gms | German Medical Science

Background: Dose verification in heavy ion therapy is frequently performed using stacks of ionization chambers (ICs) for absolute dosimetry together with x-ray verification films as a check of the field geometry. However, IC-stacks have the disadvantage of only few positions per measurement, while a reconstruction of the dose distribution from the irradiated film is generally not possible. Scintillating screens show a linear dependency between the signal and the energy loss of the ion beam for medium and high energies and hence might serve as a substitute for IC or film measurements in dose verification. For low energies they can be used in verification after a suitable calibration procedure. They can further be useful in beam diagnosis and quality assurance measurements. This work aims to establish and investigate scintillating screens at the Heidelberg Ion Therapy (HIT) center for dosimetric applications.

Material and Methods: This detector setup contains a scintillating screen (Gd₂O₂S:Tb, P43, Proxitronic) and a CCD camera (IEEE1394 Digital Camera, C4742-80-12AG, Hamamatsu Photonics K.K.) that detects the emitted light distribution. The software controlling the camera, the lens-settings and the data readout and analysis is written object oriented in C/C++ and therefore provides a flexible interface for possible changes in the detector system. Together with the detector, it will present a stand alone system for ion dosimetry. Measurements will be performed at the Heidelberg Ion Beam Therapy Centre (HIT) using carbon ions as well as other ions.

Results: Software to operate the scintillating screen is currently under development and an IEEE 1394 interface to the camera has been established. First measurements, serving to optimize the sensitivity of the scintillating screen detector system, will be presented. The first results will be acquired by integrating over one slice during delivery by raster scanning.

Conclusion: After the operating system is established, further investigation of the dosimetric properties of the scintillating screen's behavior will be required for different homogeneous and inhomogeneous dose distributions as well as for several beam energies, especially at low energies typical for the bragg peak region.

Acknowledgements: The authors acknowledge the collaboration with J. Nauman, A. Peters and T. Haberer from the Heidelberg Ion Therapy (HIT) facility for helpful discussions and their technical support.