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252 poster A novel, rigorous approach to dosimetric characterisation of CCD-based electronic portal imaging devices
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Posters
ues than distributions of values. Also, the relative importance of the modelling parameters determining the. slope of the TCP curve is different if distributions of values rather than single values are used. Therefore, distributions of values are needed for realistic TCP modelling. 250
poster
Optimisation of 3D conformal radiotherapy for prostate cancer: a radiobiological analysis P. Diez 1, P.J. Hoskin 2, P.J. Bownes 1, P.J. Ostler 3, E.G.A. Aird t 1Mount Vernon Cancer Centre, Clinical Physics, Middlesex, United Kingdom 2Mount Vernon Cancer Centre, Marie Curie Research Wing, Middlesex, United Kingdom 3Mount Vernon Cancer Centre, Oncology, Middlesex, United Kingdom External beam radiotherapy (EBRT) is commonly used for the management of adenocarcinoma of the prostate. A typical treatment, such as that proposed in the RT-01 UK prostate trial protocol, would involve delivering 64 Gy in 32 fractions to the PTV, plus a 10 Gy boost in 5 fractions to the GTV, all in 2 Gy per fraction. A similar dose escalation may also be achieved using high dose-rate brachytherapy (HDR-BT). This study aims to determine whether HDR-BT is indeed an optimisation tool in radiotherapy of prostate cancer by comparing predicted local control rates and rectal toxicity for both dose-escalating techniques. Ten random intermediate4o-high risk patients have been planned for different treatment schedules: RT-01 (high-dose arm) [as described above]; EBRT alone [55 Gy in 20f] and EBRT+HDR-BT [35.75 Gy in 13f + 17 Gy in 2f] as delivered at MVCC; HDRBT alone according to protocols used at the William Beaumont, Michigan, [38 Gy in 4f] and in Osaka, Japan [48Gy in 8f or 54Gy in 9f]. Local control is defined by predicted tumour control probabilities (TCP) and normal tissue complication probabilities are calculated using rectum as the critical normal tissue. Different cd~ ratios for prostate cancer have been considered. Boosting of the target volume using HDR-BT has significant advantages for tumour control if the ed[3 ratio for prostate cancer is low, comparable to that of late-reacting tissue, without compromising rectal morbidity. HDR-BT as monotherapy does not show a clear benefit for TCP over the combined modality but both schedules using HDR are superior to EBRT alone if cd~ is low. An assessment of the impact of dose optimisation with external beam using IMRT is under way. 251
poster
Treatment precision and treatment effect in conformal radiotherapy of urinary bladder cancer - preliminary results L.P. Muren 1,2, R. Ekerold2, R. Smaaland2, O. Dahl 1 1University of Bergen, Section of Oncology, Institute of Medicine, Medical Faculty, Bergen, Norway 2Haukeland University Hospital, Dept of Oncology and Medical Physics, Bergen, Norway Purpose: Geometrical uncertainties may compromise the treatment precision of conformal radiotherapy (CRT) and thereby cause local treatment failures. This study aimed to correlate treatment precision to treatment effect in radical CRT of urinary bladder cancer (UBC). Materials and methods: Treatment precision and treatment effect data for 20 UBC patients were analysed. Patients were prescribed a dose of 60-64 Gy, and treated with a four-field CRT technique using CTV-ITV margins of 1.5-2.0 cm. Target coverage measures were derived from treatment planning data supplemented with the actual treatment Iocalisation of the bladder from co-registration of weekly repeat CT scans. The outcome of the treatment in terms of local control was quantified from the routine follow-up cystoscopies, commencing 3 months post-treatment. Results: In 105 of the 128 repeat scans (82%), at least 99% of the CTV received at least 95% of the prescribed dose (CTV_95% > 99%). 15 of the 20 patients (75%) had adequate CTV coverage, defined as a CTV_95% > 99% in all except one of the repeat scans. Local control was achieved in 11 of the 17 patients (65%) that could be evaluated (12 months median followup). In the 11 cases with local control, the CTV_95% > 99% in 64 of 73 repeat scans (88%), compared to in 31 of 39 repeat scans (79%) in the six cases with local failure (p=0.38). Nine of the 11 cases with local control had adequate CTV coverage (81%), compared to four of the six patients with local failure (67%) (p=0.92). There were also no statistically significant differences between patients with local control and patients with local failure in the average CTV coverage measures, including minimum CTV dose and the average CTV fraction receiving less than 90% or 95% of the dose. Con¢lusi0n$: Local control was achieved in 65% of patients with UBC prescribed 60-64 Gy to the bladder with wide margins. Patients with local con-
trol tended to have slightly higher CTV coverage rates than patients with local failures, but the differences were not of statistical significance. Future studies will investigate whether the inclusion of information about the exact tumour Iocalisation and patient positioning data derived from repeat electronic portal imaging will improve the correlation between target coverage and local control.
I M A G I N G FOR RT 252
poster
A novel, rigorous approach to dosimetric characterisation of CCD-based electronic portal imaging devices J.C.J. de Beet E.M. Franken, B.J.M. Heijmen Erasmus MC, Daniel den Hoed Oncology Center, Rotterdam, The Netherlands Introduction: The excellent long term response reproducibility of CCD-camera based electronic portal imaging devices (CEPIDs) renders them highly suited for accurate portal dosimetry. Five of our Theraview-NT CEPIDs are equipped with cooled (-20C) CCDs. By suppressing the deleterious effect of radiation damage, cooling has boosted the CCD-lifetime by years and improved signal-to-noise ratios (SNR) markedly over previous generation CEPIDs. However, precise CEPID-based dosimetry requires knowledge of the low-amplitude but spatially extended tails of the point-spread functions (PSFs), mainly stemming from optical cross-talk between the mirror and the fluorescent screen (FS). We present a method to measure these PSFs directly, without making any assumptions on their properties. Methods and Materials: Position dependent, two-dimensional PSFs were extracted from CEPID images (EPI) as follows. Using the linac collimators, EPI were obtained for a series of small fields (side < 3 cm) positioned on a grid covering the FS. For high SNR, the CEPID software allowed for videoframe summation up to large exposures. The small fields thus imaged effectively constitute input pencil-beams. A transmission EPI, obtained with collimators closed, was subtracted. The resultant EPI directly measures the PSF for a given field position. This procedure was executed for a low and an intermediate elbow CEPID (distance between FS and mirror of 5 respectively 13 cm). In addition, the optical propagation in the CEPID was modelled to predict the PSFs. Results: For both elbows, PSF-tails could be confidently measured using 2000 MU per EPI, even for tail magnitudes of 0.1% of the PSF-peak. The PSFs were asymmetrical, dependent on position on the FS, and were quite different for the low and intermediate elbow. These features were in good agreement with the model predictions. Hence, a small number of PSF measurements (-9) in combination with our model was sufficient for a complete PSF characterisation of both CEPID types. Conclusions: The high SNR offered by present CEPIDs, together with a basic optical cross-talk model, allowed for a complete experimental determination of PSFs using a few calibration measurements per CEPID. We are now implementing robust and fast deconvolution schemes to translate EPI into portal dose images for position dependent PSFs. Hence, a general framework for precise portal dosimetry with CEPIDs, even in case of highly asymmetrical IMRT fields, is established. 253
poster
Effects of geometric distortion in pelvic MRI on radiotherapy treatment planning of prostate cancer patients B. Petersch 1, J. Bogner 1, A. Fransson 2, T. Lorang 3, R. P~tter 1 1University of Vienna, Department of Radiotherapy and Radiobiology, Vienna, Austria 2Karofinska Hospital, Department of Hospital Physics, Stockholm, Sweden 3University of Vienna, Institut for Medizinische Computerwissenschaften, Vienna, Austria Purpose: To evaluate the impact of two different methods of geometric distortion correction of MR images from a Siemens Magnetom Open Viva 0,2T resistive MR unit on the process of external beam radiotherapy treatment planning (RTTP) for prostate cancer. Material and methods: A method for correction of system related AND object induced distortions and one for correction of purely system related distortions have been evaluated. The latter used information extracted from MR images of a 3D phantom specifically designed for geometric distortion evaluation. An active shim procedure was performed prior to all phantom and patient scans For each of five patients five standard treatment plans were compared using uncorrected and corrected MR images alone (density=water) and CT images alone. Finally internal anatomical landmarks were
Posters
ues than distributions of values. Also, the relative importance of the modelling parameters determining the. slope of the TCP curve is different if distributions of values rather than single values are used. Therefore, distributions of values are needed for realistic TCP modelling. 250
poster
Optimisation of 3D conformal radiotherapy for prostate cancer: a radiobiological analysis P. Diez 1, P.J. Hoskin 2, P.J. Bownes 1, P.J. Ostler 3, E.G.A. Aird t 1Mount Vernon Cancer Centre, Clinical Physics, Middlesex, United Kingdom 2Mount Vernon Cancer Centre, Marie Curie Research Wing, Middlesex, United Kingdom 3Mount Vernon Cancer Centre, Oncology, Middlesex, United Kingdom External beam radiotherapy (EBRT) is commonly used for the management of adenocarcinoma of the prostate. A typical treatment, such as that proposed in the RT-01 UK prostate trial protocol, would involve delivering 64 Gy in 32 fractions to the PTV, plus a 10 Gy boost in 5 fractions to the GTV, all in 2 Gy per fraction. A similar dose escalation may also be achieved using high dose-rate brachytherapy (HDR-BT). This study aims to determine whether HDR-BT is indeed an optimisation tool in radiotherapy of prostate cancer by comparing predicted local control rates and rectal toxicity for both dose-escalating techniques. Ten random intermediate4o-high risk patients have been planned for different treatment schedules: RT-01 (high-dose arm) [as described above]; EBRT alone [55 Gy in 20f] and EBRT+HDR-BT [35.75 Gy in 13f + 17 Gy in 2f] as delivered at MVCC; HDRBT alone according to protocols used at the William Beaumont, Michigan, [38 Gy in 4f] and in Osaka, Japan [48Gy in 8f or 54Gy in 9f]. Local control is defined by predicted tumour control probabilities (TCP) and normal tissue complication probabilities are calculated using rectum as the critical normal tissue. Different cd~ ratios for prostate cancer have been considered. Boosting of the target volume using HDR-BT has significant advantages for tumour control if the ed[3 ratio for prostate cancer is low, comparable to that of late-reacting tissue, without compromising rectal morbidity. HDR-BT as monotherapy does not show a clear benefit for TCP over the combined modality but both schedules using HDR are superior to EBRT alone if cd~ is low. An assessment of the impact of dose optimisation with external beam using IMRT is under way. 251
poster
Treatment precision and treatment effect in conformal radiotherapy of urinary bladder cancer - preliminary results L.P. Muren 1,2, R. Ekerold2, R. Smaaland2, O. Dahl 1 1University of Bergen, Section of Oncology, Institute of Medicine, Medical Faculty, Bergen, Norway 2Haukeland University Hospital, Dept of Oncology and Medical Physics, Bergen, Norway Purpose: Geometrical uncertainties may compromise the treatment precision of conformal radiotherapy (CRT) and thereby cause local treatment failures. This study aimed to correlate treatment precision to treatment effect in radical CRT of urinary bladder cancer (UBC). Materials and methods: Treatment precision and treatment effect data for 20 UBC patients were analysed. Patients were prescribed a dose of 60-64 Gy, and treated with a four-field CRT technique using CTV-ITV margins of 1.5-2.0 cm. Target coverage measures were derived from treatment planning data supplemented with the actual treatment Iocalisation of the bladder from co-registration of weekly repeat CT scans. The outcome of the treatment in terms of local control was quantified from the routine follow-up cystoscopies, commencing 3 months post-treatment. Results: In 105 of the 128 repeat scans (82%), at least 99% of the CTV received at least 95% of the prescribed dose (CTV_95% > 99%). 15 of the 20 patients (75%) had adequate CTV coverage, defined as a CTV_95% > 99% in all except one of the repeat scans. Local control was achieved in 11 of the 17 patients (65%) that could be evaluated (12 months median followup). In the 11 cases with local control, the CTV_95% > 99% in 64 of 73 repeat scans (88%), compared to in 31 of 39 repeat scans (79%) in the six cases with local failure (p=0.38). Nine of the 11 cases with local control had adequate CTV coverage (81%), compared to four of the six patients with local failure (67%) (p=0.92). There were also no statistically significant differences between patients with local control and patients with local failure in the average CTV coverage measures, including minimum CTV dose and the average CTV fraction receiving less than 90% or 95% of the dose. Con¢lusi0n$: Local control was achieved in 65% of patients with UBC prescribed 60-64 Gy to the bladder with wide margins. Patients with local con-
trol tended to have slightly higher CTV coverage rates than patients with local failures, but the differences were not of statistical significance. Future studies will investigate whether the inclusion of information about the exact tumour Iocalisation and patient positioning data derived from repeat electronic portal imaging will improve the correlation between target coverage and local control.
I M A G I N G FOR RT 252
poster
A novel, rigorous approach to dosimetric characterisation of CCD-based electronic portal imaging devices J.C.J. de Beet E.M. Franken, B.J.M. Heijmen Erasmus MC, Daniel den Hoed Oncology Center, Rotterdam, The Netherlands Introduction: The excellent long term response reproducibility of CCD-camera based electronic portal imaging devices (CEPIDs) renders them highly suited for accurate portal dosimetry. Five of our Theraview-NT CEPIDs are equipped with cooled (-20C) CCDs. By suppressing the deleterious effect of radiation damage, cooling has boosted the CCD-lifetime by years and improved signal-to-noise ratios (SNR) markedly over previous generation CEPIDs. However, precise CEPID-based dosimetry requires knowledge of the low-amplitude but spatially extended tails of the point-spread functions (PSFs), mainly stemming from optical cross-talk between the mirror and the fluorescent screen (FS). We present a method to measure these PSFs directly, without making any assumptions on their properties. Methods and Materials: Position dependent, two-dimensional PSFs were extracted from CEPID images (EPI) as follows. Using the linac collimators, EPI were obtained for a series of small fields (side < 3 cm) positioned on a grid covering the FS. For high SNR, the CEPID software allowed for videoframe summation up to large exposures. The small fields thus imaged effectively constitute input pencil-beams. A transmission EPI, obtained with collimators closed, was subtracted. The resultant EPI directly measures the PSF for a given field position. This procedure was executed for a low and an intermediate elbow CEPID (distance between FS and mirror of 5 respectively 13 cm). In addition, the optical propagation in the CEPID was modelled to predict the PSFs. Results: For both elbows, PSF-tails could be confidently measured using 2000 MU per EPI, even for tail magnitudes of 0.1% of the PSF-peak. The PSFs were asymmetrical, dependent on position on the FS, and were quite different for the low and intermediate elbow. These features were in good agreement with the model predictions. Hence, a small number of PSF measurements (-9) in combination with our model was sufficient for a complete PSF characterisation of both CEPID types. Conclusions: The high SNR offered by present CEPIDs, together with a basic optical cross-talk model, allowed for a complete experimental determination of PSFs using a few calibration measurements per CEPID. We are now implementing robust and fast deconvolution schemes to translate EPI into portal dose images for position dependent PSFs. Hence, a general framework for precise portal dosimetry with CEPIDs, even in case of highly asymmetrical IMRT fields, is established. 253
poster
Effects of geometric distortion in pelvic MRI on radiotherapy treatment planning of prostate cancer patients B. Petersch 1, J. Bogner 1, A. Fransson 2, T. Lorang 3, R. P~tter 1 1University of Vienna, Department of Radiotherapy and Radiobiology, Vienna, Austria 2Karofinska Hospital, Department of Hospital Physics, Stockholm, Sweden 3University of Vienna, Institut for Medizinische Computerwissenschaften, Vienna, Austria Purpose: To evaluate the impact of two different methods of geometric distortion correction of MR images from a Siemens Magnetom Open Viva 0,2T resistive MR unit on the process of external beam radiotherapy treatment planning (RTTP) for prostate cancer. Material and methods: A method for correction of system related AND object induced distortions and one for correction of purely system related distortions have been evaluated. The latter used information extracted from MR images of a 3D phantom specifically designed for geometric distortion evaluation. An active shim procedure was performed prior to all phantom and patient scans For each of five patients five standard treatment plans were compared using uncorrected and corrected MR images alone (density=water) and CT images alone. Finally internal anatomical landmarks were