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346 poster The impact of a knee-support and shape of the table-top on rectum and prostate position
Posters
346
$119
poster
The impact of a knee-support and shape of the table-top on rectum and prostate position R.J.H.M. Steenbakkers 1, J.C. Duppen 1, H.T. Lotz 1, P. Remeijer 1, I. Fitton 1, P,J.C.M. Nowak2, M. van Herk 1, C.R.N. Rasch 1 1The Netherlands Cancer Institute / Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2Erasmus University Medical Center, Radiation Oncology, Rotterdam, The Netherlands Backqround and uurpose: In a previous comparison between CT and MRI based treatment planning for prostate, a rectum shift was observed. The purpose of this study was to evaluate the impact of different table-tops with or without knee-support on position of rectum, prostate and bulb of the penis. Methods: For 10 male volunteers, 5 MRI scans were made in 4 different positions; 1. Qn a flat table-top with knee-support, 2. On a flat table-top without knee-support, 3. On a rounded table-top with knee-support, 4. On a rounded table-top without knee-support. The final scan was the same as the first. The position differences of the rectum, prostate and bulb of the penis were measured at several points in a sagittal plane through the central axis of the prostate. Results: Until now, data from three volunteers have been analyzed. The mean difference between first and last scan (flat table-top with knee support) was 1 mm (SEM 1 mm) for all measured points, excluding significant time trends. By far the largest difference was observed between scans with rounded table-top without knee-support and flat table-top with knee-support, where the knee-;support had the largest effect. The posterior rectal wall shifted systematically at the middle of the prostate by 10 mm (SEM 2 mm) and 3 mm (SEM 1 ram) due to the knee-support and rounded table-top, respectively. At the apex and the top of the prostate the shift was smaller and not significant. The anterior rectal wall shifted systematically at the top of the prostate by 11 mm (SEM 2 mm) and 3 mm (SEM 4 ram), respectively. At the apex the shift was smaller ,and not significant. Due to the knee-support, the anterior and posterior edge of the prostate shifted 3 mm (SEM 1 mm) and 5 mm (SEM 1 mm), respectively. The edges of the seminal vesicles shifted 15 mm (SEM 3 mm) and 11 mm (SEM 3 mm), respectively. The rounded table-top caused no significant shift for the prostate and seminal vesicles. The bulb of the penis was not influenced by the knee-support or rounded table-top. Conclusions: The rectum, prostate and seminal vesicles were significantly shifted dorsally by using a flat table-top with knee-support compared to a rounded table-top without knee-support. The (rectal) shift due to the kneesupport was about 4 times larger than due to the rounded table-top. The seminal vesicles and rectum shifted more than the prostate. In the future, the amount of rectum sparing due to the use of a knee-support will be evaluated. 347
poster
PET-CT radiotherapy planning in malignant lymphomas M. Busetto 1, L. Stea2, L. Gallo2, E. Cracco 3, C. Marchetti 2, S. Gravili 3, G. Pizzi4 l UmbertolHospital, OncoHaematologic Radiotherapy, Mestre Venice, Italy 2Umberfo I Hospital, Medical Physics, Mestre Venice, Italy 3Umberto I Hospital, Nuclear Medicine, Mestre Venice, Italy 4Umberto t Hospital, Oncologic Radiotherapy, Mestre Venice, Italy The extension of active residual masses after chemotherapy has been an important challenge for radiotherapists. The question is which volume should be irradiated. Strictly following anatomical structures is sometimes too limited and casual extension of the margins may be excessive and lead to heavy late side effects or, if too narrow, expose the patient to high risk of relapse. Pet scans provide metabolic images of the body, but they are not directly usable by radiotherapists. A new PET/CT device provide a double imaging system (Positron Imaging and spiral CT) with reliable superimposition. And high quality images from spiral CT and PET may be directly transferred via intranet in DICOM format to our Therapy Planning System, fused together and used for radiotherapy planning, CT based. When used for radiotherapy purposes, PET/CT acquisition is done with the patient in supine and appropriate position on a hard table with markers on
the skin and copper wires and fine pipes filled with 18FDG along laser beams. Both markers are visible windowing on CT, but only radioactive pipes are visible on PET scans. PET markers are used to check the range of breathing mobility during acquisition time, lasting around 25 seconds for 180 CT images, and 25 minutes for the same number of PET images. Images obtained of 4.25 mm thick every 5 mm from CT and PET are fused, then contoured first on CT. Then prophiles are superimposed to PET images, obtaining normally good correspondance with some exceptions, like hot spots outside CT target. They are marked as PET target and if they are consistent for a possible site of disease, target contours are modified to include these new volumes. At the moment we choose to include the whole anatomical extension of the residual disease after chemotherapy, plus the eventual PET corrections. We are exploring the possibilities of dose modulation as a function of PET activity.
TREATMENT MARGINS 348
poster
A radiobiological analysis of conformal therapy boost strategies for the moving prostate A.M. Amer, R.I. MacKay, P.C. Williams Christie Hospital NHS Trust, North Western Medical Physics, Manchester, United Kingdom In an attempt to maximise local control with a minimum of normal tissue complications a dose boost to a reduced target volume is often incorporated into a treatment strategy. Often the boost volume is the GTV which can receive additional fractions during the fractionation schedule, sequential boost, or be boosted to a higher dose at each fraction, simultaneous boost. In this study a radiebiological analysis has been performed to compare the merits of the two boost strategies for prostate cancer. The TCP and NTCP have been calculated from biologically effective dose accrued by a number of points representing the GTV and the rectum as they move through a treatment course. Analysis has been performed for a conventional four field plan with a sequential boost. For the sequential boost the size of the margin around the boost volume, the GTV, and the number of fractions for which the boost was applied were varied. The effect of the fractionation schedule for a five field IMRT simultaneous boost has also been investigated. A sequential boost to the prostate alone for the last five days caused a minimal reduction in TCP whilst reducing NTCP. However it was found that the same level of TCP could be achieved and NTCP further reduced by increasing the number of boost fractions but increasing the boost volume by adding a small margin. Table t shows the size of boost margin and the number of days for the same level of TCP but differing NTCP. Boost Margin(mm) Number of boost fractions NTCP (%) 0 5 19.2 2 7 18.0 4 9 17.4 8 12 18.3 10 14 18.6 Table I Boost margins and fractions boost applied iso TCP and the resulting NTCP for a four field conformal therapy plan. The results suggest that the optimum boost strategy is to boost with a small margin of 4mm for 9 fractions. For the simultaneous boost delivered using IMRT it was found that reducing the dose to the PTV outside the boost volume to less than 2Gy per fraction could jeopardise TCP. When movement is taken into account for the same level of TOP there is no preference of one strategy over another in terms of outcome. A small advantage has been found for using a sequential boost with a 45mm margin round the GTV for the 9-10 fractions of treatment. If the prostate ed# is as low as recent studies suggest then the dose to the PTV should not be dropped below 2Gy per fraction when using a simultaneous boost. 349
poster
Effect of tissue density and target size on the ideal margin for random errors M.G. Witte, J. van der Geer, C. Schneider, M. van Herk, J. V. Lebesque Netherlands Cancer Institute/AvL Hospital, Radio Therapy, Amsterdam, The Netherlands Purpose: To evaluate analytically the ideal margin needed to incorporate day-to-day geometric variations, for different tissue densities and target
346
$119
poster
The impact of a knee-support and shape of the table-top on rectum and prostate position R.J.H.M. Steenbakkers 1, J.C. Duppen 1, H.T. Lotz 1, P. Remeijer 1, I. Fitton 1, P,J.C.M. Nowak2, M. van Herk 1, C.R.N. Rasch 1 1The Netherlands Cancer Institute / Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands 2Erasmus University Medical Center, Radiation Oncology, Rotterdam, The Netherlands Backqround and uurpose: In a previous comparison between CT and MRI based treatment planning for prostate, a rectum shift was observed. The purpose of this study was to evaluate the impact of different table-tops with or without knee-support on position of rectum, prostate and bulb of the penis. Methods: For 10 male volunteers, 5 MRI scans were made in 4 different positions; 1. Qn a flat table-top with knee-support, 2. On a flat table-top without knee-support, 3. On a rounded table-top with knee-support, 4. On a rounded table-top without knee-support. The final scan was the same as the first. The position differences of the rectum, prostate and bulb of the penis were measured at several points in a sagittal plane through the central axis of the prostate. Results: Until now, data from three volunteers have been analyzed. The mean difference between first and last scan (flat table-top with knee support) was 1 mm (SEM 1 mm) for all measured points, excluding significant time trends. By far the largest difference was observed between scans with rounded table-top without knee-support and flat table-top with knee-support, where the knee-;support had the largest effect. The posterior rectal wall shifted systematically at the middle of the prostate by 10 mm (SEM 2 mm) and 3 mm (SEM 1 ram) due to the knee-support and rounded table-top, respectively. At the apex and the top of the prostate the shift was smaller and not significant. The anterior rectal wall shifted systematically at the top of the prostate by 11 mm (SEM 2 mm) and 3 mm (SEM 4 ram), respectively. At the apex the shift was smaller ,and not significant. Due to the knee-support, the anterior and posterior edge of the prostate shifted 3 mm (SEM 1 mm) and 5 mm (SEM 1 mm), respectively. The edges of the seminal vesicles shifted 15 mm (SEM 3 mm) and 11 mm (SEM 3 mm), respectively. The rounded table-top caused no significant shift for the prostate and seminal vesicles. The bulb of the penis was not influenced by the knee-support or rounded table-top. Conclusions: The rectum, prostate and seminal vesicles were significantly shifted dorsally by using a flat table-top with knee-support compared to a rounded table-top without knee-support. The (rectal) shift due to the kneesupport was about 4 times larger than due to the rounded table-top. The seminal vesicles and rectum shifted more than the prostate. In the future, the amount of rectum sparing due to the use of a knee-support will be evaluated. 347
poster
PET-CT radiotherapy planning in malignant lymphomas M. Busetto 1, L. Stea2, L. Gallo2, E. Cracco 3, C. Marchetti 2, S. Gravili 3, G. Pizzi4 l UmbertolHospital, OncoHaematologic Radiotherapy, Mestre Venice, Italy 2Umberfo I Hospital, Medical Physics, Mestre Venice, Italy 3Umberto I Hospital, Nuclear Medicine, Mestre Venice, Italy 4Umberto t Hospital, Oncologic Radiotherapy, Mestre Venice, Italy The extension of active residual masses after chemotherapy has been an important challenge for radiotherapists. The question is which volume should be irradiated. Strictly following anatomical structures is sometimes too limited and casual extension of the margins may be excessive and lead to heavy late side effects or, if too narrow, expose the patient to high risk of relapse. Pet scans provide metabolic images of the body, but they are not directly usable by radiotherapists. A new PET/CT device provide a double imaging system (Positron Imaging and spiral CT) with reliable superimposition. And high quality images from spiral CT and PET may be directly transferred via intranet in DICOM format to our Therapy Planning System, fused together and used for radiotherapy planning, CT based. When used for radiotherapy purposes, PET/CT acquisition is done with the patient in supine and appropriate position on a hard table with markers on
the skin and copper wires and fine pipes filled with 18FDG along laser beams. Both markers are visible windowing on CT, but only radioactive pipes are visible on PET scans. PET markers are used to check the range of breathing mobility during acquisition time, lasting around 25 seconds for 180 CT images, and 25 minutes for the same number of PET images. Images obtained of 4.25 mm thick every 5 mm from CT and PET are fused, then contoured first on CT. Then prophiles are superimposed to PET images, obtaining normally good correspondance with some exceptions, like hot spots outside CT target. They are marked as PET target and if they are consistent for a possible site of disease, target contours are modified to include these new volumes. At the moment we choose to include the whole anatomical extension of the residual disease after chemotherapy, plus the eventual PET corrections. We are exploring the possibilities of dose modulation as a function of PET activity.
TREATMENT MARGINS 348
poster
A radiobiological analysis of conformal therapy boost strategies for the moving prostate A.M. Amer, R.I. MacKay, P.C. Williams Christie Hospital NHS Trust, North Western Medical Physics, Manchester, United Kingdom In an attempt to maximise local control with a minimum of normal tissue complications a dose boost to a reduced target volume is often incorporated into a treatment strategy. Often the boost volume is the GTV which can receive additional fractions during the fractionation schedule, sequential boost, or be boosted to a higher dose at each fraction, simultaneous boost. In this study a radiebiological analysis has been performed to compare the merits of the two boost strategies for prostate cancer. The TCP and NTCP have been calculated from biologically effective dose accrued by a number of points representing the GTV and the rectum as they move through a treatment course. Analysis has been performed for a conventional four field plan with a sequential boost. For the sequential boost the size of the margin around the boost volume, the GTV, and the number of fractions for which the boost was applied were varied. The effect of the fractionation schedule for a five field IMRT simultaneous boost has also been investigated. A sequential boost to the prostate alone for the last five days caused a minimal reduction in TCP whilst reducing NTCP. However it was found that the same level of TCP could be achieved and NTCP further reduced by increasing the number of boost fractions but increasing the boost volume by adding a small margin. Table t shows the size of boost margin and the number of days for the same level of TCP but differing NTCP. Boost Margin(mm) Number of boost fractions NTCP (%) 0 5 19.2 2 7 18.0 4 9 17.4 8 12 18.3 10 14 18.6 Table I Boost margins and fractions boost applied iso TCP and the resulting NTCP for a four field conformal therapy plan. The results suggest that the optimum boost strategy is to boost with a small margin of 4mm for 9 fractions. For the simultaneous boost delivered using IMRT it was found that reducing the dose to the PTV outside the boost volume to less than 2Gy per fraction could jeopardise TCP. When movement is taken into account for the same level of TOP there is no preference of one strategy over another in terms of outcome. A small advantage has been found for using a sequential boost with a 45mm margin round the GTV for the 9-10 fractions of treatment. If the prostate ed# is as low as recent studies suggest then the dose to the PTV should not be dropped below 2Gy per fraction when using a simultaneous boost. 349
poster
Effect of tissue density and target size on the ideal margin for random errors M.G. Witte, J. van der Geer, C. Schneider, M. van Herk, J. V. Lebesque Netherlands Cancer Institute/AvL Hospital, Radio Therapy, Amsterdam, The Netherlands Purpose: To evaluate analytically the ideal margin needed to incorporate day-to-day geometric variations, for different tissue densities and target