Conventional HDR Research Articles

Enhancing vibration control in stay cables: a modified damping formulation with NS-HDR damper

Cables in cable-stayed bridges have low intrinsic damping, and dampers are often used as a countermeasure for cable vibration control. This paper presents an innovative asymptotic formula for calculating the additional damping in stay cables equipped with Negative Stiffness High Damping Rubber dampers (NS-HDR). The NS-HDR damper incorporates negative stiffness through a pre-compressed spring. The analysis employs models of flexural cables with fixed-fixed or hinged-hinged ends to derive the formulation of attainable damping ratio. The results of the study reveal that the NS-HDR damper, with its negative stiffness feature, exhibits a significantly higher added damping ratio in comparison to the conventional HDR damper configuration. To quantify this increased added damping resulting from negative stiffness, a modification factor is proposed. The accuracy and effectiveness of the proposed damping formula are successfully validated using the Finite Difference Method (FDM). Subsequently, the methodology is applied to design the damping of two existing stay cables (137.82m and 167.18m in length). Field measurements reveal that the damping in these cables falls below the required threshold of 0.5%. The proposed NS-HDR damper offers a viable solution to achieve the required damping ratio. These findings contribute significantly to the understanding and optimization of damping in stay cables employing HDR dampers, presenting potential applications in the field of bridge engineering. The research opens up new possibilities for enhancing vibration control and safety in cable-stayed bridges.

Advanced design, simulation, and dosimetry of a novel rectal applicator for contact brachytherapy with a conventional HDR 192Ir source

PurposeDose escalation yields higher complete response to rectal tumors, which may enable the omission of surgery. Dose escalation using 50 kVp contact x-ray brachytherapy (CXB) allow the treatment of a selective volume, resulting in low toxicity and organs-at-risk preservation. However, the use of CXB devices is limited because of its high cost and lack of treatment planning tools. Hence, the MAASTRO applicator (for HDR 192Ir sources) was developed and characterized by measurements and Monte Carlo simulations to be a cost-effective alternative to CXB devices. Methods and MaterialsA cylindrical applicator with lateral shielding was designed to be used with a rectoscope using its tip as treatment surface. Both the applicator and the rectoscope have a slanted edge to potentially allow easier placement against tumors. The applicator design was achieved by Monte Carlo modeling and validated experimentally with film dosimetry, using the Papillon 50 (P50) device as reference. ResultsThe applicator delivers CXB doses in less than 9 min using a 20375 U source for a treatment area of approximately 20 × 20 mm2 at 2 mm depth. Normalized at 2 mm, the dose falloff for depths of 0 mm, 5 mm, and 10 mm are 130%, 70%, and 43% for the P50 and 140%, 67%, and 38% for the MAASTRO applicator, respectively. ConclusionsThe MAASTRO applicator was designed to use HDR 192Ir sources to deliver a dose distribution similar to those of CXB devices. The applicator may provide a cost-effective solution for endoluminal boosting with clinical treatment planning system integration.

A novel 169 Yb-based dynamic-shield intensity modulated brachytherapy delivery system for prostate cancer.

Intensity modulated brachytherapy (IMBT) is a novel high dose rate brachytherapy (HDR BT) technique which incorporates static or dynamic shielding to increase tumor coverage and/or spare healthy tissues. The purpose of this study is to present a novel delivery system (AIM-Brachy) design that can enable dynamic-shield IMBT for prostate cancer. The AIM-Brachy system dynamically controls the rotation of platinum shields, placed within interstitial catheters, which partially collimate the radiation emitted from an 169 Yb source. Conventional HDR BT (10Ci 192 Ir) and IMBT (18Ci 169 Yb) plans were generated for 12 patients using an in-house column generation-based optimizer, coupled to a Geant4-based dose calculation engine, RapidBrachyMC. Treatment plans were normalized to match the same PTV D90 coverage as the clinical plan. Intershield attenuation effects were taken into account. A sensitivity analysis was performed to evaluate the dosimetric impact of systematic longitudinal source positioning errors ( 1mm, 2mm, and 3mm) and rotational errors ( 5 , 10 and 15 ) on clinically relevant parameters (PTV D90 and urethra D10 ). The platinum shield reduced the dose rate on the shielded side at 1cm to 18.1% of the dose rate on the unshielded side. For equal PTV D90 coverage, the urethral D10 was reduced by 13.3% 4.7%, without change to other plan quality indices (PTV V100 , V150, V200 , bladder V75 , rectum V75 , HI, COIN). Delivery times for HDR BT and IMBT were 9.2±1.6min and 18.6±4.0min, respectively. In general, the PTV D90 was more sensitive to source positioning errors than rotational errors, while the urethral D10 was more sensitive to rotational errors than source positioning errors. For a typical range of positioning errors ( 1mm, 5 ), the overall tolerance was <2%. The AIM-Brachy system was proposed to deliver dynamic-shield IMBT for prostate cancer with the potential to create a low dose tunnel within the urethra. The urethra-sparing properties are desirable to minimize the occurrence and severity of urethral strictures or, alternatively, to provide a method for dose escalation.

Treatment of cervical cancer with electronic brachytherapy.

PurposeWe report the first cervical cancer cases treated with interstitial electronic brachytherapy (eBT) at our hospital and compare them with plans made with high‐dose‐rate interstitial brachytherapy based on Ir192 (HDR‐BT).Materials and methodsEight patients with cervical cancer were treated with the Axxent eBT device (Xoft, Inc.). Planning was with magnetic resonance imaging and computed tomography following the recommendations of the EMBRACE protocol. The dosimetry parameters of organs at risk (OAR) were evaluated for the bladder, rectum, and sigmoid colon (D2cc, D1cc, and D0.1cc). In addition, the V150 and V200 of irradiated tissue were compared for both eBT and HDR‐BT. All patients received intensity‐modulated external beam radiation therapy with a regimen of 23 sessions of 2 Gy followed by four sessions of 7 Gy of eBT performed over 2 weeks (two sessions followed by another two sessions a week later) following the EMBRACE recommendations. Each of the eight patients was followed to assess acute toxicity associated with treatment.ResultsThe doses reaching OAR for eBT plans were lower than for HDR‐BT plans. As for acute toxicity associated with eBT, very few cases of mucositis were detected. No cases of rectal toxicity and one case with grade 1 urinary toxicity were detected. The results at 1 month are equally good, and no relapses have occurred to date.ConclusionsThe first results of treatment with the Axxent eBT device are promising, as no recurrences have been observed and toxicity is very low. eBT is a good alternative for treating cervical cancer in centers without access to conventional HDR.

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.

SU‐E‐T‐583: Optimizing Dosimetric Indices for HDR Brachytherapy Using CVaR

Purpose: To develop a new model for automated optimization of HDR brachytherapy (BT) dose distributions that operates on the dose volume indices used in plan evaluation. The model can be solved to optimality, with constraints that are easy to interpret and modify for the clinical user. Recent research has shown that the optimization model hitherto used corresponds only weakly to such dosimetric indices. Alternative models that include such dosimetric indices have been presented; however, the explicit inclusion of dosimetric indices yields intractable models. Methods: An approach for optimizing HDR BT dose distributions is presented where dosimetric indices are taken into account through surrogates based on the conditional value‐at‐risk concept (CVaR). This yields a linear optimization model that is easy to solve, and has the additional advantage that the constraints are easy to interpret and modify to obtain satisfactory dose distributions. Results: Experimental comparisons have shown that our proposed model corresponds well with constraining dosimetric indices. All modifications of the parameters in our model yield the expected Result. Both our approach and those presented earlier for optimization of dosimetric indices include an approximation, earlier models solve the models approximately, and we use a model based on an approximation. This gives us better control of the optimization. The dose distributions generated by the proposed optimization model are comparable to those generated by the standard model with respect to dosimetric indices. Conclusion: Our new model is a viable surrogate to optimizing BT dosimetric indices that quickly and easily yield high‐quality dose distributions. New optimization models that are capable of optimizing dosimetric indices will become of increasing importance in new applications involving more degrees of freedom than conventional HDR 192Ir BT such as electronic BT.

Potential dose-conformity advantages with multi-source intensity-modulated brachytherapy (IMBT)

The possibilities for optimizing brachytherapy by including additional degrees of freedom in source design were investigated. This included examining optimised dose delivery with a brachytherapy source that can provide intensity-modulated dose delivery in angle about the source travel direction (to achieve intensity-modulated brachytherapy-IMBT). A prostate HDR case was selected as an example. An inverse planning algorithm was used to define how an asymmetric radiation source can be controlled in multiple source catheters to maximize tumour dose coverage and minimize urethral and rectal doses. Substantial improvements in conformity in terms of tumour coverage and urethral dose reduction could be achieved when conventional HDR source positioning was used with IMBT. With the objective definition used in the example however, rectal doses could not be improved over those delivered via conventional HDR. When source position was included as a variable in IMBT, significant conformity improvements result for all structures. IMBT would be a technically challenging form of therapy that would be strongly influenced by the type of sources that could be created for it. This study has shown however that there is a potential for improving dose conformity with such a therapy. Introduction of IMBT techniques would require conventional brachytherapy concepts to be radically modified.

TU-EE-A1-05: The Use of Directional Interstitial Sources to Reduce Skin Dose in Breast Brachytherapy

Purpose: To investigate the feasibility of reducing the skindose with temporary LDR multicatheter breast implants with the use of directional 125I interstitial sources in comparison to conventional HDR interstitial breast brachytherapy.Method and Materials: The treatment plan for a patient treated with HDR interstitial brachytherapy with 192Ir was compared to a directional 125I treatment plan in the same dataset. Directional sources contain an internal radiation shield that greatly reduces the intensity of radiation in the shielded direction. They have a similar dose distribution to non‐directional sources on the unshielded side. Several dosimetric parameters are compared including target volume coverage, dose homogeneity index, and the skin surface areas receiving 30%, 50% and 80% of the prescription dose (S30, S50 and S80, respectively). The HDR prescription dose was 34 Gy in 10 fractions. Results: Similar excellent target coverage was achieved by both directional LDR and HDR (99.2% and 97.5%, respectively). Moreover, for a 170‐cc target volume, the dose homogeneity index was 0.82 for both LDR and HDR (V100 was 211.4 cc or 225.7 cc, and V150 was 39.1 cc or 40.4 cc, respectively). However, with directional LDR, the following reductions in skindose may be achieved: S30 is reduced from 100.6 cm2 to 62.6 cm2, S50 from 50.6 cm2 to 16.1 cm2, and S80 at 2 cm2 to null. The reduction in V50 for the whole breast is more than 100 cm3 (386.1 cc vs. 489.2 cc). Conclusion: As compared to HDR, directional interstitial 125I sources allow similar dose coverage to the subcutaneous target, while significantly lowering the skindose due to a quicker fall‐off beyond the target. Directional LDR sources can produce a similar dose homogeneity index, but the biological characteristics are more tolerable to the patient and can potentially reduce the risk of late skin and subcutaneous toxicity.

SU‐EE‐A1‐04: Integrated Complementary IMRT Combined with Image‐Guided HDR Brachytherapy for Cancers of the Uterine Cervix

Purpose: Conventional HDR (CHDR) brachytherapy for cervical cancers often fails to cover the entire gross tumor volume (GTV). CT/MRI‐based 3‐D planning allows optimization of dwell times for improved target coverage, but often at the cost of substantially higher doses to organs at risk (OARs). In the present study, we integrate image‐guided HDR brachytherapy with concomitant complementary IMRT to boost target coverage and maintain low doses to OARs. Method and Materials: Treatment planning CT and MRI images of 6 patients were acquired with CT/MRI‐compatible carbon‐fiber applicator. HDR plans were obtained by modifying CHDR plans to keep bladder and rectum dose below 80%. Based on these HDR plans, IMRT plans were subsequently optimized to complement the dose coverage of GTV with 3‐mm margin (GTV+) and the uterus. To prevent anatomical deformation, IMRT plans were to be delivered immediately following image‐guided HDR treatment with applicator in place. V95% (Target volume receiving 95% prescription dose) and D2 (minimum dose in 2.0‐cm3 OAR volume receiving the highest dose) were used to compare IMRT‐HDR technique with CHDR and HDR optimized for target coverage (OHDR). Results: The MRT‐HDR technique substantially improved target coverage while maintaining bladder and rectum doses at acceptable low level. For the six patients, V95% for GTV+ and the uterus improved from the average of 56.8% and 51.4% in CHDR to 92.5% and 95.3% in IMRT‐HDR, respectively. Average D2 doses to bladder and rectum were 88.7% and 65.2% for CHDR and 72.4% and 71.4% for IMRT‐HDR. In contrast, OHDR improved target coverage, but D2 doses to bladder (&gt;119%) and rectum (87.3% to &gt; 200%) were unacceptably high. Conclusion: Integrated complementary IMRT combined with image‐guide HDR significantly improves dose coverage to the targets while maintaining low doses to OARs. It is dosimetrically and logistically feasible to clinically implement this technique for potentially improved outcome.

Biologic treatment planning for high-dose-rate brachytherapy

Purpose: Interstitial brachytherapy treatment plans are conventionally optimized with respect to total target dose and dose homogeneity, which does not account for the biologic effects of dose rate. In an HDR implant, with a stepping source, the dose rate dramatically changes during the course of treatment, depending on location, as the source moves from dwell position to dwell position. These widely varying dose rates, together with the related sequencing of the dwell positions, may impart different biologic effects at points receiving the same total dose. This study applies radiobiologic principles to account for the potential biologic impact of dose delivery at varying dose rates within an HDR implant. Methods and Materials: The model under study uses a generalized version of the linear-quadratic (LQ) cell kill formula to calculate the surviving fraction of cells subjected to HDR irradiation. Using a planar interstitial HDR implant with the dwell times optimized to produce a homogeneous dose distribution along a reference plane parallel to the implant plane, surviving fractions were compared at selected reference points subjected to the same total dose. Biologic effect homogeneity was compared to dose homogeneity by plotting the effects at the reference points. The effects were examined with LQ parameters α, β, and sublethal repair time T 1 varied over a range typical of human cells. Results: In a region in which dose is relatively uniform, surviving fraction for some values of the model parameters are found to vary by as much as an order of magnitude due to differences in the HDR irradiation profiles at different dose points. This effect is more pronounced for shorter repair times and smaller α/β ratios, and increases with increasing total irradiation time. Conclusion: Conventional HDR treatment planning currently considers dose distribution as the primary indicator of clinical effect. Our results demonstrate that plans optimized to maximize homogeneity within a target volume may not reflect the effect of the sequential nature of HDR dose delivery on cell kill. Biologic effect modeling may improve our understanding and ability to predict the adverse effects of our treatment, such as fat necrosis and fibrosis. Accounting for irradiation history and repair kinetics in the evaluation of HDR brachytherapy plans may add an important new dimension to our planning capabilities.