A study of the use of flat‐panel imagers for radiotherapy verification (in English)

Abstract

This thesis investigates the feasibility of the application of flat‐panel imagers (FPIs) based on amorphous silicon to radiotherapy verification. Intensity modulated radiotherapy imposes new demands on verification procedures. A contrast resolution of 1% may be required from portal imaging systems to resolve the target structure and the organs at risk (OAR). Furthermore a spatial resolution of 1 mm is desired. The imaging characteristics, spatial resolution, efficiency, and noise characteristics of the FPI for megavoltage imaging were investigated regarding these demands. Possible improvements from scatter rejection by a modified detector design were investigated by simulating the spectral response of the converter plates of these detectors to photons of different energies and the response to scattered photons. Since low contrast objects like the target structure and the OAR can be detected in radiographic transmission images only for some rare cases, tomographical imaging was tested for position verification. This was also done using an FPI and the megavoltage (MV) source of the treatment accelerator. A different approach to position verification is the integration of kilovoltage (kV) computed tomography (CT) into a clinical linear accelerator. Although this approach has higher demands on the hardware side, it will deliver higher contrast images at a lower dose. A comparison of MVCT and kVCT and an outlook to new possibilities using integrated CT therefore completes this thesis.

Consequences of the spectral response of an a‐Si EPID and implications for dosimetric calibration

“…It has been established that a-Si EPIDs that employ high-Z scintillation screens such as Gd 2 O 2 S : Tb have an increased sensitivity to low energy ͑Ͻ1 MeV͒ photons. 3,[7][8][9][10][11] This is in contrast with the fairly linear response vs photon energy observed above 1 MeV where the Compton interaction dominates. Generally, EPID pixel values are calibrated against ion chamber readings in an open reference field where the energy spectrum is essentially that which exits the head of the linac.…”

Methods for improving limited field‐of‐view radiotherapy reconstructions using imperfect a priori images

“…One option is to use the megavoltage linear accelerator ͑linac͒ as the CT radiation source in conjunction with a detector system to measure the transmitted fluences. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Online kVCT has been explored with an additional kV x-ray tube, [23][24][25] using CT-on-rails, 26 and with a dual-energy-capable linac head. 27 Their numerous differences aside, one characteristic com-mon amongst many of these systems is a limited field-ofview ͑LFOV͒.…”

Development of Fast and Highly Efficient Gas Ionization Chamber For Patient Imaging and Dosimetry in Radiation Therapy

“…It was shown that the low-frequency drop of MTF pre of the investigated detector was caused by the scattered radiation created within the detector. One author observed the low-frequency drop experimentally for a flat panel imager (Groh 2001).…”