Seminarium Wydziału Fizyki Technicznej i Matematyki Stosowanej
SEMINARIUM WYDZIAŁU FIZYKI TECHNICZNEJ I MATEMATYKI STOSOWANEJ
8 listopada (piątek) o godz. 11:15 w sali 121 (GG)
dr inż. Marek Maryański, prof. PG
Zakład Inżynierii Materiałowej Wielowymiarowych Detektorów Promieniowania Jonizującego
Instytut Nanotechnologii i Inżynierii Materiałowej, Politechnika Gdańska
wygłosi wykład pt.: ”Laser transmission tomography of tissue-equivalent polymer gels: micro- and macro- dosimetry of hadron beams, from conventional to ultra-high dose rates in radiation oncology”
Abstract:
When I invented 3D polymer gel dosimeters for radiation oncology in the summer of 1991 as a freshman post-doc at Yale, my immediate goal was just to find a replacement for the so-called Fricke gels that had suffered from blurring of T1-weighted MRI images of radiation dose distributions due to fast diffusion of ferric ions in the aqueous medium. Fast-forward three decades later, I found myself being again at Politechnika Gdańska, this time under the NAWA Polish Returns program, working with my young NAWA PP Project Group on 3D mapping of 1) radiobiologically significant microscopic densities of ionization as well as 2) the physical dose delivered at ultra-large range of dose rates (“FLASH”, known for extraordinary sparing of normal tissues) by hadron beams in polymer gels. The two projects were funded by IDUB grants ARGENTUM and AURUM.
From the start I decided to pursue the first goal by applying an innovative modification to my original 1993 invention of 3D imaging of dose distributions in gels by laser-transmission CT. The innovation was based on modulating the reception angle of the photodetector aperture in my confocal lens-pinhole laser-transmission CT version, which had already generated very encouraging preliminary experimental data from my work in the US. Here at PG, we embarked on using the Fraunhofer diffraction part of the light transmitted by the gel and elastically forward-scattered by micrometer-size clusters of the radiation-initiated polymer and collected by the photodetector, to determine the voxel-averaged sizes of the clusters and to reconstruct their 3D distributions. Why? Because my first hypothesis was that the average cluster size grows as the ionization density increases, due to the aggregation of individual polymer chains that are electrically charged as well as grafted to the flexible gel matrix. My second hypothesis was that the clusters are approximately spherical and well-enough separated from each other, such that they should scatter light according to the Mie-Debye theory. My third hypothesis was that the relative refractive index of the hydrated clusters in the gel medium exceeds unity by only a small fraction ("optically soft particles"). Therefore, Van de Hulst approximation ("anomalous diffraction") of the Mie scattering efficiency factor could be applied, thus significantly simplifying computations across millions of voxels in laser CT reconstructed map of apparent optical extinction coefficients. Over the past two years, we have successfully verified all three hypotheses using optical microscopy, spectrophotometry, our own in-house built confocal laser-light scattering measurement system, my original laser-CT data from experiments performed over a decade before at MD Anderson Cancer Center - Orlando and UPenn's Roberts Proton Therapy Center, along with a Monte Carlo simulation, performed at UPenn and based on the “treatment plan” created there and then for my “CrystalBall” gel phantoms. For our studies here at PG, gel samples were made in our lab and exposed at IFJ PAN in Kraków to 60 MeV protons, at NCBJ in Otwock to thermal neutrons and products of their capture by nuclei of nitrogen, hydrogen and boron-10 present in the gels at controlled concentrations, at GUMED to 6MV linac photons and at ZIMWDPJ to 150 kVp x-rays. Computations were performed at ZIMWDPJ and at UPenn.
The second goal of our research required us to determine the polymer gel formulation suitable for 3D End-End patient-specific quality assurance in ultra-high dose-rate (“FLASH”) proton therapy. The light-scattering gels described above proved to not work, showing significant decrease in sensitivity at ultra-high dose rates. That was not surprising, as the fast-coming free-radical initiators were likely to meet nascent polymer chains, contributing to their premature termination. Therefore, my second approach was to use a different proprietary formulation I had developed first in 2013 and then refined in 2015. There, I used a polymerization inhibitor and a dye marker that attaches to the site where the oligomer chain is terminated. The dye is then spontaneously reduced in the gel but lasts long enough to perform laser CT scanning. This formulation has the advantage of being reusable multiple times. We also made it into a do-it-yourself kit, to further simplify its use in the clinic. The result was excellent. We repeatedly saw unmeasurable dependence on proton dose-rate up to 175 Gy/s, a dose rate 4 times higher than the 40Gy/s threshold normally used for FLASH therapy. However, we also repeatedly saw a puzzling effect that may look like some instability of the proton FLASH delivery system in the middle of the sequence of irradiations. Since transverse profiles of all beams as seen in the gel by our laser CT (installed temporarily at UPenn) remain identical, the source of this instability is unlikely to be related to gel properties. This quest to identify the source continues to be investigated in collaboration with UPenn.
We have presented our most important results in 2023 and 2024 at international conferences in Madrid, Spain (PTCOG), in Aarhus, Denmark (IC3DDOSE) and in Los Angeles, CA, US (AAPM). Patents and publications will follow in the next 2 years as per agreement with NAWA. So will several international grants that I am currently working on and that will fund both basic and applied collaborative research that has emerged from our work. When and where exactly they will be performed, time will tell.
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