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Image Guided Stereotactic Body Radiotherapy for Lung Cancer Using 4D Treatment Planning and On-line Cone-beam CT
I. J. Radiation Oncology d Biology d Physics
S690
3131
Volume 75, Number 3, Supplement, 2009
Imaging Guided Stereotactic Radiosurgery and its Accuracy Compared to Frame-based Localization
Z. Wang, J. P. Kirkpatrick, Q. J. Wu, Z. Chang, C. G. Willett, F. Yin Duke University Medical Center, Durham, NC Purpose/Objective(s): To investigate the accuracy of cone-beam CT (CBCT) guided stereotactic radiosurgery (SRS) and compares its accuracy to conventional frame-based localization. Materials/Methods: A SRS geometric phantom was used to verify the localization accuracy based on a SRS localizer (BrainLAB) and CBCT images (NovalisTx, Varian Medical Systems). 70 patients with 86 SRS treatments were retrospectively analyzed (11 with fixed head rings and 75 with removable U-frames and face masks). Patients were localized with the SRS localizer first. CBCT images with 1 mm slice thickness were then acquired to match planning CT. Results: The SRS geometric phantom showed that the SRS localizer and CBCT image-based localizations agreed within 1 mm. The magnitudes of shifts between the SRS localizer and CBCT images based localizations for SRS with fixed head rings were 0.04 ± 0.05 cm along x (lateral), 0.09 ± 0.06 cm along y (vertical), and 0.08 ± 0.08 cm along z (longitudinal) direction with 0.06 ± 0.14 degrees of couch rotation. For SRS patients using removable U-frames with face masks, the magnitudes of shifts between the SRS localizer and CBCT image-based localizations were 0.11 ± 0.10 cm along x, 0.12 ± 0.09 cm along y, and 0.18 ± 0.13 cm along z direction with 0.34 ± 0.53 degrees of couch rotation. The 95% probability shifts for the removable frame were 0.3 cm along x, 0.3 cm along y, and 0.4 cm along z direction with 1.6 degrees of couch rotation. Conclusions: The results from both the phantom test and the patients with a fixed head ring show that the CBCT based localization is reliable and accurate to within 1 mm. For patients with removable U-frames, differences up to 34 mm were detected between CBCT and frame localization. These differences may be caused by the position change of the patient head relative to the removable frame since the frame based localization assumes that the patient head position relative to the U-frame is the same at the time of treatment as the one at simulation. Author Disclosure: Z. Wang, None; J.P. Kirkpatrick, None; Q.J. Wu, None; Z. Chang, None; C.G. Willett, None; F. Yin, None.
3132
Image Guided Stereotactic Body Radiotherapy for Lung Cancer Using 4D Treatment Planning and On-line Cone-beam CT
T. P. Wong, M. Rao, D. Cao, J. Ye, D. Shepard, V. Mehta Swedish Cancer Institute, Seattle, WA Purpose/Objective(s): This study investigates the geometric accuracy of stereotactic body radiotherapy (SBRT) for lung cancer using 4D based treatment planning, on-line volumetric cone-beam CT (CBCT) image guidance and couch position adjustment for geometric error correction. Materials/Methods: From 7/2008 to 3/2009, 6 patients with lung cancer were treated with fixed-field 3D conformal SBRT using a linear accelerator equipped with an on-board CBCT (Elekta Synergy). Prescribed doses ranged from 9 to 20 Gy per fraction with 3 to 5 fractions. 4D CT was used for treatment planning. Patients were immobilized with a vacuum cushion and cover sheet (BodyFIX) to limit voluntary and involuntary motions. An abdominal compressor was used if the mean tumor motion was larger than 10 mm. The internal target volume (ITV) was determined based on the maximum intensity projection (MiP) 4DCT dataset. The planning target volume (PTV) was defined by adding a 5-mm margin to the ITV. The target volumes were fused to the average intensity projection (Ave-iP) 4DCT dataset for the dose calculation which was performed using a superposition algorithm (CMS Xio) that included tissue heterogeneity corrections. CBCT was used for on-line image guided radiotherapy. For each fraction, patient setup error was corrected based on CBCT guidance prior to treatment. A minimum of two additional CBCTs wereP acquired between beams to analyze and correct for intrafraction errors. For our patient group, the SD of the systematic error ( ) and the SD of the random error (s) for inter- and intra-fraction were analyzed. A single-arc volumetric modulated arc therapy (VMAT) plan was retrospectively generated for two of the patients using the SmartArc inverse planning module (Pinnacle) for plan quality and delivery efficiency comparison. P Results: In the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions, the interfraction areP 7.6 mm, 5.9 mm and 8.1 mm respectively and the interfraction s are 1.5 mm, 3.7 mm and 1.5 mm respectively. The intrafraction and s along these three directions are 0.6 mm, 0.8 mm and 1.1 mm and 0.4 mm, 0.6 mm and 0.7 mm respectively. Results show that online CBCT guidance is needed to correct for interfraction errors. Intrafraction errors are small and well within the 5-mm PTV margin. VMAT produced compatible dose coverage to the PTV and ITV and dose sparing to the organs at risk but with a shorter beam delivery time over 3DCRT of 70 4400 vs. 100 5500 for 12 Gy/fx and 90 3800 vs. 140 1000 for 20Gy/fx. Conclusions: With the use of an improved patient immobilization system, 4D based treatment planning and CBCT for IGRT, high geometric accuracy can be achieved for lung patients using SBRT. The use of VMAT will reduce patient treatment time and improve work flow. Author Disclosure: T.P. Wong, Elekta, B. Research Grant; M. Rao, Elekta, B. Research Grant; D. Cao, Elekta, B. Research Grant; J. Ye, Elekta, B. Research Grant; D. Shepard, Elekta, B. Research Grant; V. Mehta, Elekta, B. Research Grant.
3133
Evaluation of Block Margin and Dose Heterogeneity in Extracranial Stereotactic Radiotherapy of the Lung with 1cm MLC Leaves
W. Logan, J. Ye, V. Mehta, B. Douglas Swedish Medical Center, Seattle, WA Purpose/Objective(s): A decreasing block margin in radiotherapy treatment planning geometrically reduces the volume of tissue being irradiated outside the PTV due to beam divergence. This tissue sparing effect is of particular interest in stereotactic radiotherapy where the dose per fraction is much higher than conventional radiotherapy. However, to maintain prescription isodose
S690
3131
Volume 75, Number 3, Supplement, 2009
Imaging Guided Stereotactic Radiosurgery and its Accuracy Compared to Frame-based Localization
Z. Wang, J. P. Kirkpatrick, Q. J. Wu, Z. Chang, C. G. Willett, F. Yin Duke University Medical Center, Durham, NC Purpose/Objective(s): To investigate the accuracy of cone-beam CT (CBCT) guided stereotactic radiosurgery (SRS) and compares its accuracy to conventional frame-based localization. Materials/Methods: A SRS geometric phantom was used to verify the localization accuracy based on a SRS localizer (BrainLAB) and CBCT images (NovalisTx, Varian Medical Systems). 70 patients with 86 SRS treatments were retrospectively analyzed (11 with fixed head rings and 75 with removable U-frames and face masks). Patients were localized with the SRS localizer first. CBCT images with 1 mm slice thickness were then acquired to match planning CT. Results: The SRS geometric phantom showed that the SRS localizer and CBCT image-based localizations agreed within 1 mm. The magnitudes of shifts between the SRS localizer and CBCT images based localizations for SRS with fixed head rings were 0.04 ± 0.05 cm along x (lateral), 0.09 ± 0.06 cm along y (vertical), and 0.08 ± 0.08 cm along z (longitudinal) direction with 0.06 ± 0.14 degrees of couch rotation. For SRS patients using removable U-frames with face masks, the magnitudes of shifts between the SRS localizer and CBCT image-based localizations were 0.11 ± 0.10 cm along x, 0.12 ± 0.09 cm along y, and 0.18 ± 0.13 cm along z direction with 0.34 ± 0.53 degrees of couch rotation. The 95% probability shifts for the removable frame were 0.3 cm along x, 0.3 cm along y, and 0.4 cm along z direction with 1.6 degrees of couch rotation. Conclusions: The results from both the phantom test and the patients with a fixed head ring show that the CBCT based localization is reliable and accurate to within 1 mm. For patients with removable U-frames, differences up to 34 mm were detected between CBCT and frame localization. These differences may be caused by the position change of the patient head relative to the removable frame since the frame based localization assumes that the patient head position relative to the U-frame is the same at the time of treatment as the one at simulation. Author Disclosure: Z. Wang, None; J.P. Kirkpatrick, None; Q.J. Wu, None; Z. Chang, None; C.G. Willett, None; F. Yin, None.
3132
Image Guided Stereotactic Body Radiotherapy for Lung Cancer Using 4D Treatment Planning and On-line Cone-beam CT
T. P. Wong, M. Rao, D. Cao, J. Ye, D. Shepard, V. Mehta Swedish Cancer Institute, Seattle, WA Purpose/Objective(s): This study investigates the geometric accuracy of stereotactic body radiotherapy (SBRT) for lung cancer using 4D based treatment planning, on-line volumetric cone-beam CT (CBCT) image guidance and couch position adjustment for geometric error correction. Materials/Methods: From 7/2008 to 3/2009, 6 patients with lung cancer were treated with fixed-field 3D conformal SBRT using a linear accelerator equipped with an on-board CBCT (Elekta Synergy). Prescribed doses ranged from 9 to 20 Gy per fraction with 3 to 5 fractions. 4D CT was used for treatment planning. Patients were immobilized with a vacuum cushion and cover sheet (BodyFIX) to limit voluntary and involuntary motions. An abdominal compressor was used if the mean tumor motion was larger than 10 mm. The internal target volume (ITV) was determined based on the maximum intensity projection (MiP) 4DCT dataset. The planning target volume (PTV) was defined by adding a 5-mm margin to the ITV. The target volumes were fused to the average intensity projection (Ave-iP) 4DCT dataset for the dose calculation which was performed using a superposition algorithm (CMS Xio) that included tissue heterogeneity corrections. CBCT was used for on-line image guided radiotherapy. For each fraction, patient setup error was corrected based on CBCT guidance prior to treatment. A minimum of two additional CBCTs wereP acquired between beams to analyze and correct for intrafraction errors. For our patient group, the SD of the systematic error ( ) and the SD of the random error (s) for inter- and intra-fraction were analyzed. A single-arc volumetric modulated arc therapy (VMAT) plan was retrospectively generated for two of the patients using the SmartArc inverse planning module (Pinnacle) for plan quality and delivery efficiency comparison. P Results: In the medial-lateral (ML), superior-inferior (SI), and anterior-posterior (AP) directions, the interfraction areP 7.6 mm, 5.9 mm and 8.1 mm respectively and the interfraction s are 1.5 mm, 3.7 mm and 1.5 mm respectively. The intrafraction and s along these three directions are 0.6 mm, 0.8 mm and 1.1 mm and 0.4 mm, 0.6 mm and 0.7 mm respectively. Results show that online CBCT guidance is needed to correct for interfraction errors. Intrafraction errors are small and well within the 5-mm PTV margin. VMAT produced compatible dose coverage to the PTV and ITV and dose sparing to the organs at risk but with a shorter beam delivery time over 3DCRT of 70 4400 vs. 100 5500 for 12 Gy/fx and 90 3800 vs. 140 1000 for 20Gy/fx. Conclusions: With the use of an improved patient immobilization system, 4D based treatment planning and CBCT for IGRT, high geometric accuracy can be achieved for lung patients using SBRT. The use of VMAT will reduce patient treatment time and improve work flow. Author Disclosure: T.P. Wong, Elekta, B. Research Grant; M. Rao, Elekta, B. Research Grant; D. Cao, Elekta, B. Research Grant; J. Ye, Elekta, B. Research Grant; D. Shepard, Elekta, B. Research Grant; V. Mehta, Elekta, B. Research Grant.
3133
Evaluation of Block Margin and Dose Heterogeneity in Extracranial Stereotactic Radiotherapy of the Lung with 1cm MLC Leaves
W. Logan, J. Ye, V. Mehta, B. Douglas Swedish Medical Center, Seattle, WA Purpose/Objective(s): A decreasing block margin in radiotherapy treatment planning geometrically reduces the volume of tissue being irradiated outside the PTV due to beam divergence. This tissue sparing effect is of particular interest in stereotactic radiotherapy where the dose per fraction is much higher than conventional radiotherapy. However, to maintain prescription isodose