Speaker
Description
Carbon-ion radiotherapy (CIRT) is a lifesaving technique for treating radioresistant cancer. Recent clinical evidence shows that besides the applied radiobiological effective (RBE) dose, the averaged values of the linear energy transfer (LET) in the irradiated volume are correlated to the tumor control. Probability density distributions of LET are currently based on simulations and show high inter-center variations. Good clinical
practice requires verification through measurements, and microdosimetric detectors are under investigation for this purpose.
We used Geant4/GATE simulations to obtain the energy deposition spectra from carbon ions deposited into a microdosimetric sensor consisting of a 12 μm-thick SiC PIN diode. The temporal structure of the carbon beam at MedAustron was measured with a time resolution of 0.3 ns. A scalable model for the particle arrival time (PAT) probability density function (PDF) was designed and validated against measurements. Together with the energy deposition spectra obtained from Geant4/GATE simulations, this PAT-PDF model enabled the generation of realistic time domain sensor signals as input for the front-end ASIC simulations. An array of charge-sensitive amplifiers (CSA) was designed and taped out in the IHP 130nm CMOS process.
The time-domain sensor signal was transformed into input stimuli for SPICE simulations of the CSA. Using ngspice, the CSA output transient and noise spectral densities were simulated for the generated input stimuli. Noise was generated based on the simulated output noise spectra of the CSA and added to the simulated transient output signal. The generated time-domain signal was post-processed in Python using a dual shaper approach. One shaper was used for pileup detection, the second was used for amplitude measurements. The applied peaking times for the shapers were 5 ns for pile-up rejection and 150 ns for amplitude measurement. Microdosimetric spectra were extracted for different locations along the Bragg peak for a microdosimetric sensor covering an area of 200 x 200 μm², taking into account the beam’s time structure and the behavior of the CSA circuit, including added noise.