Investigation of timepix radiation detector for autoradiography and microdosimetry in targeted alpha therapy

Abstract

The Timepix detector developed by CERN is a novel and sophisticated particle detector. It consists of a semiconductor layer divided into an array of pixels. This array of pixels is bumpbonded to an electronics integrated layer (i.e. the readout chip). Timepix can be used for a wide range of measurements of electromagnetic radiation and particles and their applications in different fields such as space physics, nuclear physics, radiotherapy physics, imaging and radiation protection. The Timepix detector used in this work was purchased from Amsterdam Scientific Instruments, the Netherlands, in order to investigate its use for microdosimetry purposes, in particular in targeted alpha therapy. The device has the following properties: 256 x 256 pixels of 55 x 55 μm2 area each, the chip is effective for positive or negative charge and can be used to detect electrons, X-rays, neutrons and heavy charge particles. It can work as an energy spectrometer, has good spatial resolution and reason1ble detection efficiency. The device can operate in three common modes: Timepix mode, Medipix mode, and Time-Over-Threshold (TOT) mode. Targeted alpha therapy (TAT) is a novel type of radionuclide therapy in which an alpha emitting radioisotope is attached to a cancer cell seeking vector (so called radioimmunoconjugate (RIC)). Once attached to a cancer cell, it causes localized damage due to traversal and energy deposition high LET a-particles. There is, however, a lack of data related to a-particle distribution in TAT. These data are required to more accurately estimate the absorbed dose on a cellular level. As a result, this work aims to develop a microdosimetry technique, using Timepix detector that will estimate, or better yet determine the absorbed dose deposited by a-particles in cells as well as will measure the biodistribution of the radioisotope in a tumour. Initially, extensive Timepix characterization and testing has been done to evaluate the detector's response, including linearity, reproducibility, and sensitivity to low doses of radiations (μGy-mGy dose region) and energy dependence. 1-125 seeds and superficial X-rays (below 70 kVp), produced by the Gulmay superficial X-ray unit, were used. The measured Timepix pixel value was correlated with the known dose (based on the irradiation time used and TLD-100 measurements) and a pixel-value-to- dose calibration curve was obtained. It was confinned that Timepix value increased linearly with the dose delivered. The dose calibration curves using the superficial X-ray beams showed that the pixel value, however, depended on the energy of the X-ray beam. The application of Timepix to measure radioisotope biodistribution (i.e. autoradiography) was investigated. Mice with Lewis lung (LL2) tumours were treated with about 18 kBq oP27Thlabelled DAB4 murine monoclonal antibody that bounds to necrotic tumour cells. The rationale is to develop a-particle-mediated bystander kill of nearby viable tumour cells. To generate more necrotic tumour cells for 227Th-DAB4 binding, some mice also received chemotherapy before being injected with Th-227-DAB4. Finally, 5 mm tumour sections were cut from treated mice for autoradiography with Timepix. Each tumour section was mounted onto a slide with front face uncovered to allow emission of a-particles from the tumour section. Simple steel collimator (I cm radius, 2 cm length) was manufactured in-house and positioned around the tumour section. The slide was placed 2 cm away from the Timepix detector. Bias voltage of 7 V was applied, and a-particle filter was selected for acquisition. Detector cover was removed, exposing the Si layer, to allow the emitted a-particles ( - 6 Me V) to reach the detector. Image acquisition took -14 h. Good resolution autoradiographs of radiolabelled tumour sections were acquired, showing a-particle, electron and X-ray tracks. Timepix measurements also showed an increased Th-227-DAB4 uptake following chemotherapy due to increase in necrotic tissue volume. Timepix was also used to measure the uptake of Cr-51 by A549 cells (lung carcinoma cell line) for different pH levels and the dependence of uptake on pH was investigated. Timepix was observed to be sensitive to detect small changes in the activity/uptake of radioactive sources depending on the environmental condition and the number of cells. The last part of this thesis deals with the development of a transmitted a-particle microdosimetry technique. First, A549 cells were grown in vitro using standard protocols and were irradiated using a 6 MY photon beam with different doses varying between 2-8 Gy and Ra-226 source was used for a-particle irradiation to evaluate A549 radiation sensitivity using clonogenic assay and MTT assay. The cell line was found radiosensitive, with 050 of~ 2 Gy for X-ray irradiation. For transmitted dosimetry, A549 cells were either unirradiated (control) or irradiated for ~2, 1, 2 or 3 hours with a-particles emitted from a Ra-223 source positioned below a monolayer of A549 cells. The HTS Transwell" 96 well system (Corning, USA), consisting of 2 compartments, was used to develop a method for tracking a-particles through a cell mono layer. This system comprises of two compartments, with liquid Ra-223 evaporated in the lower compartment to avoid a-particle self-absorption inside the liquid. The measured activity of 5 kBq was unifonnly distributed, as confirmed by Timepix detector. The second compartment consists of a flat bottom polycarbonate membrane (I 0 μm thick) where cells are plated. It is sufficiently thin to allow a-particles to penetrate through and hit the cells. Fifteen thousand A549 cells were seeded in the upper compartment that was then inserted into the lower compartment containing the evaporated Ra-223. The transwell system was positioned under the Timepix detector. Transmitted a-particles were detected for 1;2, I, 2 or 3 hour irradiation times. Additionally, DNA double strand breaks (DSBs) in the form of y-H2AX foci, were examined by fluorescence microscopy. The number of transmitted a-particles was correlated with the observed DNA DSBs and the delivered radiation dose was estimated. Additionally, the dose deposited was calculated using Monte Carlo code SRIM. Approximately 20% of a-particles were transmitted and detected by Timepix. The frequency and number of y-H2AX foci increased significantly following a-particle irradiation as compared to unirradiated controls. The RBE equivalent dose delivered to A549 cells was estimated to be approximately 0.66 Gy, 1.32 Gy, 2.53 Gy and 3. 96 Gy after Y2, I, 2 and 3 h irradiation, respectively, considering a relative biological effectiveness of a-particles of 5.5. In summary, the Timepix detector can be used effectively for autoradiography in TAT, providing high resolution images and excellent spatial resolution of detected a-particles, as well as a transmitted a-particle microdosimetry detector. If cross-calibrated using biological dosimetry, this method will give a good indication of the biological effects of a-particles without the need for repeated biological dosimetry which is costly, time consuming and not readily available.