- Email: [email protected]
Online Intrafraction Spine Motion Monitoring Using Digital Tomosynthesis: Preliminary Phantom Results
I. J. Radiation Oncology d Biology d Physics
S772
Volume 81, Number 2, Supplement, 2011
scan in Trilogy, and MV portal images for a head and a prostate patient treatment site. For a head scan the dose to eye, brain, brain stem, and spinal cord are all less than 0.15 cGy which are approximately 38% lower compared to Standard Head scan in OBI 1.4 and doses to bones are shown to be 2 - 4 times higher than that to soft tissues in kV x-rays. However, the doses resulting from a conventional portal imaging system (an anterior of 1515 cm2 and a lateral of 1515 cm2 fields of 6 MV photon beam) are found to be 1 - 5 cGy which are 10 - 30 times higher than that from kV-CBCT images for soft tissues. There is little dose variations between bones and soft tissues resulting from MV portal images. For a pelvis kV-CBCT scan in TrueBEAM, the dose to prostate, rectum, bladder, and femoral heads are 0.8, 0.86, 0.87 and 1.5 cGy, which are approximately 43% lower compared to Pelvis scan in OBI 1.4. The dose resulting from conventional MV portal imaging system using two orthogonal (an anterior of 2520 cm2 and a lateral of 2020 cm2) fields are shown to be 1 - 8 cGy which are 2 - 5 times higher than that from kV-CBCT images for soft tissues. Conclusions: The imaging dose from kV radiographs and kV-CBCT scans is much less than from conventional MV portal imaging. Using kV imaging rather than MV imaging reduces patient doses and results in higher quality images. Reference: 1. Rogers, D.W.O., et al. BEAM: a Monte Carlo code to simulate radiotherapy treatment units. Medical Physics 1995;22:503 524. Author Disclosure: G.X. Ding: None. P. Munro: None.
3176
Delivered Target Dose Reconstruction in Actual and Simulated SBRT Liver Treatments with MV Image Guided Dynamic MLC Tracking
P. R. Poulsen1, E. Worm1, W. Fledelius1, M. Høyer1, P. Keall2,3 1 Department of Oncology, Aarhus University Hospital, 8000 Aarhus C, Denmark, 2Central Clinical School, University of Sydney, Sydney, Australia, 3Department of Radiation Oncology, Stanford University, Stanford, CA
Purpose/Objective(s): Intrafraction tumor motion during stereotactic body radiotherapy (SBRT) may deteriorate the tumor dose distribution. The tumor motion and its impact on the dose distribution can be determined from intra-treatment MV portal images when gold markers are implanted as tumor position surrogates. In this study we use continuous MV images from three liver SBRT treatment fractions to reconstruct the tumor dose distribution (1) for the actually delivered treatments, and (2) for realistic simulations of MV image guided dynamic MLC tracking treatments based on the same MV images. Materials/Methods: A clinical target volume (CTV) was delineated for a liver tumor patient with implanted gold markers. A planning target volume (PTV) was formed by adding 10 mm margins in the cranio-caudal (CC) directions and 5 mm margins in the axial plane. A 5-field conformal treatment plan ensuring 45 Gy mean CTV dose (= 100% dose) and minimum target doses of 95% (CTV) and 66.7% (PTV) was delivered in three fractions. At treatment, cone-beam CT was used for patient set-up based on the marker positions. During treatment continuous MV images were acquired at 7.8 Hz. Offline, the marker positions were segmented in each image and the tumor motion within the MLC aperture was determined. For each fraction, the actually delivered target dose distribution was estimated in a treatment planning system (Eclipse, Varian Medical Systems) by convolving the planned fluence with the observed tumor motion and calculating the resulting 3D dose from the motion blurred fluences. For comparison, the would-be MLC motion in real-time DMLC tracking treatments guided by the MV images and using the same treatment fields was simulated. The latency of a prototype DMLC tracking system (290 ms) was accounted for by target position prediction starting 8 seconds into each field delivery. The target dose distribution was estimated similarly to the actual nontracking treatments. Results: The mean peak-to-peak tumor motion during field delivery was 19 mm in the CC direction and 6 mm perpendicular to the CC direction. For the delivered non-tracking treatments the mean (max) tumor-field mismatch was 4.0 mm (20.7 mm) in the CC direction and 0.8 mm (8.3 mm) perpendicular to the CC direction. For the simulated tracking this was reduced to 0.1 mm (12.3 mm) (CC) and 0.1 mm (5.3 mm) (perpendicular to CC). For the delivered non-tracking treatments, 4.4 cm3 (12%), 3.7 cm3 (10%), and 1.4 cm3 (3%) of the CTV received \95% dose due to tumor motion at the three treatment fractions, respectively. For the simulated tracking treatments, 0% of the CTV received \95% dose (as in the original plan). Conclusions: The CTV dose in SBRT was deteriorated by intrafractional tumor motion. MV image guided DMLC tracking would allow restoration of the planned CTV dose. Author Disclosure: P.R. Poulsen: B. Research Grant; Varian Medical Systems. E. Worm: None. W. Fledelius: B. Research Grant; Varian Medical Systems. M. Høyer: None. P. Keall: None.
3177
Online Intrafraction Spine Motion Monitoring Using Digital Tomosynthesis: Preliminary Phantom Results
W. Verbakel, M. Dahele, J. R. van Sornsen de Koste, S. Senan VU University Medical Center, Amsterdam 1007 MB, Netherlands Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) using volumetric modulated arc therapy delivers high doses to the tumor during 1 - 2 gantry rotations. Steep dose gradients close to organs at risk frequently require intrafraction motion to be limited to 1 - 2 mm. Patients treated at our department do not undergo rigid immobilization during SBRT delivery. Surrogate external markers are often used to monitor motion but their relationship to critical structures is often uncertain. In spine SBRT it would be desirable to monitor and display the position of the spine itself during treatment delivery. We investigated a novel method for doing this with Digital Tomosynthesis (DTS) and present the first results on a moving phantom. Materials/Methods: DTS requires multiple kV images at small angle intervals. In a clinical situation, these could be acquired generating a CBCT scan during RapidArc delivery. For this study, a CBCT scan of the thoracic part of an Alderson phantom was acquired. The CBCT acquisition was paused 4 times to manually translate the phantom, simulating intrafractional shifts. The phantom was equipped with infrared reflective markers (ExacTrac, Brainlab AG) that quantified the translations. The
Proceedings of the 53rd Annual ASTRO Meeting CBCT-scan resulted in 691 projections. A non-clinical research version of DTS software (Varian Medical Systems) was used to generate 120 sets of DTS images from the CBCT projections using 3 arc segments every 3 , and then to register these with reconstructed DTS images from the CT-scan of the phantom. In this preliminary study, groups of four consecutive DTS image sets (representing a 12 arc) were triangulated to determine the shift of the spine with respect to the CT scan. Deviations between actual phantom position according to ExacTrac and DTS results were calculated. Results: Phantom off-sets were between 0.7 and 2.7 mm, in three directions. The average deviations (and range) between DTS results and actual phantom positions in lateral, longitudinal and vertical directions were -0.23 (-1.3 to 0.6), 0.02 (-0.4 to 0.3) and -0.05 (-1.9 to 0.8) mm, respectively. Standard deviations (SD) were 0.50, 0.17 and 0.55 mm, which includes the uncertainty of the imager positioning. The accuracy was best in longitudinal direction because this direction is always perpendicular to the imaging direction. Currently, the combination of four DTS datasets can be generated and processed in less than 10 seconds, enabling new possibilities to perform online setup monitoring while projection data is collected during radiation delivery. Conclusions: Online intrafractional motion using DTS is feasible. A preliminary version of the DTS software was possible to detect the phantom position within 0.5 mm, 1 SD. This supports further development of DTS for online spine tracking during RapidArc SBRT delivery. Author Disclosure: W. Verbakel: D. Speakers Bureau/Honoraria; Varian Medical Systems. M. Dahele: None. J.R. van Sornsen de Koste: None. S. Senan: C. Other Research Support; Varian Medical Systems. D. Speakers Bureau/Honoraria; Varian Medical Systems.
3178
Evaluation Of Visual CT Artifacts While Using Gold, Carbon Or Polyetheretherketone (PEEK) Fiducials In Image Guided Radiotherapy (IGRT)
R. Jacob, S. Shen, R. Popple, W. T. Sobol, D. Dubay University of Alabama, Birmingham, AL Purpose/Objective(s): Implanted fiducials are increasingly used in the IGRT of various organ sites. An ideal fiducial should be easy to implant, readily visible on imaging, show minimum reconstruction artifacts, and not significantly affect radiation dosedistribution. This study evaluates the image perturbations of 4 different fiducials: Gold seed, Gold coil, Carbon and Polyetheretherketone (PEEK) in CT, Cone beam CT (CBCT) and MRI. Materials/Methods: The fiducials used in this study measured 1 mm x 3 mm. They were oriented in a superior-inferior direction in a cylindrical Jazack water-filled phantom. Planning CT images were acquired on a Philips Big Bore CT scanner at 3 mm slice thickness to simulate standard planning imaging, and repeated on a Varian CBCT at 2 mm slice thickness and OBI kV images to simulate daily imaging for setup alignment. MRI images were acquired on a GE scanner at 5 mm slice thickness. Four parameters were evaluated for CT images: 1) identification of fiducial 2) visual inspection of steak artifacts 3) maximum CT value deviated from water due to artifacts 4) volume of artifacts in 10x10 cm in the center slice 5) maximum length of streak artifacts from the fiducial. Results: All fiducials were visualized on planning CT images. The streak artifacts were highest for gold seed, followed by gold coil, and least for PEEK and Carbon. The maximum CT value deviation was -174, -127, -77 and -24 for gold seed, gold coil, Carbon and PEEK respectively. The volume of artifacts in a 10x10 cm 3 mm slice was 0.369, 0.132, 0.046 and 0.074 cm3 and the maximum length of streak artifact from the seed was 80mm, 50 mm, 5 mm and 7 mm for gold seed, gold coil, Carbon and PEEK respectively. On CBCT, the maximum CT value deviation from water was -900, -900, -180, and -231, and the maximum length of streak artifact from the seed was 70mm, 55 mm and 10 mm for gold seed, gold coil, and Carbon/PEEK respectively. As 2 standard deviation for CBCT pixel value for water was 80 - 100, volumes of streak artifacts could not be reliably compared. All fiducials were visualized on OBI kV images. Image contrast was highest for gold seed, followed by gold coil, and least for PEEK and Carbon fiducials. All fiducials were visualized on MRI images as black spots. No obvious image artifacts were seen with any fiducial. Small artifacts were observable when imaged with high gradient magnetic fields. Conclusions: Our results confirm that streak artifacts on planning CT and verification CBCT associated with use of gold fiducials can be avoided by using Carbon or PEEK fiducials, without significantly losing kV image contrast. This has clinical significance in the IGRT of organ sites such as the liver, where post-therapy imaging studies are routinely performed. Author Disclosure: R. Jacob: None. S. Shen: None. R. Popple: None. W.T. Sobol: None. D. Dubay: None.
3179
Intrafractional Prostate Mobility and CTV Margins Depending On Radiotherapy Technique (RapidArc vs. IMRT) of Prostate Cancer Patients
D. Bodusz, L. Miszczyk Maria Sklodowska-Curie Memorial, Cancer Center and Institute of Oncology, Gliwice Branch, Radiotherapy Department, Gliwice, Poland Purpose/Objective(s): IGRT of prostate cancer patients is very precise treatment modality. Daily prostate localization based on 2D/2D kV technique and gold markers (GoldAnchor) requires more time but allows reducing CTV margins and dose in organs at risk. The problem is an intrafractional prostate mobility caused by patient moves, filling of the bladder and rectum or breathing. The main aim of this study is comparison of intrafractional prostate mobility during volumetric modulated arc radiotherapy (RapidArc - RA) and intensity modulated radiotherapy (IMRT). Materials/Methods: 381 measurements of prostate mobility during RA and 60 during IMRT were done. All patients were irradiated in supine position using thermoplastic masks and plate with head support and four fixation points. The 2D/2D kV technique was used to acquire pre-treatment and post-treatment images in lateral and anterior-posterior projection. The marker was localized and appropriate corrections before each fraction were done. The value of prostate intrafractional displacement was calculated on the base of post-treatment images.
S773
S772
Volume 81, Number 2, Supplement, 2011
scan in Trilogy, and MV portal images for a head and a prostate patient treatment site. For a head scan the dose to eye, brain, brain stem, and spinal cord are all less than 0.15 cGy which are approximately 38% lower compared to Standard Head scan in OBI 1.4 and doses to bones are shown to be 2 - 4 times higher than that to soft tissues in kV x-rays. However, the doses resulting from a conventional portal imaging system (an anterior of 1515 cm2 and a lateral of 1515 cm2 fields of 6 MV photon beam) are found to be 1 - 5 cGy which are 10 - 30 times higher than that from kV-CBCT images for soft tissues. There is little dose variations between bones and soft tissues resulting from MV portal images. For a pelvis kV-CBCT scan in TrueBEAM, the dose to prostate, rectum, bladder, and femoral heads are 0.8, 0.86, 0.87 and 1.5 cGy, which are approximately 43% lower compared to Pelvis scan in OBI 1.4. The dose resulting from conventional MV portal imaging system using two orthogonal (an anterior of 2520 cm2 and a lateral of 2020 cm2) fields are shown to be 1 - 8 cGy which are 2 - 5 times higher than that from kV-CBCT images for soft tissues. Conclusions: The imaging dose from kV radiographs and kV-CBCT scans is much less than from conventional MV portal imaging. Using kV imaging rather than MV imaging reduces patient doses and results in higher quality images. Reference: 1. Rogers, D.W.O., et al. BEAM: a Monte Carlo code to simulate radiotherapy treatment units. Medical Physics 1995;22:503 524. Author Disclosure: G.X. Ding: None. P. Munro: None.
3176
Delivered Target Dose Reconstruction in Actual and Simulated SBRT Liver Treatments with MV Image Guided Dynamic MLC Tracking
P. R. Poulsen1, E. Worm1, W. Fledelius1, M. Høyer1, P. Keall2,3 1 Department of Oncology, Aarhus University Hospital, 8000 Aarhus C, Denmark, 2Central Clinical School, University of Sydney, Sydney, Australia, 3Department of Radiation Oncology, Stanford University, Stanford, CA
Purpose/Objective(s): Intrafraction tumor motion during stereotactic body radiotherapy (SBRT) may deteriorate the tumor dose distribution. The tumor motion and its impact on the dose distribution can be determined from intra-treatment MV portal images when gold markers are implanted as tumor position surrogates. In this study we use continuous MV images from three liver SBRT treatment fractions to reconstruct the tumor dose distribution (1) for the actually delivered treatments, and (2) for realistic simulations of MV image guided dynamic MLC tracking treatments based on the same MV images. Materials/Methods: A clinical target volume (CTV) was delineated for a liver tumor patient with implanted gold markers. A planning target volume (PTV) was formed by adding 10 mm margins in the cranio-caudal (CC) directions and 5 mm margins in the axial plane. A 5-field conformal treatment plan ensuring 45 Gy mean CTV dose (= 100% dose) and minimum target doses of 95% (CTV) and 66.7% (PTV) was delivered in three fractions. At treatment, cone-beam CT was used for patient set-up based on the marker positions. During treatment continuous MV images were acquired at 7.8 Hz. Offline, the marker positions were segmented in each image and the tumor motion within the MLC aperture was determined. For each fraction, the actually delivered target dose distribution was estimated in a treatment planning system (Eclipse, Varian Medical Systems) by convolving the planned fluence with the observed tumor motion and calculating the resulting 3D dose from the motion blurred fluences. For comparison, the would-be MLC motion in real-time DMLC tracking treatments guided by the MV images and using the same treatment fields was simulated. The latency of a prototype DMLC tracking system (290 ms) was accounted for by target position prediction starting 8 seconds into each field delivery. The target dose distribution was estimated similarly to the actual nontracking treatments. Results: The mean peak-to-peak tumor motion during field delivery was 19 mm in the CC direction and 6 mm perpendicular to the CC direction. For the delivered non-tracking treatments the mean (max) tumor-field mismatch was 4.0 mm (20.7 mm) in the CC direction and 0.8 mm (8.3 mm) perpendicular to the CC direction. For the simulated tracking this was reduced to 0.1 mm (12.3 mm) (CC) and 0.1 mm (5.3 mm) (perpendicular to CC). For the delivered non-tracking treatments, 4.4 cm3 (12%), 3.7 cm3 (10%), and 1.4 cm3 (3%) of the CTV received \95% dose due to tumor motion at the three treatment fractions, respectively. For the simulated tracking treatments, 0% of the CTV received \95% dose (as in the original plan). Conclusions: The CTV dose in SBRT was deteriorated by intrafractional tumor motion. MV image guided DMLC tracking would allow restoration of the planned CTV dose. Author Disclosure: P.R. Poulsen: B. Research Grant; Varian Medical Systems. E. Worm: None. W. Fledelius: B. Research Grant; Varian Medical Systems. M. Høyer: None. P. Keall: None.
3177
Online Intrafraction Spine Motion Monitoring Using Digital Tomosynthesis: Preliminary Phantom Results
W. Verbakel, M. Dahele, J. R. van Sornsen de Koste, S. Senan VU University Medical Center, Amsterdam 1007 MB, Netherlands Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) using volumetric modulated arc therapy delivers high doses to the tumor during 1 - 2 gantry rotations. Steep dose gradients close to organs at risk frequently require intrafraction motion to be limited to 1 - 2 mm. Patients treated at our department do not undergo rigid immobilization during SBRT delivery. Surrogate external markers are often used to monitor motion but their relationship to critical structures is often uncertain. In spine SBRT it would be desirable to monitor and display the position of the spine itself during treatment delivery. We investigated a novel method for doing this with Digital Tomosynthesis (DTS) and present the first results on a moving phantom. Materials/Methods: DTS requires multiple kV images at small angle intervals. In a clinical situation, these could be acquired generating a CBCT scan during RapidArc delivery. For this study, a CBCT scan of the thoracic part of an Alderson phantom was acquired. The CBCT acquisition was paused 4 times to manually translate the phantom, simulating intrafractional shifts. The phantom was equipped with infrared reflective markers (ExacTrac, Brainlab AG) that quantified the translations. The
Proceedings of the 53rd Annual ASTRO Meeting CBCT-scan resulted in 691 projections. A non-clinical research version of DTS software (Varian Medical Systems) was used to generate 120 sets of DTS images from the CBCT projections using 3 arc segments every 3 , and then to register these with reconstructed DTS images from the CT-scan of the phantom. In this preliminary study, groups of four consecutive DTS image sets (representing a 12 arc) were triangulated to determine the shift of the spine with respect to the CT scan. Deviations between actual phantom position according to ExacTrac and DTS results were calculated. Results: Phantom off-sets were between 0.7 and 2.7 mm, in three directions. The average deviations (and range) between DTS results and actual phantom positions in lateral, longitudinal and vertical directions were -0.23 (-1.3 to 0.6), 0.02 (-0.4 to 0.3) and -0.05 (-1.9 to 0.8) mm, respectively. Standard deviations (SD) were 0.50, 0.17 and 0.55 mm, which includes the uncertainty of the imager positioning. The accuracy was best in longitudinal direction because this direction is always perpendicular to the imaging direction. Currently, the combination of four DTS datasets can be generated and processed in less than 10 seconds, enabling new possibilities to perform online setup monitoring while projection data is collected during radiation delivery. Conclusions: Online intrafractional motion using DTS is feasible. A preliminary version of the DTS software was possible to detect the phantom position within 0.5 mm, 1 SD. This supports further development of DTS for online spine tracking during RapidArc SBRT delivery. Author Disclosure: W. Verbakel: D. Speakers Bureau/Honoraria; Varian Medical Systems. M. Dahele: None. J.R. van Sornsen de Koste: None. S. Senan: C. Other Research Support; Varian Medical Systems. D. Speakers Bureau/Honoraria; Varian Medical Systems.
3178
Evaluation Of Visual CT Artifacts While Using Gold, Carbon Or Polyetheretherketone (PEEK) Fiducials In Image Guided Radiotherapy (IGRT)
R. Jacob, S. Shen, R. Popple, W. T. Sobol, D. Dubay University of Alabama, Birmingham, AL Purpose/Objective(s): Implanted fiducials are increasingly used in the IGRT of various organ sites. An ideal fiducial should be easy to implant, readily visible on imaging, show minimum reconstruction artifacts, and not significantly affect radiation dosedistribution. This study evaluates the image perturbations of 4 different fiducials: Gold seed, Gold coil, Carbon and Polyetheretherketone (PEEK) in CT, Cone beam CT (CBCT) and MRI. Materials/Methods: The fiducials used in this study measured 1 mm x 3 mm. They were oriented in a superior-inferior direction in a cylindrical Jazack water-filled phantom. Planning CT images were acquired on a Philips Big Bore CT scanner at 3 mm slice thickness to simulate standard planning imaging, and repeated on a Varian CBCT at 2 mm slice thickness and OBI kV images to simulate daily imaging for setup alignment. MRI images were acquired on a GE scanner at 5 mm slice thickness. Four parameters were evaluated for CT images: 1) identification of fiducial 2) visual inspection of steak artifacts 3) maximum CT value deviated from water due to artifacts 4) volume of artifacts in 10x10 cm in the center slice 5) maximum length of streak artifacts from the fiducial. Results: All fiducials were visualized on planning CT images. The streak artifacts were highest for gold seed, followed by gold coil, and least for PEEK and Carbon. The maximum CT value deviation was -174, -127, -77 and -24 for gold seed, gold coil, Carbon and PEEK respectively. The volume of artifacts in a 10x10 cm 3 mm slice was 0.369, 0.132, 0.046 and 0.074 cm3 and the maximum length of streak artifact from the seed was 80mm, 50 mm, 5 mm and 7 mm for gold seed, gold coil, Carbon and PEEK respectively. On CBCT, the maximum CT value deviation from water was -900, -900, -180, and -231, and the maximum length of streak artifact from the seed was 70mm, 55 mm and 10 mm for gold seed, gold coil, and Carbon/PEEK respectively. As 2 standard deviation for CBCT pixel value for water was 80 - 100, volumes of streak artifacts could not be reliably compared. All fiducials were visualized on OBI kV images. Image contrast was highest for gold seed, followed by gold coil, and least for PEEK and Carbon fiducials. All fiducials were visualized on MRI images as black spots. No obvious image artifacts were seen with any fiducial. Small artifacts were observable when imaged with high gradient magnetic fields. Conclusions: Our results confirm that streak artifacts on planning CT and verification CBCT associated with use of gold fiducials can be avoided by using Carbon or PEEK fiducials, without significantly losing kV image contrast. This has clinical significance in the IGRT of organ sites such as the liver, where post-therapy imaging studies are routinely performed. Author Disclosure: R. Jacob: None. S. Shen: None. R. Popple: None. W.T. Sobol: None. D. Dubay: None.
3179
Intrafractional Prostate Mobility and CTV Margins Depending On Radiotherapy Technique (RapidArc vs. IMRT) of Prostate Cancer Patients
D. Bodusz, L. Miszczyk Maria Sklodowska-Curie Memorial, Cancer Center and Institute of Oncology, Gliwice Branch, Radiotherapy Department, Gliwice, Poland Purpose/Objective(s): IGRT of prostate cancer patients is very precise treatment modality. Daily prostate localization based on 2D/2D kV technique and gold markers (GoldAnchor) requires more time but allows reducing CTV margins and dose in organs at risk. The problem is an intrafractional prostate mobility caused by patient moves, filling of the bladder and rectum or breathing. The main aim of this study is comparison of intrafractional prostate mobility during volumetric modulated arc radiotherapy (RapidArc - RA) and intensity modulated radiotherapy (IMRT). Materials/Methods: 381 measurements of prostate mobility during RA and 60 during IMRT were done. All patients were irradiated in supine position using thermoplastic masks and plate with head support and four fixation points. The 2D/2D kV technique was used to acquire pre-treatment and post-treatment images in lateral and anterior-posterior projection. The marker was localized and appropriate corrections before each fraction were done. The value of prostate intrafractional displacement was calculated on the base of post-treatment images.
S773