321 Estimation of dose distribution from the fluoroscopy in real-time tumor tracking radiotherapy (RTRT) for stereotactic body radiotherapy (SBRT)

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Posters calculated doses was assessed for more than 80 different field setups. The calculated doses for the 6MV beam were lower than the measured ones for most of the setups and for the 18MV beam were higher. In both cases the average difference was less than 1.1%, with maximum values of 2% for wedge fields and very elongated fields. Conclusion: We have shown that the doses calculated by the TPS are reliable and in the expected accuracy range. However, these results are not sufficient for the implementation of the IMRT technique. Measurements and calculations are currently being performed for very small beam apertures. Calculations will also include inhomogeneity corrections as well.

in the real-time tumor tracking radiotherapy (RTRT). However, accuracy of dose distribution should be improved further because of uncertainty in dose calculation.

321 Estimation of dose distribution f r o m the fluoroscopy in r e a l - t i m e t u m o r tracking r a d i o t h e r a p y (RTRT) f o r s t e r e o t a c t i c body r a d i o t h e r a p y (SBRT)

M. Oita I, R. Onimaru 2, K. Suzuk 2, H. Aoyama 2, M. Fujino 2, N. Kato 2, R. Kinoshita 2, K. Fujita 3, H. Shirato 2, K. Miyasaka 2 IHokkaido University, Graduated School of Medicine, Department of Radiology, Sapporo, Japan 2Hokkaido University School of Medicine, Department of Radiology, Sapporo, Japan 3Hokkaido University Hospital, Department of Radiology, Sapporo, Japan Purpose: Image-guided radiotherapy using a real-time tumor tracking (RTRT) system has been reported to be useful for precise patient setup and gating to reduce intra-fractional error in body stereotactic radiation therapy (BSRT). However, the increase of patient's fluoroscopic exposure due to the elongation of treatment time is an important issue in the use of RTRT. In this study, we have measured the dose from fluoroscopy during irradiation and estimated the dose distribution from the fluoroscopy in BSRT. M e t h o d s and Materials: From April 2002 to October 2004, 9 patients with lung tumor were treated with SBRT using forty-eight Gray in 4 fractions (48Gy/4fr) or 40Gy/4fr. One or several gold markers were inserted near the target volume before computed tomography (CT) for treatment planning. The coordinates of the each marker and its relationship to the planning target volume were transferred to the RTRT system before irradiation. All beams were gated to irradiate the tumor only when the actual three-dimensional coordinates of a marker was within +2 mm from the planned position. In this study, percentage depth dose (PDD) at the central beam axis, off-center ratio (OCR) at the depth of 0.5, 5.0, 10.0 and 15.0 cm of the fluoroscopic beam (70KVp and 100KVp) was measured by ion-chamber for the diagnostic xray (DC-300, Scanditronix Inc.) and ready-pack film (X-Omat V, Kodak Inc.) with a phantom. These data were registered to treatment planning system (FOCUS Xio Ver.4.2.0, CMS Inc.) and then they were commissioned. Fluoroscopic dose in each patient was calculated using Clarkson algorithm retrospectively. The absorbed doses from fluoroscopy to the skin and anatomical structures in whole treatment course of the patients were estimated in the TPS. Results: PDD calculation was consisted with the phantom dosimetry within 5%. Whereas OCR calculation was estimated to have the accuracy of 15% at maximum in penumbra region compared to the film dosimetry. Mean fluoroscopic beam irradiation time at the whole treatment course of 4 days was 95.7+27.7 minutes. The estimated maximum skin surface dose (point dose) was ranged from 26.7cGy to 175.2cGy. Based on DVH analysis, average maximum fluoroscopic beam doses contributed to the skin, gloss target volume (GTV), and planning target volume (PTV) were 83.5cGy, 46.6cGy, and 58.1cGy respectively. Average mean fluoroscopic beam doses contributed to the skin, GTV, and PTV were 12.1cGy, 36.8cGy, and 37.9cGy respectively. Conclusions: The absorbed dose estimation of fluoroscopic beams in the TPS is useful for fluoroscopic dose optimization

Fig.1 Fluoroscopic dose distributions in RTRT system from two sets of fluoroscopy estimated using TPS. 322 S i m u l a t i o n and validation of an Elekta SL linear a c c e l e r a t o r and CT-data i m p l e m e n t a t i o n in GEANT4 for t h e virtual p r o t o t y p i n g of t h e M R I - a c c e l e r a t o r

A. Raaiimakers, B. Raaymakers, J. Lagendijk UMC Utrecht, Radiotherapy, Utrecht, The Netherlands In the framework of the development of the integration of a MRI-scanner with a linear accelerator (in collaboration with Philips Medical Systems and Elekta) Monte Carlo simulations have to be performed to determine the influence of the magnetic field on the dose distribution. Therefore, we need a well-validated linear accelerator model, a thorough validation of our voxelised patient geometry implementation in GEANT4 and validation of the GEANT4 magnetic field module. The first two have been (partly) achieved; the third will remain for future work. An Elekta SL linear accelerator was simulated excluding the mirror and the ionization chamber. The particle distribution was stored in a phase-space file at two levels: directly over and under the jaws. To increase the randomness in the phase-space files, the symmetry of the geometry was exploited by random rotation or mirror operations on the particles before firing. High resolution lateral profiles were obtained by using a 1 x l x 5 mm grid. Ionization chamber dose volume integration was achieved by integrating the dose levels over 5x5 surrounding voxels. A parameter study was performed on the electron beam spot size, showing an optimal FWHM value of 2 rnm. To decrease calculation time, an OpenMosix cluster was established, allowing flexible membership by employee workstations. During nights and weekends up to 35 computers are available. The simulated dose distribution corresponds to measurements within 2%/ 2 mm. Also a preliminary simulation of a MLC is presented. CT-data can be imported into GEANT4 using the parameterized volume implementation. Navigation was improved by having the navigator check only adjacent voxels. To check whether the dose deposition was simulated correctly, measurements of the dose deposition in a polystyrene phantom (with 4 cm air gap) were compared to simulations on CT-data of the same phantom, using the accelerator model described above. The polystyrene phantom was also modeled geometrically, for additional comparison. Agreement was found within 2% except for the dose buildup regions, where both the CT-data implementation and the geometrical representation showed ~ower dose values (See figure). Both simulation methods do not deviate significantly