[show abstract][hide abstract] ABSTRACT: Inaccurate conversion of CT data to water-equivalent path length (WEPL) is one of the most important uncertainty sources in ion treatment planning. Dual energy CT (DECT) imaging might help to reduce CT number ambiguities with the additional information. In our study we scanned a series of materials (tissue substitutes, aluminum, PMMA, and other polymers) in the dual source scanner (Siemens Somatom Definition Flash). Based on the 80kVp/140SnkVp dual energy images, the electron densities ϱe and effective atomic numbers Zeff were calculated. We introduced a new lookup table that translates the ϱe to the WEPL. The WEPL residuals from the calibration were significantly reduced for the investigated tissue surrogates compared to the empirical Hounsfield-look-up table (single energy CT imaging) from (-1.0±1.8)% to (0.1±0.7)% and for non-tissue equivalent PMMA from -7.8% to -1.0%. To assess the benefit of the new DECT calibration, we conducted a treatment planning study for three different idealized cases based on tissue surrogates and PMMA. The DECT calibration yielded a significantly higher target coverage in tissue surrogates and phantom material (i.e. PMMA cylinder, mean target coverage improved from 62% to 98%). To verify the DECT calibration for real tissue, ion ranges through a frozen pig head were measured and compared to predictions calculated by the standard single energy CT calibration and the novel DECT calibration. By using this method, an improvement of ion range estimation from -2.1% water-equivalent thickness deviation (single energy CT) to 0.3% (DECT) was achieved. If one excludes raypaths located on the edge of the sample accompanied with high uncertainties, no significant difference could be observed.
Zeitschrift für Medizinische Physik 04/2013; · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Purpose: Medical physics aspects of gated irradiation to moving tumors using scanned ion beams. Methods: At our institute, more than 700 patients have been treated with proton and carbon ion beams since its start. The first patient has now been treated with the application of gating. For this purpose, the Anzai respiratory motion detection and gating system was interfaced with the treatment control system such as to control spill pausing. The patient presented with a liver tumor moving with peak-to-peak amplitude of ∼1cm due to breathing. Treatment planning was performed using the Siemens Syngo RT-Planning system on a 3DCT dataset. The gating window was determined from 4DCT. Plan verification was performed in a motion phantom, consisting of a PMMA block equipped with a 3D set of 24 pinpoint chambers mounted on the Quasar moving platform. The breathing motion perpendicular to the beam was simulated based on the trajectory obtained from the Anzai system during 4DCT acquisition. Dose was measured at three conditions: static phantom, moving phantom with continuous beam delivery, and moving phantom with gated irradiation. The statically delivered dose distribution was used as a reference. Further, a 4D dose calculation was performed on the 4DCT dataset using a research treatment planning system. Results: The relative rms deviations in the homogenous dose region were ∼18% without gating, and ∼6% with gating. The 4D dose calculation showed a decrease in target coverage by ∼20% and ∼6%, respectively, for the 95% isodose line. Conclusions: Gating of a scanned carbon ion beam was applied for treatment of a moving liver tumor and a workflow was developed to verify the dose to be actually delivered. With respect to continuous dose delivery, the target dose improvement achieved due to gating is considered to be significant for this patient.
Medical Physics 06/2012; 39(6):3780-3781. · 2.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: Dynamic beam delivery techniques are being increasingly used for cancer therapy. Scanning ion beams require extensive and time-demanding quality assurance procedures and beam tuning. Accordingly, fast measurement techniques improving the efficiency of the procedures and accommodating the safety requirements are highly desirable. Major requirements for a detector used for beam-shape measurements are high spatial resolution in two dimensions, reusability, online readout and easy handling. At the Heidelberg Ion Beam Therapy Facility (Germany), we examined the performance of the RID 256 L flat-panel detector for beam spot measurements. The two-dimensional beam profiles of proton and carbon ion beams measured were compared to measurements with radiographic films at intermediate energies using the index. The difference to the beam width measured with radiographic films of less than 3% demonstrates sufficient accuracy of ion beam width measurements possible with this detector for both proton and carbon ion beams. The beam shapes were also measured at different beam intensities. At both the highest and lowest energies available at the HIT, no beam spot-shape deformation was found with increasing beam intensities, as long as the boundary of the dynamic range was not exceeded. The signal leak along the readout direction was identified as an undesirable effect. However, due to small amplitudes and static beams, this effect is of minor importance for beam spot measurements. Distortion of results due to detector radiation damage was monitored. No detector radiation damage was observed over the experiments. Moreover, the observed short-time detector response stability (within ±0.1%) as well as medium term stability (within 0.5% in 15 months) was excellent. This flat-panel detector is compact and easy to use. Together with its low weight, this helps to speed up measurement procedures substantially. All these properties make this an ideal detector for the fast, high-resolution imaging of static ion beam spots needed for constancy measurements in daily beam quality assurance and for accelerator tuning. For daily use, radiation damage has to be monitored continuously and corrected for if necessary.
Physics in Medicine and Biology 01/2012; 57(2):485-97. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Increased accuracy in radiation delivery to a patient provided by scanning particle beams leads to high demands on quality assurance (QA). To meet the requirements, an extensive quality assurance programme has been implemented at the Heidelberg Ion Beam Therapy Center. Currently, high-resolution radiographic films are used for beam spot position measurements and homogeneity measurements for scanned fields. However, given that using this film type is time and equipment demanding, considerations have been made to replace the radiographic films in QA by another appropriate device. In this study, the suitability of the flat-panel detector RID 256 L based on amorphous silicon was investigated as an alternative method. The currently used radiographic films were taken as a reference. Investigations were carried out for proton and carbon ion beams. The detectors were irradiated simultaneously to allow for a direct comparison. The beam parameters (e.g. energy, focus, position) currently used in the daily QA procedures were applied. Evaluation of the measurements was performed using newly implemented automatic routines. The results for the flat-panel detector were compared to the standard radiographic films. Additionally, a field with intentionally decreased homogeneity was applied to test the detector's sensitivities toward possible incorrect scan parameters. For the beam position analyses, the flat-panel detector results showed good agreement with radiographic films. For both detector types, deviations between measured and planned spot distances were found to be below 1% (1 mm). In homogeneously irradiated fields, the flat-panel detector showed a better dose response homogeneity than the currently used radiographic film. Furthermore, the flat-panel detector is sensitive to field irregularities. The flat-panel detector was found to be an adequate replacement for the radiographic film in QA measurements. In addition, it saves time and equipment because no post-exposure treatment and no developer and darkroom facilities are needed.
Physics in Medicine and Biology 11/2011; 57(1):51-68. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: To asses carbon ion radiation therapy (RT) performed as re-irradiation in 28 patients with recurrent tumors.
Twenty-eight patients were treated with carbon ion RT as re-irradiation for recurrent chordoma and chondrosarcoma of the skull base (n=16 and n=2), one chordoma and one chondrosarcoma of the os sacrum, high-risk meningioma (n=3), adenoid-cystic carcinoma (n=4) as well as one SCCHN. All patients were treated using active raster scanning, and treatment planning was performed on CT- and MRI-basis. All patients were followed prospectively during follow-up.
In all patients re-irradiation could be applied safely without interruptions. For skull base tumors, local tumor control after re-irradiation was 92% at 24 months and 64% at 36 months. Survival after re-irradiation was 86% at 24 months, and 43% at 60 months. In all three meningiomas treated with C12 for re-irradiation, the tumor recurrence was located within the former RT-field. Two patients developed tumor progression at 6 months, and in one patient the tumor remained stable for 67 months. In patients with head-and-neck tumors, three patients developed local tumor progression at 12, 24 and 29 months after re-irradiation. Median local progression-free survival was 24 months. For sacral tumors, re-irradiation offered palliation with tumor control for 24 and 36 months.
Due to the physical characteristics particle therapy offers a new treatment modality in cases with tumor recurrences. With carbon ions, the additional biological benefits may be exploited for long-term tumor control. Further evaluation in a larger patients' cohort will be performed in the future.
Radiotherapy and Oncology 01/2011; 98(1):63-7. · 4.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: To investigate the influence of local density increase by i.v. contrast agent on dose calculation in linac-based radiosurgery (RS) of cerebral arteriovenous malformations (AVMs).
RS was performed after three-dimensional (3-D) treatment planning using a total number of nine to 14 beams. Mean target volume was 5.3 cm(3) (range, 0.1-41.2 cm(3)). Mean maximum diameter was 23.2 mm (range, 8-51 mm). Dose deviation was estimated and calculated from the enhanced and unenhanced datasets of 30 patients. Dose calculation was performed using the same RS treatment plan on both datasets. Both plans were standardized to 1 Gy at isocenter with the same dose weight for all beams.
Mean difference of Hounsfield units (DeltaHU) between enhanced and unenhanced CT was 152 HU (range, 50-350 HU). The estimated dose deviation was <or= 1% in 80% of cases with a mean deviation of 0.67% and a maximum dose deviation of 1.8%. With increasing DeltaHU and increasing maximum diameter dose deviation increased as well. The calculated overdosage in ten datasets of enhanced and unenhanced CT scans was 0.66% mean (range, 0.2-1.2%).
The use of i.v. contrast agent in 3-D treatment planning for RS of cerebral AVMs may lead to an underestimation of actual applied dose. The effect on dose calculation is rather low with dose deviations < +1% in most of the cases. However, there are cases especially in large AVMs with high DeltaHU located next to critical, radiosensitive structures in which an additional unenhanced CT scan is recommended for exact dose calculation to avoid side effects.
Strahlentherapie und Onkologie 06/2009; 185(5):318-24. · 4.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: Radiotherapy today employs complex techniques in order to achieve the best possible conformity of the delivered dose to the target volume. When using dynamic dose delivery techniques, it is especially important to verify the delivered dose at as many representative points as possible. As a continuous medium, Gafchromic EBT film offers an excellent spatial resolution which, together with improvements in sensitivity as compared to older types of radiochromic films, make it a promising candidate for this purpose. In this paper we investigated whether EBT films can be used for quantitative dosimetry in photon beams. There are many publications which discuss different aspects of the EBT film dosimetry. Unfortunately, they differ in the used protocols, scanning devices and variables used for the film darkening quantification which makes the sources of uncertainties difficult to compare. Therefore, the overall accuracy and reproducibility of the results which can be reached with Gafchromic EBT films in combination with a commercial flatbed scanner was investigated. Both the film properties and the influence of the readout system were analysed and compared. The total uncertainty in the net optical density determination due to the studied effects was estimated to be 1.6% at 0.3 Gy and 0.8% at 1 Gy for 60Co photons. Based on this analysis of uncertainties, the handling and scanning protocol was optimized and methods to reduce the influence of some of the uncertainties were proposed. Although Gafchromic EBT films have significant advantages, there are certain effects which have to be considered in order to achieve 5% accuracy in the dose delivered to a patient.
Physics in Medicine and Biology 12/2008; 53(24):7013-27. · 2.70 Impact Factor
[show abstract][hide abstract] ABSTRACT: Properties of a new type of radiochromic film—Gafchromic EBT—were investigated with respect to its use in heavy ion therapy. For this purpose not only radiation imaging, but also quantitative dosimetry is desirable. Improvements with respect to older types of Gafchromic films as well as an excellent spatial resolution make it to a promising candidate to be used in scanned medical ion beams. First, the influence of several effects and parameters on the net optical density measured using a commercial flatbed scanner was analyzed. Here the scanner resolution, the light scattering effect, the longtime development of the film and multiple evaluations of the film were investigated. Methods to reduce the influence of the largest sources of distortions were proposed. In carbon ion beams, the signal was observed to be equal or lower than in photon beams at the same dose, depending on the linear energy transfer. This behavior, known also for other dosimetric media, is caused by a local (microscopic) saturation which occurs around highly ionizing ion tracks. However, the overall darkening of the film was found not to saturate in the region of clinically used doses and beyond.
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment - NUCL INSTRUM METH PHYS RES A. 01/2008; 591(1):171-173.