368 Invited Application of an amorphous silicon flat panel imager for dosimetry purposes

Proffered papers

Satttrday, 21 September 2002 S123

Treatmentgroup Mean% Tumour Mean liver wt. Burden (±SD) 18.3 ± 16.5 13.3 ± 9.8 5.0 ± 3.0*# 7.7 ± 5.4

(gm±SD) 1.1 ± 0.2 1.7 ± 0.2* 1.6 ± 0.2" 1.5 ± 0.2"#

Meanspleen wt. (gm±SD) Control 0.10 ± 0.03 IL-2 0.23 ± 0.05* 0.075 Gy TBI+IL-2 0.20 ± 0.04* 0.75 Gy TBI+IL-2 0.19 ± O.O4*

Significantly differentfrom control group # Significantlydifferentfrom IL-2 group Conclusion: LTBI improved the therapeutic ratio of IL-2 treatment. Optimising this combination may have important clinical implications,

TRANSIT DOSIMETRY 367

Invited

IMRT dose verification in-vivo in 3D M. Partridqe, B.-M. Hesse DKFZ (German Cancer Research Institute), Medical Physics, Heidelberg, Germany A flexible and modular strategy is presented using EPIDs for in vivo verification of IMRT. A proof of principle experiment-carried out using an amorphous silicoh flat-panel imager and an Alderson-Rando phantom-is presented. Verification of three aspects of the treatment are demonstrated i) operation of the delivery hardware, ii) patient set-up using bony or soft tissue anatomy and iii) 3D dose delivery. Collimator positions can be monitored for both multiple-static and dynamic IMRT deliveries. Dose delivery can be verified by comparing measured EPID signals with pre-calculated portal dose images. This gives a useful secondary check, but is relatively insensitive to patient position or organ motion. Patient set-up using bony anatomy can be performed using single or orthogonal pairs of images; soft tissue can only be located using implanted markers. In this study CT is performed immediately pre-treatment using the MV treatment beam, although the clinical feasibility of this approach is unproven. The use of a dedicated CT scanner in the treatment room is possible, although perhaps the best solution is incorporation of a cone-beam kVCT scanner in the treatment machine. With on-line c r available, the actual 3D dose delivered to the patient can be calculated for that treatment fraction. The measured EPID signal is converted to primary fluence using an iterative technique employing pre-calculated Monte Carlo pencil beam scatter kernels and detector response functions. This primary fluence is then back-projected through the treatment-time CT cube-correcting for attenuation-to form an input fluence map. These input fluence maps can be directly used to verify each beam separately or fed back into the original dose calculation with the treatmenttime CT to calculate in vivo 3D dose. A Monte Carlo dose calculation could also be used. Results of the phantom study indicate the accuracy of the 3D dose reconstruction to be 3 mm, 3o)/o using a relative normalisation. Using a dosimetric EPID calibration the absolute dose at the reference point was within 5%. For clinical implementation, the level of complexity of verification will depend on the level of confidence each institution has for each type of treatmerit. Portal dose images may be perfectly sufficient for routine verification of a "standard" IMRT treatment in an experienced clinic, whereas full 3D dose reconstruction may be useful for verifying new class solutions or highly critical, complex or unusual treatments, 368

Invited

Application of an amorphous silicon flat panel imager for dosimetry purposes L.N. McDermott, R.J.W. Louwe, J.-J. Sonke, B. Brand, M.B. van Herk, B.J. Mijnheer The Netherlands Cancer Institute / Antoni Van Leeuwenhoek Hospital, Amsterdam, The Netherlands. Introduction: With the advent of flat panel electronic portal imaging device (EPID) technology for dosimetric purposes, new challenges have arisen to obtain accurate and time effective verification of patient dose distributions. The Elekta iView-GT EPID (Elekta Ontology Systems, Crawley, UK) is currently being examined for radiotherapy dosimetry in our institution. The EPID is an indirect detector, with a phosphorus scintillation layer converting incident radiation to optical photons. These are read by arrays of coupled amorphous silicon (a-Si) photodiodes and thin film transistors, with a sensitive area of 41cm 2 (1024x1024 pixels, 400pm pitch). A lmm copper plate over the phosphor layer is used to provide additional build-up material. Materials & Methods : We investigatedthe dose-response relationship of the EPID under a wide range of treatment conditions, the effects of image

persistence (ghosting) and the requirement of an additional build-up layer for dosimetry. The long term stability and temperature dependence of the dose-response for 4,6,8 and 18MV photon beams were also investigated. Results: The deviation in lineadty with dose-response was found to be less than 1%. However the deviation from linearity was 4% (~40 to 600MU/min) with dose rate and 20% (-0.01-0.07mGy/pulse) with dose per pulse at the imager. The relative amount of image persistence depends mostly on the number of irradiated frames and recovery time, while dose and dose rate have a much smaller effect. An additional copper, build-up layer reduced the sensitivity to low-energy photons scattered from a polystyrene phantom. The dark field image was found to have a room temperature dependence, affecting the reproducibility of EPID images. However, by regularly updating dark field correction used for calibration of each image, a standard deviation of less than 0.5% was achieved in the response behaviour for four energies and two field sizes, for a period up to 5 months. Conclusions: With a well defined response under a wide range of treatment conditions, adequate build up material and verified long-term stability, the aSi-type EPID has strong potential for applications in clinical dosimetry. 369

Oral

D y n a m i c MLC and its impact on EPID dosimetry using Monte Carlo

P. Manser 1, M.K. Fix 1, E.J. Born2, R. Mini2, P. ROegsegger 1 1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland 2Division of Medical Radiation Physics, Inselspital and University of Berne, Berne, Switzerland Introduction: The suitability of using an a-Si:H EPID for dosimetric verification of IMRT has been demonstrated in a previous study. However, the equivalence to a water reference system is questionable due to different energy responses introduced by different detector compositions. In this work, changes of the photon energy spectra due to dynamic MLC delivery and its impact on portal dosimetry were investigated by the use of Monte Carlo (MC) simulations. Methods: Based on a recently developed multiple source model for 6 and 15 MV photon beams, the energy spe(~tra were simulated for dynamic slit beam irradiations with different slit widths. In addition, spectral changes in several portal planes were evaluated withotJt any absorber and behind a water phantom. In order to determine the energy response of both a-Si:H EPID and water reference system, the energy deposition kernels were calculated for monoenergetic pencil beams (0.02-18 MeV). For each detector, the portal dose was calculated by combining the energy spectrum with the energy response. Finally, comparisons between the portal doses for different photon beam energies, slit widths, portal planes and with and without an absorber in the beam, were performed. Results: The MC simulations of the slit beams result in appreciable energy spectra changes. For all cases investigated, beam-hardening occurs for decreasing slit widths. The energy responses show different behaviours. Mainly at low energies, the equivalence of the a-Si:H EPID to water is not fulfilled. From the comparisons of the portal doses, it turns out that for the 6 MV beam, the a-Si:H EPID underestimates the dose values due to the changes of energy spectra for decreasing slit widths, whereas for the 15 MV beam, the two systems show almost similar behaviour. These results were found to be independent of locations of the portal planes and whether or not an absorber was in the beam. Conclusions: In conclusion, the study showed that dynamic MLC affects accurate EPID dosimetry, particularly for 6 MV. We hope that this study will not only improve our understanding of the detector's underlying physics but will also lead to optimized detector design. 370

Oral

Relative dose verification of dynamic collimated beams by means of an electronic portal imaging device V. Marchesi 1, 2, p. Aletti 1, D. Wolf2, A. Noel 1 1Centre Alexis Vautrin, Unit6 de Radiophysique M~dicale, Vandoeuvre les Nancy Cedex, France 21nstitut National Polytechnique de Lorraine, Cran CNRS UMR-7039, Vandoeuvre les Nancy, France Purpose : To perform dosimetric verifications of intensity-modulated beams by dynamic collimation with a liquid-filled electronic portal imaging device (EPID). Material and methods: The PortalVision system (Mark II, Varian Medical Systems) has been adapted in order to perform dose measurements of intensity-modulated beams. Acquisition sequence was optimized to reduce