- Email: [email protected]
506 poster Image-guidance strategies with integrated on-board MVCT
Posters
Methods: In 19 patients treated with IMRT for H & N cancer (11 oropharynx, 5 nasopharynx, 2 hypopharynx, 1 oral cavity; 15/19 T~_2; 10/19 No) a planning CT scan and FDG-PET scan were performed. During imaging, patients were positioned in the same plastic mould, used for treatment purposes. Patients were injected with 2 MBq/kg of FDG, after a 6 hours fasting period. A full ring PET scanner was used. Visual analysis of FDG-uptake was performed. CT- and PET data were coregistered using mutual information. Results: In 7/19 cases the gross target volume (GTV) was adjusted based on the FDG-PET scan. In three cases the GTV of the neck nodes, and in four cases the GTV of the local tumour extension was adjusted. In two clinically No cases the GTV was extended based on a positive node (1 upper jugular, 1 parapharyngeal), and in a third patient based on an additional node, visible at the PET and not detected on the CT scan. In three other cases tumour delineation was not possible based on CT, caused by lead-inlay scattering in two. In one patient with nasopharyngeal cancer PET clearly showed bilateral involvement, in contrast to the unilateral involvement seen at CT. The GTV was adjusted. Ten days after the start of radiotherapy mucositis was confined to the tumour only, showing bilateral involvement of the nasopharynx. In one patient the PET scan was false positive. On CT and clinical examination a unilateral tonsil carcinoma was seen, FDG-uptake also was positive in the contralateral tonsil. GTV was not adjusted, and after a follow-up of 1 year no contralateral recurrence is shown. In. two cases with small tumours, no tumour was visible on CT and PET. Delineation was performed based on clinical examination only.
Conclusion: FDG-PET scanning is a valuable tool for tumour delineation in image guided radiotherapy for head and neck cancer. In one out of three patients the GTV, as delineated on CT, had to be adjusted. 505 poster Adaptive radiotherapy in stereotactic radiotherapy of extracranial targets: integration of CT-verification on the treatment couch using a dedicated software to calculate the current stereotactic coordinate including evaluation of breathing mobility
K. Baler, J. Meyer, M. Flentje, J. Wulf University of Wuerzburg, Dept. of Radiotherapy, Wuerzburg, Germany Purpose: The purpose of extracranial stereotactic radiotherapy is dose escalation due to volume restriction. The most favourable approach to achieve this goal is reduction of the PTV, which is defined to address set-up inaccuracy and target mobility. CT-verification on the treatment couch allows evaluation of the current target position and breathing mobility and its correction. A new self-programmed software was introduced to calculate the most appropriate stereotactic coordinate for adaptation of the correct isocenter. Material and Methods: Patients are treated in a stereotactic body frame (SBF; ELEKTA Instr.). The SBF side walls include a system of copper wires appearing in each CT-slice as fiducials, which characterize a certain position similar to a bar-code. Accordingly the stereotactic coordinate can be derived from each CT-slice. The amount of breathing mobility is evaluated by comparing the different target shapes at a series of CT-slices from a time-study (scanning 15 sec. at the same couch position) to a series of CT-slices covering the target region at different couch positions. This process is performed for treatment planning and is repeated prior to each irradiation on the treatment couch using a mobile CT (Philips Tomoscan M). The gantry of the mobile CT moves
$227
over the carbon fiber treatment couch and the patient in the SBF. A software evaluates the most appropriate stereotactic coordinate and a correction vector to adapt the stereotactic isocenter.
Results: The CT-slices and structure sets from treatment planning as the studies from CT-verification are imported on a DICOM base. Registration of the stereotactic coordinate system is performed by recognizing the fiducials of the SBF. The planned isocenter including the structure sets (CTV, PTV) are imported and transferred to the corresponding coordinate in the verification study. According to anatomical characteristics the isocenter can be moved to the most appropriate position. The software compares the shape of the target from the time study to the target shape showing the target cranio-caudally by mutual information quantifies the most frequent matches and suggests a corrected coordinate. Subtracting this new coordinate from the planned coordinate gives the vector for correcting the couch position. Conclusion: The introduction of image guided, adaptive radiotherapy will allow for reduction of the security margins for PTV-definition and for treatment of patients with impaired lung function. 506 poster
Image-guidance
strategies
with
integrated
on-board
MVCT
K. Ruchala 1, G. Olivera 1'2, W. Ld, J. Kapatoes ~, R. Cravens ~, T. Chapman ~, E. Schnarr ~, T. Theisen ~, T. Mackie ~'2 ITomo Therapy, Madison, WI, U.S.A. 2University of Wisconsin-Madison, Medical Physics, Madison, WI, U.S.A. Introduction: The integration of on-board MVCT is a feature of TomoTherapy systems. There are presently 9 such installed systems that have cumulatively treated over 1000 patient fractions. Opportunities for image-guided radiotherapy (IGRT) with these systems are investigated. Experiment: An initial version of IGRT with MVCT was for patient verification and repositioning. In this scenario, a patient is imaged using the treatment machine immediately prior to delivery. These images can be visually evaluated, and registered using manual or automatic tools. Desired adjustments can be made prior to treatment, either by moving the patient, moving the couch, or adjusting the gantry start angle. An additional use for MVCT images is for planning. One implementation uses the MVCT to image patients for quickstart and emergency cases. These images can be contoured using online tools, planned within minutes while the patient remains on the couch, and delivered. An alternative version of MVCT for planning is to collect the images, contour, and optimize off-line. This method is amenable for patients needing IMRT but for whom artefacts qualitatively or quantitatively impair any diagnostic CT images. The MVCT system can also be used for real-time image collection during treatment delivery. In this variation, transmission data collected during treatment can be used to reconstruct, enhance, or illustrate changes in the patient's anatomy. Data and results: For pre-treatment scanning, images were generated with typical doses below 1 cGy and show soft tissue contrast adequate for patient repositioning. The use of MVCT for quick-start plans enabled imaging, contouring, planning, and treating a patient in less than 20 minutes, without need for the patient to leave the couch. It was also found that the artefact reduction in the MVCT was favourable
$228
for planning of patients who were otherwise difficult to image, such as patients with one or more prostheses. Finally, by incorporating treatment transmission data into the imaging process, tomographic images could be temporally updated to reflect real-time intra-fraction anatomical changes.
Conclusion: Several methods for IGRT were identified utilizing an on-board MVCT imaging system. These methods were successful for patient repositioning; planning for quickstart and emergency patients, as well as patients with prostheses, bony artefacts, and metallic implants; and for detecting anatomical changes during the treatment. 507 poster
Correction of non-ideal projection geometries in conebeam computed tomography using a phantom with helically arranged markers
R. Hinderer, B.M. Hesse, M. Ebert, S. Nil~, U. Oelfke German Cancer Research Center, Medical Physics in Radiotherapy, Heidelberg, Germany Cone-beam computed tomography (CT) is based on the reconstruction of a three-dimensional (3-D) image from a set of 2-D x-ray projections. The reconstruction process requires a precise knowledge of the imaging geometry for each projection. For systems with an x-ray source and a detector travelling on a perfect circular path, the geometry can be easily predicted. Most of the practical systems, however, exhibit deviations from this ideal trajectory caused by the flex of the imaging components during rotation as well as by the gantry wobble and eccentricity. If not accounted for, these nonidealities result in a degradation of the image quality including blurring artefacts. The purpose of this work was to find a simple and reliable correction scheme. For this purpose a phantom with helically arranged small steel spheres was employed. The spheres were placed on well-defined positions on the curved surface of a hollow plastic cylinder. Projections of this phantom are acquired at those angles for which projections of the object of interest are to be obtained. The data were taken on a system with a kilovoltage x-ray tube and an opposing flat panel detector mounted on a linear accelerator. Additional data were acquired with a megavoltage cone-beam CT system consisting of the accelerator and an opposing flat panel detector. After a careful alignment of the phantom by means of lasers, the locations of the spheres are well-known with respect to the isocenter of the system. Using the conic projection and assuming a circular trajectory of the imaging components, the centroids of the projected spheres in the imaging plane can be predicted. A comparison of these values with the actually measured centroids allows the derivation of a transformation matrix for each projection. The centroids of the spheres were fully automatically determined by an edge-based detection algorithm. The obtained matrices were incorporated into the Feldkamp algorithm, which was used to reconstruct the images. The algorithm employed to determine the centroids of the projected spheres proved to be highly reliable. With only minor modifications, it can be applied to projections acquired with kilovoltage as well as with megavoltage x-rays. The correction for the non-ideal projection geometry resulted in an improved overall image quality and any blurring artefacts were eliminated. In conclusion, a simple and effective procedure to correct for geometric non-idealities in conebeam CT has been developed.
Posters
508 poster
Adaptive radiotherapy (ART) of organ motion: evaluation of two imaging concepts S. Nill, L. Dietrich, J. Unkelbach, U. Oelfke German Cancer Research Center, Department of Medical Physics, Heidelberg, Germany Introduction: One aim of ART is the observation of organ motion followed by a subsequent adaptation of the treatment plan (TP). One way of achieving this goal is a kV x-ray source mounted at the linac in combination with a flat panel imager at the opposite side. Two imaging hardware configurations were evaluated for their potential of online tracking and correction of organ motion by using fluoroscopic images: x-ray tube positioned with (A) 90 degree - and (B) 180 degree offset to the MV-beam. Methods: For two lung cases an IMRT plan with 5 coplanar beams was optimized and the motion of the target was modelled by rigid translations. First a motion only in the LR and AP direction with an amplitude of 10mm was investigated followed by a realistic lung motion that is motivated by Seppenwoolde et al with amplitudes of 1, 2 and 12mm in the AP, LR and CC direction. The expected 3D dose distribution is compared for four cases: 1) no movement, 2) uncorrected movement, 3+4) correction of the leaf positions using system (A) and system (B). For the correction strategies 3 and 4 the displacement vector due to the organ movement is accounted for by correcting the projection of the displacement vector onto the plane perpendicular to the kV beam direction for each beam of the TP. Consequently for both hardware setups the full displacement can not be compensated by the online correction process since the displacement in the kV beam direction cannot be observed. The respective dose distributions were evaluated with the help of DVHs and the conformity- and coverage index using the 90% isodose level. Results: For the AP and LR simulations the dose distributions using system (B) have the same conformity and coverage index as the static dose distribution whereas for system (A) an agreement between the corrected- and the uncorrected dose distribution is observed (index diff. < 0.5%). For the simulated realistic tumour motion both correction methods lead to similar results and are in good agreement with the static dose distribution. These findings can be explained by the fact that system (A) can not observe the motion in the AP-LR plane perpendicular to the MV beam and can therefore not correct for this important direction. System (B) only fails to observe the motion in beam direction which has less impact on the dose distribution. Conclusions: Comparing the two image concepts shows that system (B) always leads to dose distributions equal or superior to system (A). 509 poster A new method for image and dose guided radiotherapy B.M. Hesse S. Nill, T. TEIcking, R. Hinderer, U. Oelfke DKFZ, Medical Physics, Heidelberg, Germany Purpose: The efficacy of radiation therapy relies on the knowledge of the possible organ movements and/or deformations at any time of the treatment beam delivery and on the accuracy of dose delivery regarding all possible changes. In this presentation, we describe a new method for a dose guided radiotherapy by means of entrance dosimetry based on on-line imaging of the anatomy and input fluence. Methods: Dose guidance in radiation therapy requires beside a 2D beam monitor anatomical image of the patient in
Methods: In 19 patients treated with IMRT for H & N cancer (11 oropharynx, 5 nasopharynx, 2 hypopharynx, 1 oral cavity; 15/19 T~_2; 10/19 No) a planning CT scan and FDG-PET scan were performed. During imaging, patients were positioned in the same plastic mould, used for treatment purposes. Patients were injected with 2 MBq/kg of FDG, after a 6 hours fasting period. A full ring PET scanner was used. Visual analysis of FDG-uptake was performed. CT- and PET data were coregistered using mutual information. Results: In 7/19 cases the gross target volume (GTV) was adjusted based on the FDG-PET scan. In three cases the GTV of the neck nodes, and in four cases the GTV of the local tumour extension was adjusted. In two clinically No cases the GTV was extended based on a positive node (1 upper jugular, 1 parapharyngeal), and in a third patient based on an additional node, visible at the PET and not detected on the CT scan. In three other cases tumour delineation was not possible based on CT, caused by lead-inlay scattering in two. In one patient with nasopharyngeal cancer PET clearly showed bilateral involvement, in contrast to the unilateral involvement seen at CT. The GTV was adjusted. Ten days after the start of radiotherapy mucositis was confined to the tumour only, showing bilateral involvement of the nasopharynx. In one patient the PET scan was false positive. On CT and clinical examination a unilateral tonsil carcinoma was seen, FDG-uptake also was positive in the contralateral tonsil. GTV was not adjusted, and after a follow-up of 1 year no contralateral recurrence is shown. In. two cases with small tumours, no tumour was visible on CT and PET. Delineation was performed based on clinical examination only.
Conclusion: FDG-PET scanning is a valuable tool for tumour delineation in image guided radiotherapy for head and neck cancer. In one out of three patients the GTV, as delineated on CT, had to be adjusted. 505 poster Adaptive radiotherapy in stereotactic radiotherapy of extracranial targets: integration of CT-verification on the treatment couch using a dedicated software to calculate the current stereotactic coordinate including evaluation of breathing mobility
K. Baler, J. Meyer, M. Flentje, J. Wulf University of Wuerzburg, Dept. of Radiotherapy, Wuerzburg, Germany Purpose: The purpose of extracranial stereotactic radiotherapy is dose escalation due to volume restriction. The most favourable approach to achieve this goal is reduction of the PTV, which is defined to address set-up inaccuracy and target mobility. CT-verification on the treatment couch allows evaluation of the current target position and breathing mobility and its correction. A new self-programmed software was introduced to calculate the most appropriate stereotactic coordinate for adaptation of the correct isocenter. Material and Methods: Patients are treated in a stereotactic body frame (SBF; ELEKTA Instr.). The SBF side walls include a system of copper wires appearing in each CT-slice as fiducials, which characterize a certain position similar to a bar-code. Accordingly the stereotactic coordinate can be derived from each CT-slice. The amount of breathing mobility is evaluated by comparing the different target shapes at a series of CT-slices from a time-study (scanning 15 sec. at the same couch position) to a series of CT-slices covering the target region at different couch positions. This process is performed for treatment planning and is repeated prior to each irradiation on the treatment couch using a mobile CT (Philips Tomoscan M). The gantry of the mobile CT moves
$227
over the carbon fiber treatment couch and the patient in the SBF. A software evaluates the most appropriate stereotactic coordinate and a correction vector to adapt the stereotactic isocenter.
Results: The CT-slices and structure sets from treatment planning as the studies from CT-verification are imported on a DICOM base. Registration of the stereotactic coordinate system is performed by recognizing the fiducials of the SBF. The planned isocenter including the structure sets (CTV, PTV) are imported and transferred to the corresponding coordinate in the verification study. According to anatomical characteristics the isocenter can be moved to the most appropriate position. The software compares the shape of the target from the time study to the target shape showing the target cranio-caudally by mutual information quantifies the most frequent matches and suggests a corrected coordinate. Subtracting this new coordinate from the planned coordinate gives the vector for correcting the couch position. Conclusion: The introduction of image guided, adaptive radiotherapy will allow for reduction of the security margins for PTV-definition and for treatment of patients with impaired lung function. 506 poster
Image-guidance
strategies
with
integrated
on-board
MVCT
K. Ruchala 1, G. Olivera 1'2, W. Ld, J. Kapatoes ~, R. Cravens ~, T. Chapman ~, E. Schnarr ~, T. Theisen ~, T. Mackie ~'2 ITomo Therapy, Madison, WI, U.S.A. 2University of Wisconsin-Madison, Medical Physics, Madison, WI, U.S.A. Introduction: The integration of on-board MVCT is a feature of TomoTherapy systems. There are presently 9 such installed systems that have cumulatively treated over 1000 patient fractions. Opportunities for image-guided radiotherapy (IGRT) with these systems are investigated. Experiment: An initial version of IGRT with MVCT was for patient verification and repositioning. In this scenario, a patient is imaged using the treatment machine immediately prior to delivery. These images can be visually evaluated, and registered using manual or automatic tools. Desired adjustments can be made prior to treatment, either by moving the patient, moving the couch, or adjusting the gantry start angle. An additional use for MVCT images is for planning. One implementation uses the MVCT to image patients for quickstart and emergency cases. These images can be contoured using online tools, planned within minutes while the patient remains on the couch, and delivered. An alternative version of MVCT for planning is to collect the images, contour, and optimize off-line. This method is amenable for patients needing IMRT but for whom artefacts qualitatively or quantitatively impair any diagnostic CT images. The MVCT system can also be used for real-time image collection during treatment delivery. In this variation, transmission data collected during treatment can be used to reconstruct, enhance, or illustrate changes in the patient's anatomy. Data and results: For pre-treatment scanning, images were generated with typical doses below 1 cGy and show soft tissue contrast adequate for patient repositioning. The use of MVCT for quick-start plans enabled imaging, contouring, planning, and treating a patient in less than 20 minutes, without need for the patient to leave the couch. It was also found that the artefact reduction in the MVCT was favourable
$228
for planning of patients who were otherwise difficult to image, such as patients with one or more prostheses. Finally, by incorporating treatment transmission data into the imaging process, tomographic images could be temporally updated to reflect real-time intra-fraction anatomical changes.
Conclusion: Several methods for IGRT were identified utilizing an on-board MVCT imaging system. These methods were successful for patient repositioning; planning for quickstart and emergency patients, as well as patients with prostheses, bony artefacts, and metallic implants; and for detecting anatomical changes during the treatment. 507 poster
Correction of non-ideal projection geometries in conebeam computed tomography using a phantom with helically arranged markers
R. Hinderer, B.M. Hesse, M. Ebert, S. Nil~, U. Oelfke German Cancer Research Center, Medical Physics in Radiotherapy, Heidelberg, Germany Cone-beam computed tomography (CT) is based on the reconstruction of a three-dimensional (3-D) image from a set of 2-D x-ray projections. The reconstruction process requires a precise knowledge of the imaging geometry for each projection. For systems with an x-ray source and a detector travelling on a perfect circular path, the geometry can be easily predicted. Most of the practical systems, however, exhibit deviations from this ideal trajectory caused by the flex of the imaging components during rotation as well as by the gantry wobble and eccentricity. If not accounted for, these nonidealities result in a degradation of the image quality including blurring artefacts. The purpose of this work was to find a simple and reliable correction scheme. For this purpose a phantom with helically arranged small steel spheres was employed. The spheres were placed on well-defined positions on the curved surface of a hollow plastic cylinder. Projections of this phantom are acquired at those angles for which projections of the object of interest are to be obtained. The data were taken on a system with a kilovoltage x-ray tube and an opposing flat panel detector mounted on a linear accelerator. Additional data were acquired with a megavoltage cone-beam CT system consisting of the accelerator and an opposing flat panel detector. After a careful alignment of the phantom by means of lasers, the locations of the spheres are well-known with respect to the isocenter of the system. Using the conic projection and assuming a circular trajectory of the imaging components, the centroids of the projected spheres in the imaging plane can be predicted. A comparison of these values with the actually measured centroids allows the derivation of a transformation matrix for each projection. The centroids of the spheres were fully automatically determined by an edge-based detection algorithm. The obtained matrices were incorporated into the Feldkamp algorithm, which was used to reconstruct the images. The algorithm employed to determine the centroids of the projected spheres proved to be highly reliable. With only minor modifications, it can be applied to projections acquired with kilovoltage as well as with megavoltage x-rays. The correction for the non-ideal projection geometry resulted in an improved overall image quality and any blurring artefacts were eliminated. In conclusion, a simple and effective procedure to correct for geometric non-idealities in conebeam CT has been developed.
Posters
508 poster
Adaptive radiotherapy (ART) of organ motion: evaluation of two imaging concepts S. Nill, L. Dietrich, J. Unkelbach, U. Oelfke German Cancer Research Center, Department of Medical Physics, Heidelberg, Germany Introduction: One aim of ART is the observation of organ motion followed by a subsequent adaptation of the treatment plan (TP). One way of achieving this goal is a kV x-ray source mounted at the linac in combination with a flat panel imager at the opposite side. Two imaging hardware configurations were evaluated for their potential of online tracking and correction of organ motion by using fluoroscopic images: x-ray tube positioned with (A) 90 degree - and (B) 180 degree offset to the MV-beam. Methods: For two lung cases an IMRT plan with 5 coplanar beams was optimized and the motion of the target was modelled by rigid translations. First a motion only in the LR and AP direction with an amplitude of 10mm was investigated followed by a realistic lung motion that is motivated by Seppenwoolde et al with amplitudes of 1, 2 and 12mm in the AP, LR and CC direction. The expected 3D dose distribution is compared for four cases: 1) no movement, 2) uncorrected movement, 3+4) correction of the leaf positions using system (A) and system (B). For the correction strategies 3 and 4 the displacement vector due to the organ movement is accounted for by correcting the projection of the displacement vector onto the plane perpendicular to the kV beam direction for each beam of the TP. Consequently for both hardware setups the full displacement can not be compensated by the online correction process since the displacement in the kV beam direction cannot be observed. The respective dose distributions were evaluated with the help of DVHs and the conformity- and coverage index using the 90% isodose level. Results: For the AP and LR simulations the dose distributions using system (B) have the same conformity and coverage index as the static dose distribution whereas for system (A) an agreement between the corrected- and the uncorrected dose distribution is observed (index diff. < 0.5%). For the simulated realistic tumour motion both correction methods lead to similar results and are in good agreement with the static dose distribution. These findings can be explained by the fact that system (A) can not observe the motion in the AP-LR plane perpendicular to the MV beam and can therefore not correct for this important direction. System (B) only fails to observe the motion in beam direction which has less impact on the dose distribution. Conclusions: Comparing the two image concepts shows that system (B) always leads to dose distributions equal or superior to system (A). 509 poster A new method for image and dose guided radiotherapy B.M. Hesse S. Nill, T. TEIcking, R. Hinderer, U. Oelfke DKFZ, Medical Physics, Heidelberg, Germany Purpose: The efficacy of radiation therapy relies on the knowledge of the possible organ movements and/or deformations at any time of the treatment beam delivery and on the accuracy of dose delivery regarding all possible changes. In this presentation, we describe a new method for a dose guided radiotherapy by means of entrance dosimetry based on on-line imaging of the anatomy and input fluence. Methods: Dose guidance in radiation therapy requires beside a 2D beam monitor anatomical image of the patient in