Abstract
Prostate biopsy (Bx) and brachytherapy (BT) are common procedures for the diagnosis and treatment of prostate cancer. However, using MRI-guidance for these techniques is in its infancy and subsequently the availability of tools to assist in confirming the accuracy of the biopsy or treatment, are limited. Examples of key limitations in commercially available devices include typical prostate motion, MRI tissue characteristics and needle/tissue forces not being representative of the clinical range. Furthermore the designs do not incorporate the integration of dosimeters to measure the radiation delivery, they have limited reusability and tend to be expensive. The primary aim of this work was to design, construct and test an anthropomorphic phantom (the PROM phantom), with a focus on mimicking the MRI imaging parameters of prostate tissue, whilst achieving mechanical properties similar to real human structures. This is important to simulate needle forces during penetration into the prostate, during Bx and BT procedures. Secondary aims included: evaluating candidate dosimeters suitable for integration into the PROM phantom, to compare intended brachytherapy dose to the delivered dose; designing the phantom to be reusable and cost-effective; and designing a second phantom for assessing the accuracy of detecting brachytherapy seeds and needle tips geometrically within MRI images.The materials used, the modular design of the PROM phantom, and the option to configure the phantom for a fixed or mobile prostate, are all aspects that make this work novel. Furthermore, the reusable formulation of PRESAGE® dosimeter (NJ, USA) used in this work had not previously been studied for prostate brachytherapy, and the Micro-Silica bead dosimeters (TRUEInvivo®, UK) utilised, had not previously been studied for low dose rate prostate brachytherapy. Materials considered for use within the phantoms were tested using a selection of methods depending on their function and importance within the phantom. Materials for the prostate, being the organ of primary importance, were tested using qualitative assessment, metrological methods, clinical feedback, quantitative MRI studies, mechanical compression testing and needle insertion force measurements. A subset of these methods was used to test the other phantom structures. Fully assembled PROM phantoms were tested for manufacturing reproducibility using metrological methods and CT imaging, and for clinically suitable mechanical properties using needle insertion force measurements. Dosimeters were studied using film scanning techniques, dose to water calculations, energy-dispersive x-ray spectroscopy and Monte-Carlo simulation. For repetitive or complex quantitative analysis, MATLAB® scripts were written and statistical methods including the Shapiro-Wilk, the Friedman and post-hoc Dunn- Bonferroni tests were applied.
A key finding from this work was that the mechanical properties of the prostate tissue mimicking material could be augmented, using Konjac powder, to mimic friction on the shaft of the needle, whilst maintaining clinically appropriate MRI relaxation characteristics. For example the addition of 12 g of Konjac powder into the PTMM lead to an increase, by a factor of 43 (0.0012 to 0.052 mm.N-1), in force on the needle with insertion distance into the prostate, whilst the Young’s Modulus at 10% strain increased by a factor of 5 (to 52 N.mm-2) and the T1 relaxation remained relatively constant averaging 1294 mS (range 1165 to 1472 mS). The mobile configuration of the PROM phantom demonstrated an inferior to superior shift of 10 ±1 mm and a posterior to anterior shift of 4 ±1 mm, in-line with clinically reported data. The final PROM phantom design has been shown to work with both robotic and manual biopsy techniques under live MRI, demonstrating a significant difference (P = 0.03) in biopsy accuracy between two clinical users. Regarding dosimetry, the Micro-Silica beads, tested using Iodine-125 seeds in water, measured between 3 0 to 5.7% of the expected dose (± 10.7% k =2), with uncertainties less than that expected from dose calculation methods via post-implant CT imaging.
The PROM phantom is unique compared to others available commercially or reported in the literature. It is more advanced in terms of mimicking prostate tissue properties on MRI, it offers more flexibility to mechanically model the range of clinical scenarios, it can be used for quantitative assessment of biopsy accuracy and has been designed with reusability in mind. The modular design adversely affects the use of the phantom with ultrasound imaging. This can be overcome by placing the phantom in water, but this is less convenient than ultrasound specific prostate phantoms. In conclusion, the objectives of this research have been met and new knowledge has been generated in the field of tissue mimicking materials and manufacturing methods for phantoms, LDR seed planning for MRI guided implants, Micro-Silica LDR dosimeters, and assessing accuracy of biopsy techniques.
Key words: Prostate, Brachytherapy, Biopsy, Phantoms, MRI, Mechanical Properties, Dosimeters
| Date of Award | 14 Nov 2024 |
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| Original language | English |
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| Supervisor | Andrea Bucchi (Supervisor) & Petko Petkov (Supervisor) |