Henning Braess

Implementation and workflow for PET monitoring of therapeutic ion irradiation: a comparison of in-beam, in-room, and off-line techniques.

An independent assessment of the dose delivery in ion therapy can be performed using positron emission tomography (PET). For that a distribution of positron emitters which appear as the result of interaction between ions of the therapeutic beam and the irradiated tissue is measured during or after the irradiation. Three concepts for PET monitoring implemented in various therapy facilities are considered in this paper. The in-beam PET concept relies on the PET measurement performed simultaneously to the irradiation by means of a PET scanner which is completely integrated into the irradiation site. The in-room PET concept allows measurement immediately after irradiation by a standalone PET scanner which is installed very close to the irradiation site. In the off-line PET scenario the measurement is performed by means of a standalone PET/CT scanner 10-30 min after the irradiation. These three concepts were evaluated according to image quality criteria, integration costs, and their influence onto the workflow of radiotherapy. In-beam PET showed the best performance. However, the integration costs were estimated as very high for this modality. Moreover, the performance of in-beam PET depends heavily on type and duty cycle of the accelerator. The in-room PET is proposed for planned therapy facilities as a good compromise between the quality of measured data and integration efforts. For facilities which are close to the nuclear medicine departments off-line PET can be suggested under several circumstances.

On the effectiveness of ion range determination from in-beam PET data

At present, in-beam positron emission tomography (PET) is the only method for in vivo and in situ range verification in ion therapy. At the GSI Helmholtzzentrum für Schwerionenforschung GmbH (GSI) Darmstadt, Germany, a unique in-beam PET installation has been operated from 1997 until the shut down of the carbon ion therapy facility in 2008. Therapeutic irradiation by means of (12)C ion beams of more than 400 patients have been monitored. In this paper a first quantitative study on the accuracy of the in-beam PET method to detect range deviations between planned and applied treatment in clinically relevant situations using simulations based on clinical data is presented. Patient treatment plans were used for performing simulations of positron emitter distributions. For each patient a range difference of + or - 6 mm in water was applied and compared to simulations without any changes. The comparisons were performed manually by six experienced evaluators for data of 81 patients. The number of patients required for the study was calculated using the outcome of a pilot study. The results indicate a sensitivity of (91 + or - 3)% and a specificity of (96 + or - 2)% for detecting an overrange, a reduced range is recognized with a sensitivity of (92 + or - 3)% and a specificity of (96 + or - 2)%. The positive and the negative predictive value of this method are 94% and 87%, respectively. The interobserver coefficient of variation is between 3 and 8%. The in-beam PET method demonstrated a high sensitivity and specificity for the detection of range deviations. As the range is a most indicative factor of deviations in the dose delivery, the promising results shown in this paper confirm the in-beam PET method as an appropriate tool for monitoring ion therapy.

Measurement of radiation hardness of PET components

IN-BEAM PET has given valuable feedback on treatment quality over the 11 years of operation time between 1997 and 2008 of the heavy ion treatment facility at the Gesellschaft fu¨r Schwerionenforschung (GSI) Darmstadt [1]. Based on this technical expertise a next generation of in-beam PET scanners will be developed. An experiment addressing the question whether the detectors and electronic components used in state-of-the-art PET-systems are suitable regarding radiation hardness for configuring a future in-beam PET scanner was performed at the medical beam line of GSI. A 12C beam of an energy of E =430.10 AMeV was stopped in a PMMA phantom. The primary particle fluence of this irradiation was equivalent to about 5300 patient fractions (3 GyE per fraction, one fraction means one treatment per patient per day). Several parts of the equipment were placed in the forward direction of the cone of the secondary particles arising from nuclear reactions in the phantom. The equipment for a new in-beam PET scanner should be tested whether it will resist the fluence of secondaries arising from the patient treatment of about 5 years.

Influence of the time of flight information on the reconstruction of in-beam PET data

Positron emission tomography (PET) is the only available technique for an in-situ, non-invasive monitoring of the dose delivery precision in highly conformal ion beam therapy. Significant improvement in the quality of the images reconstructed using time of flight (TOF) information has recently been reported for usual conditions of nuclear PET (relatively high statistics, full ring geometry of the scanner). In order to analyze the advantages for a new in-beam PET system with detectors allowing timing resolution of at least 1.2 ns FWHM, we investigate how the reconstruction of in-beam PET data can profit from the TOF information taking into specific account issues of an in-beam PET system.

System solution for particle therapy PET

AT present, positron emission tomography (PET) is the only available technique for an in-vivo, non-invasive monitoring of the dose delivery precision in highly conformal ion beam therapy. The successful exploitation of in-beam PET at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany during the last decade [1, 2] and a rising number of built or planned proton and ion therapy facilities worldwide makes the development of a particle therapy PET (PT-PET) system of the next generation reasonable. The in-beam PET installation at GSI is a double-head positron scanner with a very limited solid angle which results in severe artifacts in the reconstructed images and in a low counting statistics. Thus, it is highly desirable to have larger solid angle coverage for PT-PET scanners of the next generation [3]. However, increasing the effective area of a scanner might be limited by several requirements for the equipment of a radiotherapy treatment unit. Possible configurations of a prospective PT-PET scanner as well as a methodology for the evaluation of concurrent designs of the scanner taking into account the requirements of a therapy facility and integration costs are discussed in this paper.

Comparison of PET Concepts for Dose Delivery Monitoring of Particle Therapy

Tumor therapy using particle beams is highly precise and requires methods for monitoring of dose delivery. Solutions based on positron emission tomography (PET) are successfully implemented for the radiation therapy with carbon ions and protons at the Gesellschaft fur Schwerionenforschung, Darmstadt, Germany, at the Heavy Ion Medical Accelerator at Chiba, Japan, at the Massachusetts General Hospital, Boston, USA, at the Hyogo Ion Beam Medical Center, Japan as well as at the National Cancer Center Kashiwa, Japan. Furthermore, the PET monitoring technique has been tested experimentally for 3He, 7Li, and 16O. The main requirements for the PET monitoring are (1) to produce images containing relevant information for the evaluation of dose delivery and (2) to reduce the additional time required for imaging as much as possible. There are three technical realizations: (1) Inbeam PET (measurement during the irradiation), (2) In-room PET (measurement immediately after the irradiation of each portal using a PET scanner placed in the therapy room), (3) Off-line PET (measurement after the complete irradiation by means of a PET scanner located in a different room). These three concepts have been evaluated concerning workflow, counting statistics, and imaging. The treatment workflow is mostly affected by off-line PET. In the case of in-room and offline PET, the counting statistics is approximately one half of that for in-beam PET if reasonable measuring times (≤ 15 min) are assumed. Furthermore, there is an information loss using off-line PET. Thus, in-beam and in-room PET are the most feasible concepts to integrate PET into the particle therapy for dose monitoring.