In vivo dosimetry for Synchrotron Stereotactic Radiation Therapy

The first clinical study of therapeutic applications of Contrast-Enhanced Synchrotron Stereotactic Radiation Therapy (SSRT) is underway since June 2012 at the European Synchrotron Radiation Facility (ESRF). The phase I-II clinical trial is designed to test the feasibility and safety of SSRT through a dose escalation protocol. So far, 10 patients suffering from brain metastasis of medium to small volume have already been treated using this modality. A localized dose enhancement is obtained due to higher photoelectric effect rate in the target and is directly related to the amount iodine located in a given voxel of tissue (about 10% per mg/mL of iodine). The 3D iodine content in a given patient is derived from the 3D CT acquisition obtained during the dosimetry CT procedure and associated treatment planning [1] . In vivo dosimetry (i.e., experimental dosimetry in real time during the treatment) would be a serious added value to the project, in terms of online dose monitoring and quality assurance. It is challenging to perform in vivo dosimetry with the currently available conventional clinical techniques [2] . Entrance measurements using semiconductor detectors lead to significant incoming beam perturbations (medium energy x-rays) whereas it’s complicated to set-up portal imaging techniques, as the iodine content is not taken into account. The idea is to measure the iodine fluorescence X-rays yield emitted from the target during the irradiation. This can be achieved using spectrometry techniques and iodine Kα peaks analysis. Our first approach consists in developing a 0D model with a CZT detector pointing on the irradiation isocenter to characterize the relationship between peak contents and average iodine concentration obtained in the tumor during irradiation [3]. Fluorescence from iodine tubes of different concentrations (0.5 to 20 mg/mL) were acquired in air and inserted into an anthropomorphic radiosurgery phantom. The same setup has been recently used on the last patient treated in SSRT at the ESRF. The detector was pointing the isocentre and the spectra were recorded during the three irradiation incidences. The iodine concentration was plotted versus the number of counts in the fluorescence channel for the tubes in air. We notice a non-linearity in the curve due to self-absorption [4]. Concerning the signal obtained in patients, it should be furthered analyzed in order to retrieve the concentration. For this purpose, simulations should be used, using a priori information from the dosimetry CT scans.. A potential improvement of the technique would be its transfer to a 3D modality using a pixelated spectrometric detector. References [1] L. Obeid,et al. J. Cereb. Blood Flow Metab., vol. 34, no. 4, pp. 638–45, Apr. 2014. [2] B. Mijnheer,et al. Med. Phys., vol. 40, no. 7, p. 070903, Jul. 2013. [3] A. M. Rene Van Grieken, “Handbook of X-Ray Spectrometry, Second Edition,.” [Online]. [4] C. Hall, J. Instrum., vol. 8, no. 06, pp. C06007–C06007, 2013.