A novel approach to infectious disease control and radiotherapy risk management.

BACKGROUND: Infectious disease outbreaks have always presented challenges to the operation of healthcare systems. In particular, the treatment of cancer patients within Radiation Oncology often cannot be delayed or compromised due to infection control measures. Therefore, there is a need for a strategic approach to simultaneously managing infection control and radiotherapy risks. PURPOSE: To develop a systematic risk management method that uses mathematical models to design mitigation efforts for control of an infectious disease outbreak, while ensuring safe delivery of radiotherapy. METHODS: A two-stage failure mode and effect analysis (FMEA) approach is proposed to modify radiotherapy workflow during an infectious disease outbreak. In stage 1, an Infection Control FMEA (IC-FMEA) is conducted, where risks are evaluated based on environmental parameters, clinical interactions, and modeling of infection risk. occupancy risk index (ORI) is defined as a metric for infection transmission risk level in each room, based on the degree of occupancy. ORI, in combination with ventilation rate per person (Rp ), is used to provide a broad infection risk assessment of workspaces. For detailed IC-FMEA of clinical processes, infection control failure mode (ICFM) is defined to be any instance of disease transmission within the clinic. Infection risk priority number (IRPN) has been formulated as a function of time, distance, and degree of protective measures. Infection control measures are then systematically integrated into the workflow. Since the workflow is perturbed by infection control measures, there is a possibility of introducing new radiotherapy failure modes or increased likelihood of existing failure modes. Therefore, in stage 2, a conventional radiotherapy FMEA (RT-FMEA) should be performed on the adjusted workflow. RESULTS: The COVID-19 pandemic was used to illustrate stage 1 IC-FMEA. ORI and Rp values were calculated for various workspaces within a clinic. A deep inspiration breath hold (DIBH) CT simulation was used as an example to demonstrate detailed IC-FMEA with ICFM identification and IRPN evaluation. A total of 90 ICFMs were identified in the DIBH simulation process. The calculated IRPN values were found to be progressively decreasing for workflows with minimal, moderate, and enhanced levels of protective measures. CONCLUSION: The framework developed in this work provides tools for radiotherapy clinics to systematically assess risk and adjust workflows during the evolving circumstances of any infectious disease outbreak.

Outcomes of 3D MRI based HDR brachytherapy with hybrid multichannel vaginal cylinder applicator and freehand needles for treatment of vaginal disease.

Freehand needles can be used with multichannel vaginal cylinders (MCVC) to cover vaginal cancer >7 mm thick or with supra-vaginal extension. We report our institutional outcomes using this novel hybrid technique. Patients with vaginal malignancies treated with HDR BT using MCVC plus freehand needles from 2014-2021 at our institution were identified. Clinical characteristics, details of brachytherapy, initial response, and overall local control (LC) outcomes were recorded. LC was analyzed via Kaplan-Meier method. 34 patients were identified with median follow-up 1.9 years. 19 patients had primary endometrial cancer with vaginal recurrence/disease, and remaining had primary vaginal cancer or other primaries. 7 patients had recurrence after previous RT course. 25 patients received EBRT with median dose 45 Gy in 25 fractions, and rest received BT alone. Median HR-CTV D90 for patients treated with EBRT plus BT was 77.4 Gy. 30 patients had complete local response to BT on initial examination and/or follow-up imaging. 1 and 2-year LC rates in patients without prior RT treated with EBRT + BT were 94.1% and 94.1%, respectively. 1 and 2-year LC rates for those without prior RT were 88.1% and 76.4%, respectively. 1 and 2-year LC rates for those with prior RT were 68.6% and 34.3%, respectively. 1 patient had vaginal laceration requiring surgical repair, and 1 patient developed small bowel obstruction 1 month after BT, with no additional acute grade 3+ toxicities identified. Our approach with MCVC plus freehand needles with MRI-based planning was feasible and safe, with excellent initial local response and low rate of serious toxicities.

Effect of inter-electrode RF coupling on heating patterns of wire-like conducting implants in MRI.

PURPOSE: In this study, the effects of RF coupling on the magnitude and spatial patterns of RF-induced heating near multiple wire-like conducting implants (such as simultaneous electrical stimulation of stereoelectroencephalography electrodes) during MRI were assessed. METHODS: Simulations and experimental measurements of RF-induced temperature increases near partially immersed wire-like conductors were performed using a phantom with a transmit/receive head coil on a 3T MRI system. The conductors consisted of either a pair of wires or a single simultaneous electrical stimulation of stereoelectroencephalography electrode with multiple contacts, and the locations and lengths of the conductors were varied to study the effect of electromagnetic coupling on RF-induced heating. RESULTS: The temperature increase near a wire within the phantom was dependent not only on its own location and length, but also on the locations and lengths of the other partially immersed wires. In the configurations that were studied, the presence of a second implant could increase the heating near the tip of the conductor by as much as 95%. CONCLUSION: The level of RF-induced heating during an MR scan is affected significantly by RF coupling when more than one wire-like implant is present. In some of the configurations studied, the heating was increased by the presence of a second conductor partially immersed in the phantom. Thus, RF coupling is an important factor to consider in the assessment of safety issues for MRI when multiple implants are present.

Can we reduce dose to ureters as avoidance organs for MRI based brachytherapy for cervical cancer? A dosimetric feasibility study.

BACKGROUND AND PURPOSE: Ureteral stenosis (US) is an underreported complication of brachytherapy (BT) for cervical cancer (CC), with limited data on toxicity risk reduction. A previous study demonstrated ureter EQD2 D0.1cc > 77 Gy correlated with US development. We sought to assess feasibility of this constraint while maintaining similar HR-CTV coverage. MATERIALS AND METHODS: Patients with locally advanced CC treated with EBRT plus HDR MRI-based brachytherapy boost without hydronephrosis at diagnosis and with ureter dose EQD2 D0.1cc > 77 Gy were included. Replan was attempted to achieve HR-CTV D90 ≥ 80-85 Gy and ureter dose reduction. Ureter distance from lateral margin of HR-CTV and tandem was recorded. t-test was performed to compare ureteral dose and HR-CTV D90. RESULTS: Of 25 patients were identified. Hundred percent received 45 Gy in 25 fractions to the pelvis ± paraaortic lymph nodes and 80% receiving median additional parametrial dose of 5.4 Gy. Replan meeting ureteral dose of ≤77 Gy was feasible in 18 of 25 patients, with a reduction in median ureter D0.1cc from 82.3 to 76.8 Gy (p < 0.001). Median HR-CTV D90 was similar (84.7 vs. 85.0 Gy). Replan achieved D0.1cc ≤77 Gy in 56% of patients who experienced US. All unilateral US cases occurred in the ureter closest to HR-CTV. CONCLUSIONS: Optimization to reduce ureter dose to ≤77 Gy is feasible when ureters are visible and contoured. Ureters may be considered as potential OAR during MRI-based brachytherapy treatment. Reduced ring to tandem total reference air kerma (TRAK) ratio may provide an additional metric by which to lower US risk.

Feasibility of improving patient's safety with in vivo dose tracking in intracavitary and interstitial HDR brachytherapy.

PURPOSE: The in vivo dosimetric monitoring in HDR brachytherapy is important for improving patient safety. However, there are very limited options available for clinical application. In this study, we present a new in vivo dose measurement system with a plastic scintillating detector (PSD) for GYN HDR brachytherapy. METHODS: An FDA approved PSD system, called OARtrac (AngioDynamics, Latham, NY), was used with various applicators for in vivo dose measurements for GYN patients. An institutional workflow was established for the clinical implementation of the dosimetric system. Action levels were proposed based on the measurement and system uncertainty for measurement deviations. From October 2018 to September 2019, a total of 75 measurements (48 fractions) were acquired from 14 patients who underwent HDR brachytherapy using either a multichannel cylinder, Venezia applicator, or Syed-Neblett template. The PSDs were placed in predetermined catheters/channels. A planning CT was acquired for treatment planning in Oncentra (Elekta, Version-4.5.2) TPS. The PSDs were contoured on the CT images, and the PSD D90% values were used as the expected doses for comparison with the measured doses. RESULTS: The mean difference from patient measurements was -0.22% ± 5.98%, with 26% being the largest deviation from the expected value (Syed case). Large deviations were observed when detectors were placed in the area where dose rates were less than 1 cGy/s. CONCLUSIONS: The establishment of clinical workflow for the in vivo dosimetry for both the intracavitary and interstitial GYN HDR brachytherapy will potentially improve the safety of the patient treatment.

Refining complex re-irradiation dosimetry through feasibility benchmarking and analysis for informed treatment planning.

PURPOSE/OBJECTIVES: The purpose of this study is to dually evaluate the effectiveness of PlanIQ in predicting the viability and outcome of dosimetric planning in cases of complex re-irradiation as well as generating an equivalent plan through Pinnacle integration. The study also postulates that a possible strength of PlanIQ lies in mitigating pre-optimization uncertainties tied directly to dose overlap regions where re-irradiation is necessary. METHODS: A retrospective patient selection (n = 20) included a diverse range of re-irradiation cases to be planned using Pinnacle auto-planning with PlanIQ integration. A consistent planning template was developed and applied across all cases. Direct plan comparisons of manual plans against feasibility-produced plans were performed by physician(s) with dosimetry recording relevant proximal OAR and planning timeline data. RESULTS AND DISCUSSION: All re-irradiation cases were successfully predicted to be achievable per PlanIQ analyses with three cases (3/20) necessitating 95% target coverage conditions, previously exhibited in the manually planned counterparts, and determined acceptable under institutional standards. At the same time, PlanIQ consistently produced plans of equal or greater quality to the previously manually planned re-irradiation across all (20/20) trials (P = 0.05). Proximal OAR exhibited similar to slightly improved maximum point doses from feasibility-based planning with the largest advantages gained found within the subset of cranial and spine overlap cases, where improvements upward of 10.9% were observed. Mean doses to proximal tissues were found to be a statistically significant (P < 0.05) 5.0% improvement across the entire study. Documented planning times were markedly less than or equal to the time contributed to manual planning across all cases. CONCLUSION: Initial findings indicate that PlanIQ effectively provides the user clear feasibility feedback capable of facilitating decision-making on whether re-irradiation dose objectives and prescription dose coverage are possible at the onset of treatment planning thus eliminating possible trial and error associated with some manual planning. Introducing model-based prediction tools into planning of complex re-irradiation cases yielded positive outcomes on the final treatment plans.

Mechanical analysis of an MgB2 1.5 T MRI main magnet protected using Coupling Loss Induced Quench.

Mechanical analysis of the stress and strains developed in the coils were calculated for a ten coil 1.5 T MRI magnet design with magnesium diboride (MgB2) wire protected with Coupling Loss Induced Quench (CLIQ). The temperature distribution inside the coils was first simulated in MATLAB to solve the governing heat and circuit equations. Simulations were performed on the magnet, in which each coil was divided into two subsections, with two CLIQ units while the capacitor ranged from 5 to 20 mF and the initial charging voltage ranged from 2.6 kV to 1.3 kV in order to keep the total stored energy in the CLIQ system constant. The wire's filamentary twist pitch remained constant at 5 cm for all simulations. The exported temperature distribution was expanded to form a representative unit cell (RUC) representing the wire composite and then imported into ANSYS to calculate the 1st principle strain in the MgB2 filament and shear stress across the epoxy for the coils. A peak temperature of 191 K occurred inside the coil with the initial quench when the CLIQ unit had a 20 mF capacitor charged to 1.3 kV. According to the mechanical simulations, the largest resulting peak strain in the wire was 0.034%, and peak shear stress was 44 MPa.

Measurements and simulation of RF heating of implanted stereo-electroencephalography electrodes during MR scans.

PURPOSE: To assess RF-induced heating during MRI of patients with implanted stereo-electroencephalography electrodes. METHODS: Simulations and experimental measurements using phantom and a head-only transmit/receive coil on a 3T MR system were performed to evaluate temperature increases at the tip of an 8-contact stereo-electroencephalography electrode and an insulated wire partially immersed into the phantom. The lengths of wire producing maximum (resonant condition) and minimum (anti-resonant condition) heating were evaluated for different entry modes and penetration depths. RESULTS: For both wire and stereo-electroencephalography electrode, resonant lengths were close to odd integral multiples of RF quarter wavelength in air and antiresonant length close to even integral multiples of RF quarter wavelength, both being unaffected by the entry mode. In the resonant condition, temperature increased by as much as a factor of 10 higher than that at antiresonant condition. Larger penetration depths did not change resonant length, but did lead to increased RF heating. CONCLUSION: For the partially immersed implants like stereo-electroencephalography electrode, the resonant lengths were found to be independent of the penetration depths and entry modes, although the temperature increases may vary. Avoiding such lengths of cables can reduce the risk of tissue heating during in vivo MRI.

Conceptual designs of conduction cooled MgB2 magnets for 1.5 and 3.0T full body MRI systems.

Conceptual designs of 1.5 and 3.0 T full-body magnetic resonance imaging (MRI) magnets using conduction cooled MgB2 superconductor are presented. The sizes, locations, and number of turns in the eight coil bundles are determined using optimization methods that minimize the amount of superconducting wire and produce magnetic fields with an inhomogeneity of less than 10 ppm over a 45 cm diameter spherical volume. MgB2 superconducting wire is assessed in terms of the transport, thermal, and mechanical properties for these magnet designs. Careful calculations of the normal zone propagation velocity and minimum quench energies provide support for the necessity of active quench protection instead of passive protection for medium temperature superconductors such as MgB2. A new 'active' protection scheme for medium Tc based MRI magnets is presented and simulations demonstrate that the magnet can be protected. Recent progress on persistent joints for multifilamentary MgB2 wire is presented. Finite difference calculations of the quench propagation and temperature rise during a quench conclude that active intervention is needed to reduce the temperature rise in the coil bundles and prevent damage to the superconductor. Comprehensive multiphysics and multiscale analytical and finite element analysis of the mechanical stress and strain in the MgB2 wire and epoxy for these designs are presented for the first time. From mechanical and thermal analysis of our designs we conclude there would be no damage to such a magnet during the manufacturing or operating stages, and that the magnet would survive various quench scenarios. This comprehensive set of magnet design considerations and analyses demonstrate the overall viability of 1.5 and 3.0 T MgB2 magnet designs.

A modified formula for defining tissue phantom ratio of photon beams.

Tissue phantom ratio (TPR), for square fields of various dimensions has been determined at varying depths in water. The dose in water has been measured at a fixed source-to-surface distance (SSD) of 100 cm and reference depth of 5 cm for 6 MV photon beam of Siemens Linear Accelerator Primus 11 in German Cancer Research Center (DKFZ), Heidelberg, Germany. A modified formula has been developed to calculate the TPR value for isocentric treatment. The present article describes the conversion of the measured data values into a comprehensive and consistent data set by the modified formula, that gives the TPR from Percentage Depth Dose (PDD) with depth as a function of field sizes from 10 mm x 10 mm upto 300 mm x 300 mm) and depth (from 0 mm to 300 mm).

Active-passive gradient shielding for MRI acoustic noise reduction.

An important source of MRI acoustic noise-magnet cryostat warm-bore vibrations caused by eddy-current-induced forces-can be mitigated by a passive metal shield mounted on the outside of a vibration-isolated, vacuum-enclosed shielded gradient set. Finite-element (FE) calculations for a z-gradient indicate that a 2-mm-thick Cu layer wrapped on the gradient assembly can decrease mechanical power deposition in the warm bore and reduce warm-bore acoustic noise production by about 25 dB. Eliminating the conducting warm bore and other magnet parts as significant acoustic noise sources could lead to the development of truly quiet, fully functioning MRI systems with noise levels below 70 dB.