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1 11. DSMF Symposium 2-3. maj 2013 ABSTRACTS - POSTERS - Titel Comparison of breath correlated and free breathing technique radiation therapy for left sided breast cancer Patients perspective on feasibility of breath correlated radiation therapy versus free breathing 2 Electron trees produced with an Elekta linear accelerator 3 Inter- and intra-fraction Geometric errors in Daily Image Guided radiotherapy of Breast cancer patients 4 Combined PET/MR brain imaging: a spatially dependent bias in PET activity results if bone attenuation is not accounted for Time-evolution of the regional lung density changes following IMRT for NSCLC 7 Adaptive radiotherapy for lung cancer patients: Large scale clinical implementation 8 What the gamma: Finding the correlation between QA and clinically based outcomes for prostate RapidArc plans IV contrast-enhanced CT can be used for attenuation correction in clinical 111-In octreotide SPECT/CT A protocol for comparative small field dosimetry at Danish radiotherapy clinics 11 Clinac MVD sensitivity and Truebeam MVD sensitivity at different dose rates 12 Dose and image quality in pediatric CT of thorax and abdomen in 4 different CT scanners 13 DTU dosimetri: Nyt laboratorium under opbygning 14 Konform linseskånende 8-felts elektron-behandling til retinoblastom-patienter 15 Quantitative clinical image quality comparison of pelvic CBCT for two imaging systems 16 Evaluating the usefulness of EPID for daily output verification by comparison to ionization chamber measurements The impact of a Dixon sequence in creating a pseudo CT scan from MR images using a Gaussian mixture regression model Flattening filter free beam dosimetry with organic scintillators, ionization chambers and diamonds 22 2D setup verification on CT versus segmented MR generated digitally reconstructed radiographs 24 Patientsikkerhed: Risikobilledet på et dansk radiocenter 2012 belyst vet nyt kategoriseringsværktøj for utilsigtede hændelser Side

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3 Comparison of breath correlated and free breathing technique radiation therapy for left sided breast cancer Karina Lindberg Gottlieb 1, Christian Rønn Hansen 1, Rasmus Lübeck Christiansen 1, Charlotte Kristensen 2,Mette Møller Nielsen 2 and Marianne Ewertz 2 1 Laboratory of Radiation Physics, Odense University Hospital, Odense, Denmark 2 Department of Oncology, Odense University Hospital, Odense, Denmark Introduction: An increasing number of patients with breast cancer receive post-operative radiotherapy with potential irradiation of volumes of the heart and ipsilateral lung. This may result in a higher risk of ischaemic heart disease and heart related deaths. Using a respiration correlated technique gives the possibility of reducing irradiation of the organs at risk, while obtaining the prescribed dose for the CTV. Material and Methods: In 2010 the first patients with left-sided breast cancer were treated with Elekta s Active Breathing Coordination (ABC) system, at Odense University Hospital. In the first 18 patients, both a CT scan in moderate deep-inspiration breath hold (mdibh) and a CT scan in free breathing (FB), was made. Dose plans were made and evaluated for each image set, allowing an evaluation of differences and possible advantages of using the ABC device for these patients. Results: Distance between the heart and breast is generally increased when using ABC. In this way, the volume of the CTV covered by 95% of the prescribed dose can be increased without compromising dose constraints to the heart. Also, the total lung volume increases using ABC which decreases the relative dose to the lung. In a few patients no improvements were found. However, this seems related to the anatomy of the particular patients rather than the breath hold volume. Conclusions: Most patients seem to benefit from mdibh compared to FB by increased coverage of the CTV and reduced dose to the organs at risk. Therefore, mdibh has been implemented as part of the standard radiotherapy for all left sided breast cancer patients in this department

4 Patients perspective on feasibility of breath correlated radiation therapy versus free breathing Christiansen R.L. 1, Gottlieb K.L. 1, Hansen C.R. 1, Ewertz M. 2 1 Laboratory of Radiation Physics, Odense University Hospital, Odense, Denmark 2 Department of Oncology, Odense University Hospital, Odense, Denmark Introduction: Moderate deep inspiration technique (mdibh) has been introduced to increase the quality of radiation therapy (RT) for breast cancer patients compared to conventional, free breathing (FB) RT. In order to receive treatment in mdibh the patients are required to follow breath hold instructions. This study investigates the patients perspectives on the feasibility of mdibh versus FB RT. Method and material A questionnaire was answered by 25 mdibh patients and a control group consisting of 25 FB patients. All patients underwent treatment with 50Gy/25F, 5 F/week. The DIBH patients all had left sided breast cancer, the control group consists of both left- and right sided breast cancers. Both groups include mastectomised as well as lumpectomised patients. The participants were asked to evaluate the level of ease, with which they underwent planning CT and RT, week by week, rated on a scale of 0-10 (0: treatment imposible to cope, 10 being treatment undergone effortlessly). Patients were also asked to evaluate the quality of information given prior to, and at the planning CT. Space was given in the questionnaire for elaborations on scores and further comments. Results The mean average feasibility scores for CT simulation and treatment week by week, for FB and mdibh respectively, are shown in the graph. Three patients scored feasibility of mdibh low (2-4), which was explained in the comments as caused by either childhood drowning trauma or problems with the mouth piece due to lack of teeth. The mean score for Quality of information was higher for mdibh (8.6) than that for FB (8.2). Skin reactions accounted for the highest number of negative comments for both groups of patients (5 mdibh, 4 FB). Acceptability Acceptality of planning/treatment 0 CT-sim Fraction 1-5 Fraction 6-10 Fraction Free Breathing mdibh Fraction Fraction Conclusion Patients generally founnd mdibh assisted RT as freasible as FB RT. Those patients, who struggled with mdibh had particular issues to cause this.. Some of these issues may be solved by individual mouth pieces.for both FB and mdibh patients, skin reactions was their main concern during the treatment - 2 -

5 Title: Electron trees produced with an Elekta linear accelerator. Authors: H.S. Rønde and A.L. Appelt. Vejle Hospital, Department of Medical Physics, Vejle, Denmark. Purpose Electron trees (Lichtenberg figures) can be produced by irradiating polymethyl methacrylate (PMMA) with an intense high energy electron beam and thereafter discharging the PMMA. This causes a dielectric breakdown, where the electrons create branching chains of fractures on leaving the PMMA, producing a tree pattern. However, accessing the primary electron beam on a clinical accelerator designed for hospital use is not simple, as it requires circumvention of monitor chamber, jaws, flattening filter, inhibits, etc. Here we describe the procedure for production of electron trees with a decommissioned Elekta linear accelerator (Linac) (Elekta, Crawley, UK). Materials and Methods Using a decommissioned Elekta Precise Linac with Desktop Pro 7.0 sp 2 software, we irradiated different sized pieces of PMMA. As access to the primary electron beam was required to ensure a sufficiently intense beam and at the same time avoiding damage to the monitor chamber and mylar mirror, we removed the entire head, leaving the target exposed. With the gantry placed in the 180 position, the head was left next to the gantry, such that all electrical connections to the Linac head were kept intact. In service mode the Linac was operated at clinical settings for 6 MV and 18 MV photons, respectively. The target position was then overwritten, to allow for emission of electron beams with energies up to 6 MeV and 18 MeV through the electron window. Cuboidal PMMA pieces from (3 cm x 4 cm x 9 cm) to (15 cm x 20 cm x 20 cm) and cylindrical pieces from (d = 3 cm, l = 9 cm) to (d = 8 cm, l = 11.5 cm) were irradiated. We irradiated the PMMA with 6 MeV, 18 MeV or both, from multiple sides. The PMMA was discharged with a centre punch tool grounded to the machine. Results As the ion chamber was removed from the beam, it was not possible to measure the delivered number of monitor units. But an approximate irradiation of the squared PMMA of 5 x 1 s from each side allowed for good results slightly more for the larger pieces and a little less for the smaller ones. The round pieces were irradiated on the curved side for about 1 s, turned a bit, irradiated and turned again, etc. The PMMA had to be kept electrically insulated during the irradiation to prevent spontaneous discharging, and the less time used to charge the PMMA the better, as it reduced the risk further. For the PMMA shapes used here, the optimal results were achieved with 6 MeV. After 6 hours of electron tree production, clear changes to the electron window was observed, most probably due to soot. A total of 24 electron trees were created in a variety of PMMA blocks. Conclusions We have demonstrated the feasibility of electron tree production with an Elekta Linac, provided the head can be removed from the gantry. The beam steering is sufficiently stable at beam start for the accelerator to irradiate without the monitor chamber in place. This experiment should never be performed with a clinical accelerator due to the strong possibility of damage to the equipment

6 INTER- AND INTRA-FRACTION GEOMETRIC ERRORS IN DAILY IMAGE GUIDED RADIOTHERAPY OF BREAST CANCER PATIENTS Mette Skovhus Thomsen 1, Ulla Harrov 2, Walther Fledelius 1, Per Rugaard Poulsen 2,3 1 Department of Medical Physics, Aarhus University Hospital, Denmark 2 Department of Oncology, Aarhus University Hospital, Denmark 3 Institute of Clinical Medicine, Aarhus University, Denmark Purpose Daily image guided radiotherapy (IGRT) may improve patient setup accuracy of radiotherapy treatments. However, for respiratory moving targets, setup based on two orthogonal setup images may be inaccurate since the target could be imaged in a position that is not representative for its mean position. In this study, the accuracy of daily IGRT with two orthogonal setup images was measured for breast cancer (BC) patients by continuous portal images acquired during treatment delivery. Methods Daily IGRT based on two orthogonal setup images was performed for eight right sided (RS) and eight left sided (LS) BC patients receiving radiotherapy with two tangential fields in either 25 (n=12) or 15 fractions (n=4). The setup procedure involved an MV portal image acquired in the direction of the first tangential field (gantry angle ~60 (RS) or ~120 (LS)) and an orthogonal kv image acquired with the gantry in the same position. The setup images were manually on-line registered with DRRs using the chest wall for the MV image and the anterior part of the ribs for the kv image. The resulting couch correction was calculated and performed automatically prior to treatment. During treatment continuous portal images were acquired at 5 or 8 Hz for both tangential fields showing the position and motion of the chest wall relative to the field aperture. After the treatments the chest wall position was determined semi-automatically in each portal image by an in-house built computer program and used to calculate the time-resolved geometrical setup error in beam s eye view of the treatment fields. For each treatment fraction, the mean setup error and the peak-topeak motion of the chest wall were determined in the direction of the portal imager, which is a linear combination of the left-right direction (~⅓) and anterior-posterior direction (~⅔) in patient coordinates. Results Portal image movies from 346 (first field) and 342 (last field) of the 360 fractions were available for analysis. The table shows statistics of the setup errors. Here, a negative sign indicates that less lung than planned was included in the treatment field. Setup errors Group mean (M) Random (σ) Systematic (Σ) First field -0.7 mm 1.5 mm 1.1 mm Last field -0.2 mm 1.7 mm 1.4 mm In about 2.5% of the fractions the setup error was found to be larger than 5 mm. These deviations were found to be due to setup imaging at an unusually deep expiration or inspiration at these specific fractions. The intra-treatment peak-to-peak motion amplitudes were patient dependent with mean values for the 16 patients of 2.1 ± 0.7 mm [range mm] and 1.9 ± 0.7 mm [range mm] for the first and last field, respectively. The most extreme peak-to-peak motion during a treatment was 8 mm caused by a single deep inspiration during treatment. Conclusions Intra-treatment motion of the chest wall was found to be patient specific and was in general quite small. Setup imaging with the chest wall in a non-representative breathing phase was only observed in a few per cent of the fractions treated. Thus, daily image guided radiotherapy of respiratory moving targets based on two orthogonal setup images is feasible

7 Combined PET/MR brain imaging: a spatially dependent bias in PET activity results if bone attenuation is not accounted for FL Andersen 1, C Ladefoged 1, T Beyer 1,2, S H Keller 1, AE Hansen 1, L Højgaard 1, I Law 1 and S Holm 1 1. Dept. of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, Copenhagen University Hospital, Copenhagen, DK 2. cmi-experts GmbH, Zurich, CH Part of this material has been presented at the EANM 2012 in Milan. INTRODUCTION. Commercially available combined PET/MR systems are not equipped with standard transmission sources, and attenuation coefficients must be derived from the MR images. With conventional MRI, little or no signal is detectable from bone. Instead, commercially implemented methods for MR-based attenuation and scatter correction (AC) will assign bone tissue an attenuation coefficient of water, fat or air. Here, the effect of neglecting bone during AC in PET/MR brain imaging is assessed using phantom and patient data. METHODS. 19 dementia patients were included in the study. Patients were injected with 200 MBq [ 18 F]FDG and imaged with PET/CT (Siemens Biograph mct) and PET/MR (Siemens Biograph mmr) in random order. Four sets of PET images were reconstructed: PET(/MR) DWFS, i.e. PET images obtained from PET/MR using MR-AC based on Dixon- Water-Fat segmentation (DWFS); PET(/MR) DWFS+BONE, using MR-AC based on DWFS with co-registered and segmented CT bone values superimposed; PET(/MR) fullct with coregistered full CT-based attenuation image; PET(/CT) CT i.e. standard PET/CT reconstruction

8 All PET images were reconstructed using AW-OSEM, with neither resolution recovery nor time-of-flight, and were decay-corrected to the start time of the first examination. PET images following AC were evaluated quantitatively on a voxel-by voxel basis and also using concentric regions of interest (ROIs) drawn on transverse MR images including the basal ganglia. We report the relative difference (%) of PET(/MR) DWFS, PET(/MR) DWFS+BONE and PET(/MR) fullct in reference to PET(/CT) CT ; and also PET(/MR) DWFS in reference to PET(/MR) DWFS+BONE and PET(/MR) fullct. In a separate phantom experiment a plastic bottle was layered with Gypsum plaster to mimic skull bone. The phantom was imaged on PET/CT only and standard MR-AC was simulated by replacing hyperdense CT attenuation values corresponding to bone (plaster) with attenuation values of water. RESULTS. PET(/MR) DWFS compared to PET/CT showed for all patients a spatial dependency with relatively lower activity values close to the skull. ROI mean values for PET(/MR) DWFS compared to PET/CT were ~15 percentage points lower in outermost 5 mm of the brain than the centre of the brain. ROI mean values for PET(/MR) DWFS+BONE or PET(/MR) fullct ) compared to PET/CT did not show this dependency. When comparing ROI values for PET(/MR) reconstructions, PET(/MR) DWFS relative to PET(/MR) fullct was reduced by 25% close to the skull and 10% in the central regions. Observations in patients were supported by the phantom study. CONCLUSION. Phantom and patient data show a spatial bias of the reconstructed and attenuation-corrected PET(/MR) images of the brain when bone attenuation is not accounted for. This has immediate implications for combined PET/MR brain imaging in research and clinical practice

9 Time-evolution of the regional lung density changes following IMRT for NSCLC Uffe Bernchou 1,2, Tine Schytte 1,3, Anders Bertelsen 2, Søren M. Bentzen 4, Olfred Hansen 1,3, Carsten Brink 1,2 1 Institute of Clinical Research, University of Southern Denmark, DK-5000 Odense, Denmark 2 Laboratory of Radiation Physics, Odense University Hospital, DK-5000 Odense, Denmark 3 Department of Oncology, Odense University Hospital, DK-5000 Odense, Denmark 4 Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53792, USA Purpose: To study the time-evolution of the regional dose-response following intensity modulated radiation therapy (IMRT) for Non-Small-Cell Lung Cancer (NSCLC). Methods and Materials A total of 387 follow-up CT scans in 131 NSCLC patients treated with IMRT delivered as static field step-and-shoot or VMAT to a prescribed dose of 60 or 66 Gy in 2 Gy fractions were investigated. 3D maps of lung density changes were created by a voxel-by-voxel subtraction of the planning CT from the deformable co-registered follow-up CT. Doseresponse signals were generated by averaging the density changes for regions of healthy lung tissue receiving the same planned dose at 5-Gy intervals. The temporal evolution of the response was analyzed using a parametric model containing the sum of a time-dependent transient component in the early phase and a persistent component in the late phase. Results Panel A of the figure shows the population average dose response at different time intervals after radiotherapy. In the early phase (time<3 months), a significant increase in lung density is observed for all dose intervals (P<0.0001). In this time interval the density change from baseline increases as function of dose and reaches a plateau around 45 Gy. The subsequent evolution of the response differs in the high- and low-dose region. In regions lower than 45 Gy the response decreases, while the plateau is replaced by a transient peak at doses above 45 Gy. In the late phase (time>12 months) the response stabilizes at all dose intervals. The model shows a significant (P=0.022) increase in the onset of the transient component of the response as function of dose (see panel B of the figure). Conclusion NSCLC patients treated by modern radiotherapy techniques exhibit a significant increase in density of healthy lung tissue even at low dose levels (5-10 Gy). The transient nature of the response in the early phase might be indicative of radiation pneumonitis, while the eventual stabilization of the response likely is associated with fibrosis. The temporal evolution of the response differs in the high- and low-dose region. This finding suggests distinct dose-dependent time characteristics for the development of radiation pneumonitis and fibrosis. (A) Transient and persistent response (B) Transient + persistent response Transient Persistent - 7 -

10 L. Hoffmann1, D.S Moeller1, A.A. Khalil2, M.M. Knap2 1Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark. 2Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Adaptive radiotherapy for lung cancer patients: : Large scale clinical implementation Overall survival for lung cancer patients is poor partly due to local recurrence. Dose escalation may improve local control and hence improve the overall survival for lung cancer patients However, this demands a high precision in delivery of the daily RT treatment in order to minimize margins and normal tissue irradiation. To achieve this adaptive radiotherapy (RT) for lung cancer patients was implemented in a large scale clinic with a comprehensive educational program for the radiation therapists. Daily soft tissue matching using GTV T is performed. For each match, GTV T is evaluated using an evaluation structure (GTV T+2mm) formed by adding a symmetric margin of 2mm to GTV T. The pass criterion is that GTV T on CBCT shall be within GTV T+2mm. The lymph nodes, GTV N is evaluated using a surrogate structure. Normal tissue within 2cm from GTV N is used as surrogate structures, named MatchN. A deviation of maximum 5mm is accepted for these structures. Deviations in the position of the spinal cord is evaluated by use of the vertebral column. Finally, normal tissue changes, e.g. appearance of an atelectasis or changes >1cm in the position of the heart are considered important. When three consecutive deviations or changes are observed, the deviation or change is considered systematic and the patient gets an adaptive plan. An education programme for the RTTs were made, including 6 e-learning modules of an estimated duration of 4 hours and 3 x4 hours of hands on training in the learning centre using CBCTs from 22 patients. If deviations above the criteria were seen, the RTTs should note that the match was not within tolerance. The RTTs scored deviations similar to an expert evaluation and were ready for the clinic. After the implementation in the clinic our conclusions are: Implementation of adaptive RT for lung cancer patients is feasible, but requires daily examination of the CBCTs based on criteria for tumour and normal tissue

11 Purpose This work provides a link between quality assurance (QA) parameters and the expected clinical effect of these for prostate RapidArc (RA) plans. Methods and Materials For 8 clinical plans, and 160 plans with introduced errors, QA measurements were carried out and the result of these correlated with the values of tumour control probability (TCP) and probability of > grade 2 rectal bleeding (NTCP) arising from the plans. A QA parameter set consists of 3 parameters: the dose difference (DD: the difference between the measured and calculated dose at a point); the distance to agreement (DTA: the distance in mm between points of identical dose value in the measured and calculated data set) and A, the percentage of the QA data set which must pass the DD and DTA criteria for the QA result to be a "pass". The sensitivity and specificity of the QA criteria quantify the true positive rate and the true negative rate of the QA results. 10 QA criteria were assessed. Results Of the 10 QA criteria sets investigated, one was not significantly better than the others. However, three of the criteria were significantly worse than the best. For DD=3%; DTA=3mm; a range of A was used in the analysis. A=95% was found to maximise the sum of the sensitivity and the specificity (S+S). QA parameters of DD=3%, DTA=3mm and A=94% correspond to changes of 1% in TCP and 80% in NTCP. Conclusions Prostate RapidArc plans are rather robust with respect to QA criteria. A range of QA criteria suitable for analysis of RapidArc prostate plans were identified. For the acceptable QA parameters (DD=3%; DTA=3mm), the best value for A was determined. The QA parameters (DD=3%; DTA=3mm; A=94%) used at our clinic correspond to maximum changes of 1% in TCP and 80% in NTCP

12 IV contrast-enhanced CT can be used for attenuation correction in clinical 111-In octreotide SPECT/CT Klausen TL, Andersen FL, Mortensen J, Beyer T, Højgaard L, Holm S. Department of Clinical Physiology, Nuclear Medicine and PET, Copenhagen University Hospital Rigshospitalet, Denmark Aim: To assess the effect of CT contrast-enhancement (cect) on SPECT image quality and quantification following CT-based attenuation correction (CT-AC) using phantom and patient studies. Methods: A 20cm cylinder phantom (5.15L) was filled with In-111 in aqueous solution (20kBq/mL). Three scans were performed: (A) no IV contrast in the solution, (B) with 100mL IV contrast (Optiray, 350mg/mL iodine) and (C) with 200mL IV contrast added. Our clinical SPECT/CT (Philips Precedence-16) protocol included a topogram, a low-dose CT (140kVp, 38mA), a diagnostic CT (120kVp, 469mA) and a single-bed SPECT emission scan (128 projections, scan time 1320s). 5 patients referred for octreotide scintigraphy were scanned using the clinical SPECT/CT protocol above with the diagnostic CT being performed in arterial and venous phase following mono-phasic IV injection of 125mL Optiray (4.5ml/s). All SPECT data were reconstructed iteratively using the Astonish algorithm: 4 iter/8 sub following CT-AC using the low-dose and diagnostic CT images. We report the average ROI value of a 10cm central circle placed on 11 central image planes. Patient images with low-dose CT and diagnostic cect were examined for artifacts and quality (score 0-2). Reference circle ROIs were placed on healthy liver tissue. Results: Phantoms: In uncorrected emission data mean count density decreased with increasing IV concentration: (A) 181±14, (B) 173±12 and (C) 167±12. After CT-AC using low-dose CT data the mean activity concentration increased by 2.1% (B) and 3.1% (C) when compared to no IV contrast in the phantom (A). When performing CT-AC with diagnostic CT the mean activity concentration decreased slightly: -3.7% (A), -3.3% (B) and -2.7% (C) compared to low-dose CT. Patients: There were no visible artifacts in SPECT following CT-AC with cect. The average score of image quality was 2.0±0.0 for both low-dose and diagnostic cect. Across all 5 patients, reconstructed counts in the liver were reduced by (10±7)% when CT-AC was based on diagnostic cect (arterial phase) compared to low-dose CT, possibly caused by the difference in kvp-levels. Using the same kvp level, the reconstructed counts in the liver following CT-AC with diagnostic cect was (1.6±0.5)% higher in the venous than the in the arterial phase. Conclusion: In SPECT/CT imaging of phantoms and octreotide patients the use of IV CT contrast did neither degrade SPECT image quality nor the reconstructed counts significantly following CT-AC based on the cect images. Thus, no separate low-dose CT is essentially required for CT-AC in SPECT/CT in these patients

13 A protocol for comparative small field dosimetry at Danish radiotherapy clinics A.R. Beierholm 1, C.F. Behrens 2, H.S. Rønde 3, S.B.N. Biancardo 4, J.B. Thomsen 5, L. Nyvang 6, H.L. Riis 7, K.M. Petersen 8, J. Helt-Hansen 1, C.E. Andersen 1 1 Center for Nuclear Technologies, Technical University of Denmark, Roskilde 2 Department of Oncology R, Copenhagen University Hospital, Herlev 3 Department of Medical Physics, Vejle Hospital, Vejle 4 The Finsen Center Department of Oncology, Copenhagen University Hospital, Copenhagen 5 Department of Oncology, Aalborg University Hospital, Aalborg 6 Department of Medical Physics, Aarhus University Hospital, Aarhus 7 Radiofysisk Laboratorium, Odense University Hospital, Odense 8 The National Institute for Radiation Protection, Danish Health and Medicine Authority, Herlev The use of small photon fields has proliferated with the introduction of dynamic radiotherapy modalities such as intensity-modulated radiotherapy and volumetricmodulated arc therapy. The dosimetry of small fields is complicated and can lead to ambiguous results, as described in the Report No. 103 by the Institute of Physics and Engineering in Medicine (IPEM). The International Atomic Energy Agency (IAEA) has introduced a formalism for dosimetry in small and non-reference fields, but there is still no consensus on which detectors that can be considered suitable for small fields. Successful treatment outcome depends on the accuracy of the beam data loaded into treatment planning systems - especially for treatments where the major part of the dose delivery originates from small fields. A comparison between dosimetry methods, and between clinics, is therefore required to assess the uncertainties on dose estimates. We present a protocol for comparative dosimetry which involves measurements of relevant dosimetric parameters, obtained using the same set of dosimetry equipment. Measurements are performed at six Danish radiotherapy clinics, using medical linear accelerators and water phantoms situated at each clinic. The measurements concern output factors, TPR20:10 ratios, dose per pulse, absolute dosimetry, and field profiles. The detectors used are an IBA Dosimetry Farmer ionization chamber, a PTW Pinpoint ionization chamber, a PTW Diamond detector, a DTU Nutech ME40 scintillator dosimetry system, and Gafchromic EBT2 film. Output factors are measured using both ionization chambers, the diamond and the scintillator system, while TPR20:10 ratios are measured using only the Farmer chamber and the scintillator system. Dose per pulse is measured using the scintillator system, while absolute dosimetry is performed using the Farmer chamber. Field profiles are acquired using EBT2 film. All measurements except those for the film are carried out in water, while film measurements are carried out in solid water. The main objective of the protocol is to ensure that the small field dosimetry performed at Danish radiotherapy clinics is of high quality. The results obtained using the protocol are compared with reference beam data, and with dose calculations performed using the treatment planning system at each clinic. Furthermore, an inter-comparison between the results of all clinics is made. This work was financially supported by a grant from the Danish Cancer Society

14 Clinac MVD sensitivity and Truebeam MVD sensitivity at different dose rates Introduction According to the Danish legislation every radiotherapy treatment must be checked by an independent system to insure that the planned dose is within the prescribed tolerances. In addition to the beam modulation achieved by the multileaf collimator (MLC) motion patterns in intensity modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT) employs a variable dose rate and gantry speed during beam delivery. To check the radiotherapy treatment the calculated and measured 2D doses of a VMAT plan is often evaluated using a gamma analysis. During these analyses, it was noticed that the modulation of the field is correct, but that the measured doses are often higher (<5%) than the calculated doses. The purpose of this study is to investigate the sensitivity of the megavoltage detector (MVD) for the variable dose rates used in VMAT radiotherapy. Method and materials The measurements are performed with the MVD system on a Varian Clinac accelerator (Clinac MVD) and the MVD system at a Varian Truebeam accelerator (Truebeam MVD). The MVD is a 2D onboard amorphous silicon detector, and the doses are recorded in the integrated image mode. The Clinac MVD is calibrated at two dose rates: 300 MU/min and 600 MU/min. The Truebeam MVD can only be calibrated at one dose rate and we have chosen 600 MU/min. When setting up a VMAT plan, one chooses the maximum dose rate allowable in the plan, which determines the dose rate of the MVD calibration. However, the actual dose rate is controlled by the dose and the length of the arc. Open VMAT fields at different dose rates were measured by beginning the arc at gantry angle 0 and stopping at the gantry angles described in the table below, leading to the shown constant dose rates. Gantry stop [ ] , , ,8 24 Dose rate [MU/min] The measurements were repeated five times with 50 MU for a 10 cm 10 cm field with the MLC parked outside the beam for both 6 MV and 15 MV. The average dose to the central pixels (4 mm 4 mm) is analysed in the ImageJ software and the dose deviations relative to the output at dose rate 600 MU/min and at the fixed gantry angle 0 are calculated. The output during the arcs from the Varian Clinac accelerator at 6 MV was checked independently by performing two sets of the above measurement with the IC Profiler array from Sun Nuclear placed in an iso-centric mount. The dose integrator is checked on a monthly basis by measuring the dose at gantry 0 for dose rate 100 MU/min and 600 MU/min. Results The sensitivity of the Clinac MVD is up to +3.5% and +1.2% higher for lower dose rates compared to the calibration dose rate for 6 MV for 15 MV, respectively. The sensitivity variation of the Truebeam MVD for the same measurements is only +0.5% and +0.3%, respectively. Similar measurements and analysis with the IC profiler on the Varian Clinac showed a -0.8% dose deviation. The deviation for the dose integrator test was within ±0.2% except for 6 MV on the Clinac which showed a deviation of -0.6%. Conclusion For a simple non-modulated arc field the sensitivity of the Clinac MVD increases significantly at lower dose rates compared to the dose rate used for calibration. On the contrary, the sensitivity of the Truebeam MVD is almost independent of the dose rate. Mai-Britt Kyed Jørgensen, Ulrik Vindelev Elstrøm, Christian Skou Søndergaard, Lars Hjorth Præstegaard Afdelingen for Medicinsk Fysik, Aarhus Universitetshospital

15 Dose and image quality in pediatric CT of thorax and abdomen in 4 different CT scanners. Purpose: Image quality is of great importance in CT scanning of children. Often the choice of scanner is based on comparison of scans acquired with scan parameters defined by vendors. But as the vendors might have different levels of image quality and dose levels the comparison will not be objective. In the present work dose and image quality is compared by scanning an ATOM standard dosimetri verification phantom (pediatric 10 years old) with 4 different CT scanners with vendor defined scan protocols for pediatric thorax and pediatric abdomen using both FBP and iterative reconstruction algorithms. Methods: To evaluate image quality CT imaging QA plug in s (1 with line pair pattern and 2 with low contrast patterns) were placed in different positions (liver and lung) in the phantoms. The corresponding organ doses were measured by placing TLD in the same position as the QA plug in but on the contra lateral side of the phantom. Determination of the image quality was done by scoring (4 persons) the low and high contrast of the inserts in anonymized images and by calculating the SD (Standard Deviation) and CNR (Contrast-to-Noise-Ratio) in ROI s (Region Of Interest) in predefines positions in the images. Results: Scoring of the QA plug in s showed that 2 out of 3 plug in s in this case could not be used to determine image quality as either too less or all targets were visible. Scoring of one plug in showed no coherence between dose and image quality (low contrast). Using SD (mean of 10 samples) as a parameter of image quality the expected coherence between SD and dose was shown for 3 out of 4 scanners. As the study lacks information about spatial resolution, this might indicate a higher amount of smoothening in the reconstruction algorithm of the 4 th scanner. Two of the scanners use both FBP and iterative reconstruction. In one of these the goal is to keep the image quality constant and lower the dose when using iterative reconstruction. T-test confirmed this with a p-level of 0,59. Further work: The QA plug in s are manufactured for adult phantoms and the study indicates that there is a need to adjust them to pediatric use. The study lacks information about spatial resolution. Therefore an analysis of image quality will be repeated using Catphan 700 with Auto QA light software

16 DSMF abstract (2013) DTU dosimetri: Nyt laboratorium under opbygning Claus E. Andersen, Jakob Helt-Hansen, Lars Lindvold, Bent Lauritzen og Mikael Jensen Center for Nukleare Teknologier, Danmarks Tekniske Universitet, DK-4000 Roskilde Center for Nukleare Teknologier (DTU Nutech) er ved at etablere et Laboratorium for Fundamental Medicinsk Dosimetri på Risø Campus nær Roskilde. Laboratoriet bygger på en stor donation fra the John and Birthe Meyer Foundation, og det kommer til at omfatte state-of-the-art udstyr bestående af en medicinsk accelerator, et reference kobolt-60 bestrålingsanlæg samt avanceret instrumentering, fuld indeklimastyring og anden infrastruktur. Acceleratoren bliver en Varian Truebeam med fem konventionelle fotonenergier (4, 6, 10, 15 og 18 MV), to fotoenergier uden flattening filter (6 og 10 MV) og syv elektronenergier (6, 9, 12, 15, 16, 18 og 20 MeV). Laboratoriet skal understøtte udvikling og indførsel af nye former for stråleterapi på hospitalerne, og det skal være med til at sikre en fortsat høj standard for patientsikkerhed ved moderne strålebehandling. Laboratoriet skal således bl.a. forske i nye primære standarder og målemetoder til brug ved behandling med små strålingsfelter. Formålet med posteren er kort at redegøre for status for opbygningen af laboratoriet og fremtidsperspektiverne i relation til nye danske bidrag og samarbejder vedr. undervisning og forskning indenfor dosimetri og metrologi

17 Titel: Konform linseskånende 8-felts elektron-behandling til retinoblastom-patienter. Person(er) Kirsten Johansen, Institut for Fysik og Astronomi, AU Karsten Riisager, Institut for Fysik og Astronomi, AU Harald Spejlborg, Afdeling for Medicinsk Fysik, AUH Tekst:: Formål og baggrund: Pædiatrisk retinoblastom er en sjælden malign sygdom i øjet, der optræder med en incidens på 1 ud af levendefødte. Kurative behandlingsmuligheder er kemoterapi, kirurgi eller strålebehandling. De almindeligt anvendte teknikker til strålebehandling af retina giver enten ikke tilstrækkelig beskyttelse af linsen, uden at gå på kompromis med targetdækning, eller beskytter linsen, med en stor integraldosis til cerebrum som resultat. Formålet med dette arbejde er således at udvikle en velegnet linseskånende strålebehandlingsteknik med minimal integral cerebral dosis til øjenbestråling af børn med retinoblastom, der kræver ekstern bestråling af hele retina. Metode og materialer: I dette arbejde valgtes elektronbestrålingsteknik mhp. at minimere integraldosis til cerebrum. Alle dosisfordelinger er målt med Gaf-Chrom film i Solid Water testfantomer. Alle elektronfelter er fra en Varian Clinac lineær accelerator udstyret med standard elektronapplikator. Projektets 1.del: Optimering og minimering af elektronfeltets randbredde (penumbra): Med et 12MeV elektronfelt (4cmx4cm kvadratisk felt, SSD= cm) måltes elektronfeltets penumbra på overfladen og i 1cm dybde, i et fantom under variation af en ekstern kollimators afstand fra fantomoverfladen Projektets 2.del: I et specialfremstillet orbita-fantom, med film placeret i flere dybder, undersøgtes eksperimentelt hvilken konfiguration af elektronfelter, der gav det mest optimale resultat. Projektets 3. del: De eksperimentelle dosisprofiler blev sammenlignet med dosisprofiler udregnet i dosisplanlægningssystemet Varian Eclipse. Resultater: Vi fandt en lineær sammenhæng mellem elektronfeltets penumbra og den eksterne kollimators afstand fra fantomoverfladen. Kollimatorens effekt er således størst, når den anbringes så tæt på fantomet som muligt. Afstanden er i det efterfølgende sat til 1cm, som giver god kollimering samtidig med, at der gives plads til næsen ved klinisk anvendelse. Det er fundet at en konfiguration af 8 non-coplanare 12MeV felter med en vinkling på 65 i forhold til fantomoverflade er velegnet til at bestråle retina samtidig med at linsedosis minimeres. Den optimale kollimatorgeometri er en cirkulær åbning med delvis afdækning af orbita incl. linse i den ene kvadrant De eksperimentelle dosisprofiler tæt på overfladen og linsen havde en skarpere dosisgradient end de Eclipse udregnede. Dette er forventeligt, da Eclipsesystemets udregning forudsætter en kollimering 5cm over fantomet. Konklusion: Dette studie på testfantomer demonstrerer, at en 8-felts non-coplanar elektronbestrålingsteknik er velegnet til sufficient bestråling af retina samtidig med at øjets linse skånes. Forbedret ekstern kollimering af elektronfelterne er essentielt for at opnå bedst mulig eksperimentel konformitet i dosisfordelingen. Det findes således at, ved optimering af ekstern kollimering af elektronfelter er konformiteten af den eksperimentelt bestemte dosisfordeling bedre end den beregnede i et standard dosisplanlægningssystem

18 Quantitative clinical image quality comparison of pelvic CBCT for two imaging systems T.B. Andreasen, H. Lindberg, D. Sjöström, A. Vestergaard, C.F. Behrens Purpose Quantitative objective analyses are widely used in radiology. These are relevant in oncology as well, since the use of pelvic CBCT for adaptive RT requires a certain level of image quality. The purpose of this study is to objectively evaluate the clinical image quality of two systems; a state-of-the-art CBCT system and a new CBCT system with improved reconstruction and hardware. Materials/Methods The patients included in this retrospective study had a planning CT and CBCT s from Clinac ix as well as Truebeam (Varian Medical Systems). Based on European Guidelines provided by CEC seven quality criteria in relation to the bladder on CBCT were defined along with an ordinal rating scale reflecting the fulfillment of a particular criterion. The corresponding author and a number of physicians rated in a randomized order the CT s and the pelvic CBCT s. The resulting data were evaluated by a statistical analysis called Visual Grading Characteristics (VGC) in the free software DBM MRMC 2.32 Build 3. The difference in image quality between the two modalities was evaluated by the area under the curve (AUC) and ANOVA. An AUC of 0.5 indicated equally image quality whereas higher values indicated superior image quality. If 0.5 was not included in the 95% confidence interval the difference in image quality of the systems was significant. A VGC curve comprising the total image quality criteria was found for each observer. Furthermore, the impact of the individual criteria was demonstrated by a VGC curve and the respective AUC. The Image Criteria Score (ICS) was calculated for the total and individual criteria and ideally ICS would equal 1.0. As a reference the VGC analysis of the CT was performed. Results An excerpt of the results of the corresponding author is included for five criteria (Figure 1). The VGC curves clearly illustrate better performance of Truebeam than Clinac ix for criterion I-IV, whereas the performance is more equivocal regarding criterion V. The AUC was 0.68 for the total quality criteria and the 95% confidence interval was [0.55, 0.80]. For criterion I the AUC was 0.72, criterion II yielded an AUC of 0.71, criterion III an AUC of 0.73, criterion IV an AUC of 0.70 and for criterion V AUC was The total ICS for Truebeam and Clinac ix was 0.49 and 0.27, respectively. For the individual criteria the ICS was higher for Truebeam than Clinac ix. The percentage difference ranged between 11.1 and 33.3 percentage points

19 Conclusion The applicability of the VGC analysis on CBCT used for adaptive RT was investigated and found useful. Regarding the difference in image quality between the two systems Truebeam was found superior to Clinac ix. The VGC analysis might find its use for any two imaging systems in the clinic

20 Evaluating the usefulness of EPID for daily output verification by comparison to ionization chamber measurements Authors: P. Andersson 1, S.K. Buhl 1. 1 University Hospital Herlev, Department of Oncology (R), Herlev, Denmark. Purpose/Objective To investigate the long term stability of the amorphous silicon electronic portal imaging device (asi EPID) for monitoring medical linear accelerator output, by a) acquiring output data on a daily basis using the EPID, b) correlating EPID data to weekly measurements using an ionization chamber in a Perplex phantom and c) correlating as well as calibrating EPID response to absolute measurements with an ionization chamber in water. Materials and Methods A test patient, including two open fields (25x25 cm 2, 6 and 15 MV) intended for output measurements with the EPID, was defined. During a period of > 8 months, daily measurements were performed on 9 different medical linear accelerators (8 x Varian Clinac 2300iX and 1 x Varian TrueBeam), equipped with asi EPIDs (Varian as1000). Data from the EPID measurements was extracted, using a in-house software developed in MATLAB, and compared to weekly output measurements with an ionization chamber in a Perplex phantom as well as to quarterly (or on indication) measurements with an ionization chamber in water. Calibration of the EPIDs were performed in conjunction with the ionization chamber measurements in water, also at which point the LINAC output was adjusted to within ±0.3% of the reference data based on the measurements in water. Results The ability of the EPID to detect output variation was confirmed by the correlation between the EPID measurements and the ionization chamber measurements in the Perplex phantom as well as in water (Figure 1), for both the Varian Clinac 2300iX as well as for the TrueBeam. However, a variation in performance between some EPIDs is present, possibly in some extent as a result of the differences in the wear and tear of the EPIDs. It is also clear that a daily variation of the EPID data exists and needs to be considered when selecting tolerance levels. Some of the EPIDs detected a greater increase in output over time in comparison to the increase in output detected by the ionization chamber measurements. However, regular calibration of the EPIDs in conjunction with the measurements in water proved to be a solution for this exception