a large effect on the x‐ray spectrum, which may account for the pronounced discrepancy in empirical and theoretical RBE values. Other possible factors include cell type and endpoint used for assessment. Our study, therefore, underscores the importance of defining and understanding the effects of the therapeutic setup to achieve optimal outcome. Research sponsored by Xoft Corporation

Comparison of organ doses for patients undergoing balloon brachytherapy of the breast with HDR or electronic sources using Monte Carlo simulations in a heterogeneous human phantoma)

Accelerated partial breast irradiation via interstitial balloon brachytherapy is a fast and effective treatment method for certain early stage breast cancers. The radiation can be delivered using a conventional high-dose rate (HDR) 192Ir gamma-emitting source or a novel electronic brachytherapy (eBx) source which uses lower energy x rays that do not penetrate as far within the patient. A previous study [A. Dickler, M. C. Kirk, N. Seif, K. Griem, K. Dowlatshahi, D. Francescatti, and R. A. Abrams, "A dosimetric comparison of MammoSite high-dose-rate brachytherapy and Xoft Axxent electronic brachytherapy," Brachytherapy 6, 164-168 (2007)] showed that the target dose is similar for HDR 192Ir and eBx. This study compares these sources based on the dose received by healthy organs and tissues away from the treatment site. A virtual patient with left breast cancer was represented by a whole-body, tissue-heterogeneous female voxel phantom. Monte Carlo methods were used to calculate the dose to healthy organs in a virtual patient undergoing balloon brachytherapy of the left breast with HDR 192Ir or eBx sources. The dose-volume histograms for a few organs which received large doses were also calculated. Additional simulations were performed with all tissues in the phantom defined as water to study the effect of tissue inhomogeneities. For both HDR 192Ir and eBx, the largest mean organ doses were received by the ribs, thymus gland, left lung, heart, and sternum which were close to the brachytherapy source in the left breast, eBx yielded mean healthy organ doses that were more than a factor of approximately 1.4 smaller than for HDR 192Ir for all organs considered, except for the three closest ribs. Excluding these ribs, the average and median dose-reduction factors were approximately 28 and approximately 11, respectively. The volume distribution of doses in nearby soft tissue organs that were outside the PTV were also improved with eBx. However, the maximum dose to the closest rib with the eBx source was 5.4 times greater than that of the HDR 192Ir source. The ratio of tissue-to-water maximum rib dose for the eBx source was approximately 5. The results of this study indicate that eBx may offer lower toxicity to most healthy tissues, except nearby bone. TG-43 methods have a tendency to underestimate dose to bone, especially the ribs. Clinical studies evaluating the negative health effects caused by irradiating healthy organs are needed so that physicians can better understand when HDR 192Ir or eBx might best benefit a patient.

Modeling study for optimization of skin dose for partial breast irradiation using Xoft Axxent electronic brachytherapy applicator

Purpose Balloon brachytherapy with the MammoSite system (Hologic Inc., Bedford, MA) is a widely used approach for accelerated partial breast irradiation. Inherent to this approach, high skin doses can occur if the balloon to skin distance is small. This has been associated with late skin toxicity, particularly telangiectasia. The Xoft Axxent electronic brachytherapy balloon applicator (Xoft, Fremont, CA) is a novel device for accelerated partial breast irradiation. It is unique in that it uses an electronic 50-kV source. This source has a pronounced anisotropy with constriction of isodose distribution at the proximal end of the catheter. This anisotropy can be considered as an advantage to optimize skin dose when the cavity to skin distance is small. In this study, we simulated various balloon-insertion orientations to optimized skin surface dose. Methods Breast phantoms were constructed of tissue-equivalent material. Xoft Axxent balloon catheters were inserted at a distance of 6 mm from the surface. The catheter was placed at three different catheter to surface orientations: ( 1) perpendicular to the surface, ( 2) oblique to the surface (45°), and ( 3) parallel to the surface. Three-dimensional treatment planning was then performed using Nucletron's Plato planning system (Nucletron, Columbia, MD). Multiple dwell positions were used, and the dose was optimized to the target volume. The target volume was defined as volume from the balloon surface to 1-cm distance from the balloon surface or to the phantom surface (if less then 1 cm from the balloon surface). Target volume coverage was compared between plans using dose–volume histograms. Surface doses were compared using isodose line distribution and surface point doses. Plato planned surface doses were then verified by direct measurement using Landauer Dot InLight dosimeters (Landauer, Glenwood, IL). Results Excellent target coverage was obtained for all three catheter orientations with a D 95 of ≥95%. Surface dose was lowest for the perpendicular orientation with a calculated dose of 99%. The parallel orientation had the highest surface dose of 164%. The oblique orientation showed intermediate results with a surface dose of 117%. Measured surface doses were reproducible and correlated well with calculated values. Conclusion Optimized Xoft Axxent balloon catheter orientation using source anisotropy and multiple dwell positions can be used to minimize excessive skin dose and yet maintain optimal tumor cavity coverage when the cavity to skin distance is small. This has the potential to decrease skin late effects and improve cosmetic outcome. Further clinical study is warranted.

SU-FF-T-45: Sensitivity of the 50 KVp Xoft Axxent Electronic Brachytherapy Skin Applicator to Flattening Filter Design, Source-Filter Position, Applicator Alignment and Surface Collimation

Purpose: The Xoft Axxent Electronic Brachytherapy Skin Applicator and miniature x-ray source combination can be used for superficial skin or surface treatments. The applicator is a 25 mm SSD cone (1.0–5.0 cm diameter) with an embedded flattening filter. The applicator and source are decoupled, which could lead to variations in dose profile. Several potential problems are investigated for this system. Method and Materials: Dose profiles of the 35 mm diameter cone were measured in solid water using GAFChromic EBT film and in water using a stereotactic diode. Two flattening filter designs were examined. The effect of rotating the source or applicator independently, tilting the applicator with respect to the surface, modifying the source-filter distance, and skin collimation from 10 to 30 mm by means of tungsten aperture cutouts were examined. Results: The stepped disk filter design had flatness and asymmetry of 8.3% and 3.1%. The chamfered filter was 14.8% and 0.7% respectively. Rotation of the source and applicator produced a minimal change in flatness and asymmetry, with less than 2% change in flatness or asymmetry. The stepped-disk filter flatness changed by up to 7% with filter source displacement. Tipping the applicator off of the perpendicular axis by 6 degrees changed the symmetry by 14%. Beam profiles for cutouts ranging from 1.0–3.0 showed little change in flatness and asymmetry, and the sharp edges of the PTW were retained. Conclusion: The stepped filter design is superior. The source-filter is sufficiently concentric to prevent profile shifts, but the dose is very sensitive to the filter-source distance, as predicted by the geometric model. Small tilts of up to 2 degrees, or a 1 mm gap on one side of the cone, can be tolerated. Skin collimation preserves both profile and penumbra. Research Sponsored by Xoft, Inc.

Treatment planning considerations for electronic brachytherapy vaginal cylinder applicators

Purpose: To develop a device-specific treatment planning procedure for the Xoft Axxent (Xoft, Inc., Sunnyvale, CA) electronic brachytherapy (EBT) vaginal cylinder applicators using the following: Analysis of marker catheter positioning; verification of marker catheter/source position accuracy; verification of the dose distribution surrounding the cylinder; and, evaluation of alternative treatment planning approaches.

Interface dosimetry for electronic brachytherapy Xoft balloon applicators

Purpose: To measure dose in the region between the surface of the Xoft Axxent electronic brachytherapy (EBT) balloon applicator for accelerated partial breast irradiation (APBI) and the prescription point at 1cm depth in tissue. The previously reported 6.0% attenuation factor of the barium impregnated balloon surface was investigated.

Xoft Axxent® electronic brachytherapy—a new device for delivering brachytherapy to the breast

Balloon-based brachytherapy was developed to simplify the brachytherapy technique and make accelerated partial breast irradiation more accessible to patients with breast cancer who are suitable candidates for this technique. Xoft Axxent (Xoft, Inc., Sunnyvale, CA) electronic brachytherapy is a novel method of accelerated partial breast irradiation that uses an electronic source to produce X-rays. Xoft Axxent treatment does not require a high-dose-rate afterloader unit or a shielded vault, unlike other brachytherapy techniques that use iridium-192, such as MammoSite brachytherapy. Xoft Axxent is associated with the delivery of less radiation to normal tissues, and increased high radiation dose regions or 'hot spots' to the target volume compared with treatment with the MammoSite device. Further research will be needed to determine subgroups of patients who might benefit from treatment with Xoft Axxent electronic brachytherapy.

A dosimetric comparison of Xoft Axxent Electronic Brachytherapy and iridium-192 high-dose-rate brachytherapy in the treatment of endometrial cancer

Purpose This analysis was undertaken to dosimetrically compare iridium-192 high-dose-rate brachytherapy (IB) and Xoft Axxent Electronic Brachytherapy (XB; Xoft Inc., Sunnyvale, CA) in the treatment of endometrial cancer. Methods and Materials The planning CT scans from 11 patients previously treated with IB were used to construct hypothetical treatment plans using the source characteristics of the XB device. The mean V 95, V 100, and V 150 (percent of the planning target volume that received 95%, 100%, and 150% of the prescription dose) were calculated. For both the bladder and rectum, the V 35 (percent of the organ that received 35% of the prescription dose) and V 50 (percent of the organ that received 50% of the prescription dose) were calculated for each patient using both methods of vaginal brachytherapy. Results The mean % V 95 was 99.7% vs. 99.6% ( p = ns) and the mean % V 100 was 99.0% vs. 99.1% ( p = ns) for the IB and XB methods, respectively. The mean % V 150 was 35.8% vs. 58.9% ( p < 0.05) for the IB and XB methods, respectively. The mean bladder % V 35 was 47.7% vs. 27.4% ( p < 0.05) and the mean bladder % V 50 was 26.5% vs. 15.9% ( p < 0.05) for the IB and XB methods, respectively. The mean rectal % V 35 was 48.3% vs. 28.3% ( p < 0.05) and the mean rectal % V 50 was 27.8% vs. 17.0% ( p < 0.05) for the IB and XB methods, respectively. Conclusions The IB and XB methods of vaginal brachytherapy offer equivalent target volume coverage; however, the XB method allows increased sparing of the bladder and rectum.