Optimization and experimental characterization of the innovative thermo-brachytherapy seed for prostate cancer treatment

BACKGROUND

Adjuvant administration of hyperthermia (HT) with radiation therapy in the treatment of cancer has been extensively studied in the past five decades. Concurrent use of the two modalities leads to both complementary and synergetic enhancements in tumor management, but presents a practical challenge. Their simultaneous administration using the same implantable thermo-brachytherapy (TB) seed source has been established theoretically through magnetically mediated heat induction with ferromagnetic materials. Careful consideration, however, showed that regular ferromagnetic alloys lack the required conductivity to generate enough power through eddy current to overcome heat dissipation due to blood perfusion at clinically measured rates.

PURPOSE

We characterized the TB implant that combines a sealed radioactive source with a ferrimagnetic ceramic (ferrite) core, serving as a self-regulating HT source when placed in an alternating electromagnetic field. To increase the heat production and uniformity of temperature distribution the empty spacers between radioisotope seeds were replaced by hyperthermia-only (HT-only) seeds.

METHODS

The heat generation due to eddy currents circulating in the seed's thin metal shell, surrounding the core, depends drastically on the core permeability. We identified a soft ferrite material (MnZnFe 2 O 4 $\rm MnZnFe_2O_4$ ) as the best candidate for the core, owing to its high permeability, the HT-range Curie temperature, adjustable through material composition, and a sharp Curie transition, leading to heat self-regulation, with no invasive thermometry required. The core permeability as a function of temperature was calculated based on measured resistor-inductor (RL) circuit parameters and material B-H curves. The thickness of the shell was optimized separately for TB and HT-only seeds, having slightly different dimensions. Heat generation was calculated using the power versus temperature approximation. Finally, the temperature distribution for a realistic prostate LDR brachytherapy plan was modeled with COMSOL Multiphysics for a set of blood perfusion rates found in the literature.

RESULTS

The small size of the investigated ferrite core samples resulted in demagnetization significantly decreasing the relative permeability from its intrinsic value of ∼5000 to about 11 in the range of magnetic field amplitude and frequency values relevant to HT. The power generated by the seed dropped sharply as the shell thickness deviated from the optimal value. The optimized TB and HT-only seeds generated 45 and 267 mW power, respectively, providing a HT source sufficient for >90% volume coverage even for the highest blood perfusion rates. The toxicity of the surrounding normal tissues was minimal due to the rapid temperature fall off within a few millimeters distance from a seed.

CONCLUSIONS

The investigated TB and HT-only seed prototypes were shown to provide sufficient power for the concurrent administration of radiation and HT. In addition to being used as a source for both radiation and heat at the onset of cancer therapy, these implanted seeds would be available for treatment intensification in the setting of salvage brachytherapy for locally radiorecurrent disease, possibly as a sensitizer to systemic therapies or as a modulator of the immune response, without another invasive procedure. Experimentally determined parameters of the ferrite material cores provided in this study establish a mechanistic foundation for future pre-clinical and clinical validation studies.

A retrospective study to establish recommendations for plan quality metrics in Lung SBRT

The increased use of Stereotactic body radiation therapy (SBRT) has warranted a new method of plan evaluation. The crucial component of SBRT is the precise, conformal delivery of radiation dose to the target with rapid dose fall-off in the surrounding normal tissues.In this study, we retrospectively evaluated plan quality in lung SBRT patients by calculating conformity, homogeneity, and gradient parameters using an in-house script. The goal of this study was to establish achievable, size-dependent recommendations for these plan quality metrics such that they may be used as a guideline in our clinic. Seventy-three patients treated with lung SBRT at The University of Toledo Medical Center during the period 2017-2020 were retrospectively reviewed for this study. Plans were evaluated using dosimetric indices from respective The Radiation Therapy Oncology Group (RTOG) and International Commission on Radiation Units and Measurements (ICRU) protocols. Average values for each of the following indices were calculated: RTOG conformity index = 1.12 ± 0.13; Paddick conformity index = 0.82 ± 0.07; gradient index = 4.63 ± 0.71; and Homogeneity index = 0.3 ± 0.07, for all studied lung lesions with a mean volume of 23.2 cc. Our final recommendations are based on clinically approved plans, after having removed statistical outliers that we may not have approved had the metrics been calculated. Additionally, we observed that a sharper dose fall-off and a more homogeneous plan were found using 6 FFF compared to 10 FFF energy. Comparison between our results and RTOG0915 data shows no deviation or minor deviation for the RTOG conformity index and the ratio of 50% prescription isodose volume to the target volume. Furthermore, no statistically significant correlation between RTOG conformity index and target volume was observed which is in agreement with RTOG0915. Using various dosimetric indices to characterize dose distributions in lung SBRT is a powerful tool to assess plan quality. We recommend that these values be calculated for all plans, utilizing a script or program so as to improve clinical workflow.

High rate of contamination with Staphylococcus aureus in traditional Koozeh cheeses. A molecular typing approach

BACKGROUND

Koozeh cheese is an Iranian dairy product in rural areas, it is necessary to consider the microbial contamination in this supply.

OBJECTIVE

This study evaluates microbial contamination in Koozeh cheese by molecular tools.

MAATERIAL AND METHODS

S. aureus and its enterotoxins including type A and type B were identified by biochemical and polymerase chain reaction (PCR) and molecular typing was done by RAPD (Random Amplification of Polymorphic DNA) method. A total of 42 sheeps and cows Koozeh cheese samples were collected from random market in the cities and the surrounding villages.

RESULTS

71.42% of samples were contaminated with Staphylococcus spp. and in 50% of isolates, S. aureus specific coagulase gene "coa" was detected. High-level contamination was observed in 7.14% of samples. The SEA or SEB enterotoxins were produced in 42.84% of isolates. No clonal relationship was observed by molecular approach.

CONCLUSION

The obtained results indicate a high level of microbial contamination in Koozeh cheese. Half of isolates were enterotoxin producer and had high diversity and no clonal relationship. Long processing and manipulation are involved in contamination. Improvement in hygiene, training local manufactures of Koozeh cheese, control of products for possible contamination and developing new protocols is needed to decrement of S. aureus contamination in Koozeh Cheese products.

Optimizing MRI sequences and images for MRI-based stereotactic radiosurgery treatment planning

AIM

Development of MRI sequences and processing methods for the production of images appropriate for direct use in stereotactic radiosurgery (SRS) treatment planning.

BACKGROUND

MRI is useful in SRS treatment planning, especially for patients with brain lesions or anatomical targets that are poorly distinguished by CT, but its use requires further refinement. This methodology seeks to optimize MRI sequences to generate distortion-free and clinically relevant MR images for MRI-only SRS treatment planning.

MATERIALS AND METHODS

We used commercially available SRS MRI-guided radiotherapy phantoms and eight patients to optimize sequences for patient imaging. Workflow involved the choice of correct MRI sequence(s), optimization of the sequence parameters, evaluation of image quality (artifact free and clinically relevant), measurement of geometrical distortion, and evaluation of the accuracy of our offline correction algorithm.

RESULTS

CT images showed a maximum deviation of 1.3 mm and minimum deviation of 0.4 mm from true fiducial position for SRS coordinate definition. Interestingly, uncorrected MR images showed maximum deviation of 1.2 mm and minimum of 0.4 mm, comparable to CT images used for SRS coordinate definition. After geometrical correction, we observed a maximum deviation of 1.1 mm and minimum deviation of only 0.3 mm.

CONCLUSION

Our optimized MRI pulse sequences and image correction technique show promising results; MR images produced under these conditions are appropriate for direct use in SRS treatment planning.

Development of a Tissue-Mimicking Phantom of the Brain for Ultrasonic Studies

Constructing tissue-mimicking phantoms of the brain for ultrasonic studies is complicated by the low backscatter coefficient of brain tissue, causing difficulties in simultaneously matching the backscatter and attenuation properties. In this work, we report on the development of a polyvinyl alcohol-based tissue-mimicking phantom with properties approaching those of human brain tissue. Polyvinyl alcohol was selected as the base material for the phantom as its properties can be varied by freeze-thaw cycling, variations in concentration and the addition of scattering inclusions, allowing some independent control of backscatter and attenuation. The ultrasonic properties (including speed of sound, attenuation and backscatter) were optimized using these methods with talc powder as an additive. It was determined that the ultrasonic properties of the phantom produced in this study are best matched to brain tissue in the frequency range 1-3 MHz, indicating its utility for laboratory ultrasonic studies in this frequency range.

Machine-Specific Magnetic Resonance Imaging Quality Control Procedures for Stereotactic Radiosurgery Treatment Planning

Purpose

Magnetic resonance (MR) images are necessary for accurate contouring of intracranial targets, determination of gross target volume and evaluation of organs at risk during stereotactic radiosurgery (SRS) treatment planning procedures. Many centers use magnetic resonance imaging (MRI) simulators or regular diagnostic MRI machines for SRS treatment planning; while both types of machine require two stages of quality control (QC), both machine- and patient-specific, before use for SRS, no accepted guidelines for such QC currently exist. This article describes appropriate machine-specific QC procedures for SRS applications.

Methods and materials

We describe the adaptation of American College of Radiology (ACR)-recommended QC tests using an ACR MRI phantom for SRS treatment planning. In addition, commercial Quasar MRID^3D and Quasar GRID^3D phantoms were used to evaluate the effects of static magnetic field (B₀) inhomogeneity, gradient nonlinearity, and a Leksell G frame (SRS frame) and its accessories on geometrical distortion in MR images.

Results

QC procedures found in-plane distortions (Maximum = 3.5 mm, Mean = 0.91 mm, Standard deviation = 0.67 mm, >2.5 mm (%) = 2) in X-direction (Maximum = 2.51 mm, Mean = 0.52 mm, Standard deviation = 0.39 mm, > 2.5 mm (%) = 0) and in Y-direction (Maximum = 13. 1 mm , Mean = 2.38 mm, Standard deviation = 2.45 mm, > 2.5 mm (%) = 34) in Z-direction and < 1 mm distortion at a head-sized region of interest. MR images acquired using a Leksell G frame and localization devices showed a mean absolute deviation of 2.3 mm from isocenter. The results of modified ACR tests were all within recommended limits, and baseline measurements have been defined for regular weekly QC tests.

Conclusions

With appropriate QC procedures in place, it is possible to routinely obtain clinically useful MR images suitable for SRS treatment planning purposes. MRI examination for SRS planning can benefit from the improved localization and planning possible with the superior image quality and soft tissue contrast achieved under optimal conditions.